SPATIAL, STRUCTURAL, AND BEHAVIORAL

CHARACTERISTICS IN THE OVERLAPPING HOME RANGES f _

0F SONG AND FIELD SPAR-ROWS _ '

Thesis for the Degree of M. S. .
MICHIGAN STATE UNIVERSITY. ,
ALBERT AULEITE -
1977

 

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ABSTRACT

SPATIAL, STRUCTURAL, AND BEHAVIORAL CHARACTERISTICS IN
THE OVERLAPPING HOME RANGES OF SONG AND FIELD SPARROUS

By
Albert Aulette

This study examined the extent and consequences of interspecific
home range overlaps between Song and Field Sparrows, revealing general
patterns of coexistence during the breeding season. Two old fields on
the campus of Michigan State University served as the study sites
during the summer of 1975. Three classes of data were collected:
spatial locations of the two co-occurring species, their activities,
and habitat structure measures. The locations and activities of each
individual were plotted on scaled, gridded, transparent field maps.

By overlaying the maps, locations and activities occurring in the
home range overlap areas of each species were contrasted with the patterns
in the non-overlapping areas.

The analyses used revealed several sets of differences between
the Song'and Field Sparrows. The Field Sparrow's home range is from
l.? to 2.5 times as large as that of the Song Sparrow (both fields
considered). The overlap zone comprised 25% of the Song Sparrow's
home range area, and about 12% of the Field Sparrow's home range area.
In each of the two fields, Overlap and Overlap Proportion Indices
indicated that both species spent a disproportionate amount of time

within the overlap areas, yet maintained a high degree of spatial

Albert Aulette

separation there. Overlap and non-overlap areas for each species were
shown to differ in at least several classes of vegetation structure.
The two species differed further in their utilization of the home range
zones. Host of the activities and associated substrates were observed
more frequently in the overlap area relative to the non-overlap area
in the Song Sparrow. The Field Sparrow also showed activities and
substrates which were more frequent in the overlap area, but fewer
categories were involved. When contrasts were made between the two
species within the overlap zone, the Song Sparrow predominated in terms
of both activity and substrate usage patterns.

While not a test fer competition between the species, this
study demonstrated spatial, structural, and behavioral differences

which may allow the two species to coexist.

SPATIAL, STRUCTURAL, AND BEHAVIORAL CHARACTERISTICS IN

THE OVERLAPPING HOME RANGES OF'SONG AND FIELD SPARROHS

By

Albert Aulette

A THESIS

Submitted to
Hichigan State University
in partial fulfillment of the requirements

for the degree of

MASTER OF SCIENCE

Department of Zoology

1977

ACKNOWLEDGMENTS

I thank the members of my guidance committee, Dr. Stephen N.
Stephenson, Dr. William E. Cooper, and the chairperson, Dr. Donald
L. Beaver. Special thanks goes to Dr. Beaver, not only for
invaluable assistance on the thesis, but for continuous encouragement.
I wish to thankIJudy Aulette for not typing any of the manuscript,
and for several theoretical and practical ideas for this study.
Finally, the works of Woody Allen have helped me to keep my work in

the proper perspective.

ii

TABLE OF CONTENTS

ListofTables.........
ListofPigures ........
Introduction..........
Species and Study Area . . . . .
Methods ............

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8m 0 O O O O O O O O O O 0

Suggestions for future Research

Literature Cited 0 o o o o o o 0

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13
22
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53
5s

Table l.
T‘bl. 2.

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Table H.

T‘blc 50

Tabl. 6 o

Tabla 7 e

thC 80
Table 9.

Tabl. 10 0

LIST OF’TAHLES

Sizes and weights of the Song and Field Sparrow . .

Activities and associated substrates recorded for
C‘ChBPOCiCBoaeoooeooooeoeooeoo

Vegetation classes which were measured and used in
the analysis of home range structure . . . . . . .

mmwuocover'alnea(n) oeeeooeooao

Area (3(- 1 Signof overlapping and non-overlapping
(exclusive) me ranges of Field and Song Sparrows

Analysis of variance table for the average values
for the difference between exclusive Song Sparrow
areas and overlap areas and between exclusive
Field Sparrow areas and overlap areas within
CEChfi-Oldoa-aooaeoaoeeeoeeoeeo

Analysis of variance table for overlap index values
on species pairs within each field . . . . . . . .

Overlap proportion index (I) . . . . . . . . . . .

Analysis of variance table for the overlap
proportion indices of individual Song and
”Oldswrousaeooaoeeoeooaaeoee
Avian species seen on or near the home ranges
OfthGSOngandHoldspmrrow. o a e e o a o o e 0

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15

16
17

25

26

28

32

52

Figure 1.
figure 20

Figure 3.

figure “a

Figure 5.

Figure 6.

LIST OF'FIGURES

Water Quality Management Project Area . . . .

Two selected study sites within the Water
Quality Management Project Area . . . . . . .

Example of an observationvarea curve . . . .

Diagrammatic representation of home range
overlaparealooeooeooeaooaaoe

Significant vegetation differences for three
contrasts of home range zones . . . . . . . .

Significant activity and substrate differences

12

19

2h

35

INTRODUCTION

Even to the most casual observer it is apparent that animals
utilize space in precise and characteristic ways. Spatial patterns
are manifestations of the more subtle biotic and abiotic interactions
that occur between individuals and their environment. In this sense,
locally sympatric species have gained attention recently as the
subjects of scientific investigations into the nature of such
phenomena as resource partitioning, competition, and coexistence.

If two or more species co-occur in a particular area, the survival

of their populations (and, hence, their coexistence) may in some

manner depend upon the spatial extent of the local areas of co-occurrence.
If significant areas of spatial overlap are evident, it becomes
interesting to examine the physical and biological structure of

such areas along with the modes of behavior exhibited in these zones

in order to reveal possible ecological factors contributing to

their coexistence.

Territoriality in birds is one of the most familiar and
thoroughly studied systems whereby space is partitioned within a
particular habitat. The interest in such a system lies in the fact
that it may serve to completely separate the ecological activities of
individuals (Cody, 197A). In some cases two sympatric species which
exhibit only intraspecific territoriality may have territories that

spatially overlap and are jointly used within a local habitat. This

1

2
study investigates such a situation (with the modifications presented
below) and attempts to explain the patterns feund in terms of
coexistence mechanisms.

Territorial behavior, especially in birds, has been described
frequently since the classic book by Reward (1920). Although the
range of these behavioral interactions varies greatly between species,
the territories themselves have popularly been defined as defended
or exclusive areas (Nbble, 1939). Once established, a territory
entitles an individual to exclusive use of its resources relative to
conspecifics (Pitelka, 1959). Hence, the appeal of territories for
resource partitioning and coexistence studies is evident. These
concepts, however, require repeated observations of those sometimes
rare and subtle actions termed "defense" in order to delimit a
particular territory. To avoid these problems some researchers have
re-defined the territorial concept and for research purposes have
presented such terms as "total utilized area" (Stenger and Falls 1959),
"activity space" (Stenger Weeden, 1965), "maximum territory," and
"home range" (Odum and Kuenzler, 1955). All these terms are based on
the combined sightings of individuals engaged in a variety of activities
within a certain area and time span. The "defended" territory is
hereby considered only as some subset of a'more broadly defined area
to which an individual (or pair) is confined during the breeding
season. The term "home range" appears frequently in the scientific
literature, and refers to that broad area to which animals of a
species regularly confine their activities. Rather than basing an

investigation of coexistence mechanisms on spatial patterns resulting

3
from the rather specific behaviors called defense, home ranges may
delimit more ecologically complete areas. Territoriality, then, is
the behavioral phenomenon that initially distributes birds over a
habitat; it is in this organizational sense that territoriality is
discussed in the remainder of this study. However, once the spatial
arrangements have been established behaviorally, home ranges can then
serve as the units of investigation. The results of this study are based“
on measurements of home ranges. As will be seen below, these "home
ranges" may differ only minimally in size from their respective
"territories."

The resources actually encompassed by a territorial boundary
vary depending upon the type of territory (Hinds, 1955). The type-A
territory of Nice (l9#l) is used for mating, nesting, and.feeding of
both adults and young, and is typical of many passerine species.

In such instances determining the limits of a territory at a
particular point in time simultaneously delimits the resource space.
The type-A territorial system is characteristic of the species in the
study; and, since birds in this system rarely venture beyond the limits
of their "defended” area, a territory may have essentially the same
area as a home range (Brown, 1959).

The interest in the territorial (home range) system as a space
and resource partitioning mechanism lies in the fact that such a
system is assumed to have survival value for local populations of
a species. Intraspecific territoriality is an adaptation selected
for in the course of evolution because it aids individuals in

competing for the requisites for reproduction or survival (Brown and

I,
Oriana, 1970). The aid is rendered through the actual reduction of
competition by spatial separation within the habitat (MacArthur and
Wilson 1967).

Now consider the case of two intraspecifioally territorial bird
species coexisting in a local habitat throughout a breeding season.
The resource utilization patterns exhibited will depend upon the
ecological similarity between the two species. If ecologically very
similar, they might show interspecific territoriality and separate
spatially over the habitat. The mechanisms involved here seem to be
no different than those of intraspecific territoriality (Hinds, 1956).
This situation has been described fer many species including Red-winged
and Yellow-headed Blackbirds (Orlans and Wilson, l96h), Red-tailed
Hawks and eagles (Fitch, gt_gl, 19H6), Eastern and Western Meadowlarks
(Lanyon, 1956), and thrushes (Horse, 1971). If there are important
ecological differences between the two species such that it is no
longer necessary to maintain interspecifically exclusive areas, their
territories may overlap locally. Most species respond to such
situations of potential competition with relatively specific resource
utilization patterns (Niell, 1975). The term "ecological isolation,"
coined by Moreau (1948) and exemplified by Lack (1971), describes the
evolution of the different utilization patterns which allow local
coexistence. Ecological isolation may take place along a number of
niche dimensions.

Communities containing ecologically similar species which do not
show interspecific territoriality have been examined with respect to

general ecological isolating factors (Moreau, l9h8; Pitelka, 1951;

5
Schoener, 1968; Lack, 1971; Pianka, 1973; Cody, 197“). On a finer scale
than most of the previous investigations into coexistence patterns in
birds, this study examines the occurrence and extent of interspecific
home range overlaps by two species of sparrows in old fields. Spatial
and behavioral patterns of individuals and microhabitat structure are
compared between overlapping and distinct portions of the home ranges
to determine the way these birds use the habitat. While not a test
for competition between the species, this study may allow generalizations

to be made about potential competition and coexistence mechanisms.

SPECIES AND STUDY AREA

The two species observed were the Eastern Field Sparrow
(Spizella pusilla pusilla (Wilson)) and the Mississippi Song Sparrow
(Melospiza melodia euphonia (Wetmore)). The Eastern Field Sparrow
is one of two subspecies in North America, and breeds from central
Minnesota, north-central Michigan, and southwest Quebec south to
eastern Texas, southern Mississippi, and southern Georgia (A.O.U., 1957).
It is fairly common in old fields and bushy hedgerows. The Mississippi
Song Sparrow, one of 31 subspecies north of Mexico, breeds from
northern Wisconsin, northeastern Michigan, and western New York,
south through southeast Minnesota, Iowa; southwest Kentucky, western
North Carolina to northern Georgia, and northwestern South Carolina
(A.O.U., 1957). It is locally abundant in gardens, fields, and forest
margins. The breeding ranges of the two subspecies overlap over
much of the Midwest with Michigan being near the northern extent of
the range of each. Both species belong to the order PasserifOrmes
and family Fringillidae. Although the Song Sparrow is heavier, both
birds appear similar in size in the field (Table 1). They are both
strongly territorial (Nice's type-A) and share a similar breeding
schedule in the study area. The two sparrows can commonly be found
in local sympatry.

The study was conducted from June 9 to September 9, 1975,

6

7
within the Water Quality Management Project Area in Ingham County
on the campus of Michigan State University, East Lansing, Michigan
(Figure 1). Data were collected from two selected fields (Figure 2).
Fields I and II are 2.55 and 5.50 hectares in size, respectively, and
are separated by a 275 meter extent of deciduous woods. Each field
is bordered by either a narrow dirt road, a woodlot, or hedgerows,
and is comprised of typical old field successional vegetation. Field I
is at a slightly higher elevation than Field II, slopes to the south,
and contains a low central area with trees and shrubs. Field II is
slightly rolling but of uniform vegetative composition. Field I has
been fallow fer about 7 years, and Field II for considerably longer

(no exact determination).

Table 1. Sizes and weights of the Song and Field Sparrow.

 

 

Linear Measurements1 Song Sparrow Field Sparrow
bill to tail length 16.00 cm. 1#.45 cm.
distance from bend of
wing to end of longest
primary 6.#0 6.55
tail length 6.65 6.b8
bill length 1.24 0.91
Weights
average weight 21.} gm.2 12.6 gm.3

 

l

ZNice , 1937s

3‘Wetherbee, 1934.

Based on American Museum of Natural History specimens (Chapman, 1966).

Figure 1. Water Quality Management Project Area. The two study
fields are located within the broad area delimited by
the dashed lines.

 

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11

Figure 2. Two selected study sites within the Water Quality
Management Project Area. The shaded areas represent
woodlots. Both Fields I and II are bordered by
either roads, woodlots, or tree rows. (Disregard
other symbols in the diagram.)

 

 

 

 

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METHODS

Three classes of data were collected: spatial locations of
the two co-occurring species, their activities, and habitat structure
measures. The spatial patterns were used to elucidate home range
boundaries. The behaviors observed were used to determine if and
how activity varied within the overlapping areas relative to non-
overlapping areas; and, since the response to and the utilization
of the habitat by birds are assumed to be related to its structure,
substrate measures should reveal factors underlying home range
organization (Wiens, 1969).

Censuses taken during the previous summer indicated that both
Song and Field Sparrows occurred together in certain areas of the
Water Quality site. Fields I and II were selected for intensive study
because there were at least several pairs of overlapping territories
in each. A grid system was established in each field by placing
garden stakes with flagging at the corners of every 30.5 meter square
partition. The basic methodological unit of the study involved observing
the male of each home range for one hour periods, and plotting its
position on a scaled map (further divided into 15.25 meter squares)
at every one minute interval. From past experience, one minute intervals
seem appropriate to capture a wide range of behavior and movement for

each species. In addition, the bird's activity at that particular

13

1h

point and the associated substrate were also recorded. The data from
each of these one hour periods constitutes one observation-set. The
important activities and substrates, some of which follow Wiens (1969),
are presented in Table 2.

Most observations were done in the morning from approximately
0700 to 1200. Due to the differences in the observabilities of the
various birds, the number of one hour observation periods collected
for each home range varied. The observations were made from vantage
points within the low vegetation or behind shrubs and trees in the two
fields. There is a certain amount of difficulty involved in tracking
a bird for one hour, and there were occasions when a bird was "lost"
in the middle of one observation-set. However, most of these partial
sets with less than 60 data points were still usable in the analysis.

Due to the practical limitations on continuous sampling
(Wiens, 1969), complete habitat descriptions were taken once in each
field during the middle of the growing season (July A - Field II,
July 5 - Field I). Additional notes were kept on vegetational changes
when they were noticed. The habitat descriptions were made by mapping
each grid square separately on a scaled map, and noting average
vegetation types, clumps, low patches, bare patches, trees, and shrubs.
(See Table 5 for a list of the vegetation factors considered and the
discussion for the significance of these factors.) A subjective
assessment of percent cover for these vegetation types in relation
to each grid square was made according to the Domin scale (Kershaw,

1973) (Table A).

15

Table 2. Activities and associated substrates recorded for
each species.

 

Activity

 

singing

foraging

perching

perched while singing but between songs
aggression and/or display

flying

preening

nest associated

perched while giving alarm notes
perched with fOOd in bill
feeding young

courtship

unknown (view obstructed)

Substrate type

graminoid (narrow leaf herbaceous plant)
fbrb (broad leaf herbaceous plant)

shrub (less than six feet tall)

tree

post (log, stakes, dead tree)

bare ground

air (flying)

16

Table 3. Vegetation classes which were measured and used in the
analysis of home range structure.

 

Field I

 

aster (Aster vimineus)

bare ground - void of vegetation

chickory (Cichorium intybus)

clumps - aggregations of various plants distinctly noticeable
within a grid square

bramble (Rubus spp.)

goldenrod (Solidage spp.)

height of vegetation

low grasses - one category composed of either separate patches or
mixtures of quack grass (Agrgpyron re ens), bluegrass
(£22_app.), timothy (Phleum pratense , and.others.

'Iaplings -young deciduous trees less than 2 meters tall

shrubs - clustered woody vegetation, usually less~than.2 meters
tall.

trees - deciduous

wild carrot (Daucus carota)

 

Field II

 

aster

bramble

common mullein (Verbascum thapsus)

clumps

goldenrod ‘

height of vegetation

high grass - reed canary grass (Phalaris arundinacea)

low patches - covered with very low vegetation or worn down due
to the presence of a path

saplings

trees

wild carrot

low grasses

 

17

Table #. Domin Scale cover values (D).

 

U

 

cover about 100%

cover 75%

cover 50 - 75%

cover 33 - 50%

cover 25 - 33%

abundant, cover about 20%
Tabundant, cover about 5%
scattered, cover small
very scattered, cover small
scarce, cover small
isolated, cover small

5..-

PNW#\fiO\\lm@O

s
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18

Each observation set of 60 sequential data points (less in some
cases) per home range plotted on the scaled maps was turned into
statistically standardized measures of area by using a modification
of the method of Odum and Kuenzler (1955). This method allows a
standardized home range size to be determined once an observation-area
curve has been constructed. On a scaled map the first 10 observation
points were connected forming a convex polygon, and the area enclosed
was measured with a compensating polar planimeter. This area was now
plotted against the number of points used (10). Next, the first 20
observation points were connected, and the enclosed area was calculated
and plotted as another point on the graph. This was done until all
the points for one observation-set of a home range were plotted forming
the curve. Initially the cumulative area increased as more observations
were added, but theoretically the curve eventually levels off. The
standardized area is chosen as that point on the curve after which one
additional observation results in less than a one per cent increase
in area (Figure 3). The only difficulty involved with this method is
actually finding the equation of an observation-area curve which best
fits the data points. In the Odum and Kuenzler paper and in subsequent
studies which have used their method the.curve appears to have been
fit by eye. However, since the data suggests a curve with properties
much like those of growth curves, the method of Ricklefs.(1967) was
employed to derive equations for each curve. This is a graphical
method but is more precise and less subject to error than is the
previous procedure. Briefly, raw data were replaced by conversion

factors which transformed growth curves into straight lines. The slopes

eumlative area
(hectares x 10)

 

Fig" 30
Kuenzler 1955).

19

+ + +
\ 'one percent" point

10 20 50 no so 60

number of observations

Example of an observation-area curve (after Odum and

20
of these lines were measured by a linear regression analysis and were
proportional to the overall rate of increase in area. As a result,
for each observation-set a growth-like logistic equation was obtained
to describe the observation-area curve. From these equations the one
percent points were calculated, and the areas of the home ranges
determined. The areas were redrawn proportionately onto the field maps
with the aid of a Map-o-Graph reducing-enlarging machine. These
standardized home ranges were polygonal in shape.

The above method of generating home range sizes was used for
those observation-area curves which leveled off within the 60 observation
period. Level curves were defined as those in which at least the last
two data points had the same value for cumulative area. However, the
above methods yielded two classes of "incomplete" data. One consisted
of those observation-area curves which did not level offrwithin the
60 observations. The other class of curves also did not level off,
but were based upon less than 60 observations per set. In order to
use these incomplete data, a series of regression analyses were
performed. First, for those occasions when the level portions of an
observation-area curve were attained, the slope of the initial
(non-level) part of the curve was calculated. This was done by
determining a line of best fit to these initial points. Next, the
linear relationship between the slopes of the initial part of these
curves and the final "one per cent" areas was determined. Finally,
the slopes of the nonéleveling curves were determined. Given these
slope values, corresponding "one percent" areas were calculated from

the derived regression equation. A similar method for using otherwise

21
incomplete data on the territory size of the Ovenbird was fbllowed
by Stenger and Falls (1959).

At this point the standardized home range for each observation-
set on a particular bird was drawn on a plastic transparency of the
gridded field. By overlaying transparencies the location and extent
of home range overlaps were determined. It should be noted that because
of the difficulty in obtaining simultaneous observations of the two
species for all overlaps, the overlapping pairs were composed of two
observation-sets recorded in the field at varying times within the study
period. The fellowing analysis is based then, upon the assumption
that the birds are doing broadly similar things over the comparison
periods (see discussion). Based on these data for each overlapping
pair of home ranges, comparisons of spatial location, activity,
substrate, and micro-habitat structure could be made between overlap

and non-overlap portions of the home ranges.

RESULTS

Overlap and Hen-overlap Areas

The average area of overlap in home range and the average
non-overlap (exclusive) area for each of the two species in Field I
and II are presented in Table 5. (These are later referred to as
simply "overlap" and "non-overlap" respectively.) The pattern was
the same in each field; the zone of overlap had the smallest area,
the exclusive Song Sparrow zone had a larger area, and the exclusive
Field Sparrow zone had the largest area. Within each field, differences
in area between (1) exclusive Song Sparrow and overlap zones,
(2) exclusive Field Sparrow and overlap zones, and (3) exclusive Song
and Field Sparrow zones are all significant based on 98% confidence
intervals on the mean value of the differences for each of the above
three comparisons. Due to the nature of the statistical test used,
the third comparison is not independent and the associated significance
may not be reliable. A square root transformation was made on the
data for the difference values before calculating the confidence
intervals to obtain a normal distribution. In addition, for each set
of area measurements taken on a particular Song or Field Sparrow's home
range, an average value for the difference in areas within categories
(1) and (2) above was calculated. A two-level nested analysis of

variance was made on these average difference values for each species

22

23
within each field. The results are shown in Table 6. A significant
variance component can be seen among species, but there is no added
variance among fields.

Adding the overlap area to the two non-overlap areas (Table 5)
gives average home range sizes for the Song Sparrow and the Field
Sparrow in Field I of 0.161 hectares and 0.#08 hectares respectively.
Similarly, in Field II the average home range sizes for the Song and
Field Sparrow are 0.326 and 0.564 hectares respectively. If the
average overlap areas are expressed as fractions of the average home
ranges, the Song Sparrow has an overlap which is on the average 25%
of its home range in both fields. For the Field Sparrow the overlap
represents 10% and 15% of its home range area in Fields I and II
respectively. Within each field, then, Song Sparrows tend to have
smaller home ranges with relatively large overlap zones, while Field
Sparrows tend to have larger home ranges with relatively small overlap

zones (see schematic diagram in Figure A).

Zone of overlap

 

 

Song Sparrow of both Field Sparrow
10% 90% (I)
15% 85% (II)

 

(16:11) 75% 25%

 

 

 

 

 

 

 

Average home range Average home range
area (Estates) area (hectares)

I 0.161 I 0.14.08

II 0.326 II 0.56.;

 

I and II are the field identifications.
fi's are based an area.

Figure A. Diagramatic representation of home range overlap areas.

25

4.
Table 5. Area (I'- S—) of overlapping and non-overlapping (exclusive)
home ranges of Field and Song Sparrows. The units are given
in hectares.

 

SongFSparrow “2 Field Sparrow

 

Overlap Area Non-overlap Non-overlap N
Field 11 .040 i .001» .121 3' .006 6 .368 ‘3 .017 6
Field 111 .083 i .006 .243 3 .012 7 .481 1 .024 7

 

1Within each field all areas are significantly different. (Based on
98% confidence intervals on mean difference values - see text.)

2N: the number of individual home ranges on which repeated measurements
were taken.

26

Table 6. Analysis of variance table for the average values of the
difference between exclusive Song Sparrow areas and overlap
areas and between exclusive Field Sparrow areas and overlap
areas within each field. (See text for further explanation.)

 

 

Source of variation df SS MS Fs
Fields 1 .2987 .2987 .22831 ns
Species within ,,

fields 2 2.6176 1.3088 15.h70H
Error _jg; 1.8622 .0886
25 H.7735

 

1Fe is based on the synthesis of a new denominator mean square
(1.3086). (See Sokal and Rohlf, 1969. p. 278.)

C.
Significant at p (.01 level.

n8Non-significant

27
Co-utilization of Overlap Zone
The degree to which the two species jointly used the same
resource, the overlapping home range, served as a measure of niche
overlap. Utilization in this context refers to the frequency of
occurrence of either species in the grid squares comprising the zone
of overlap. The overlap index of Schoener (1968)

D = 1 -‘”! i=1 Px,i - gy,i

was used to calculate the degree of overlap. The overlap zone
established by a pair of home ranges contained n grid squares. The
Px,i represent that proportion of the total observations of species x
(Song Sparrow) in the overlap which were recorded in the particular
grid square (resource category) i. The Py,i represent the same
proportion for species y (Field Sparrow). The value of this index
varies from 0 (no overlap) to 1 (complete overlap). The resulting
overlap index for Field I is 0.37 3 .03 ('x' i 3;) and for Field II is
0.32 3.02. Since the value 1 would indicate that over the comparison
periods both species were recorded in every occupied grid square of
the overlap, the above values demonstrate low degrees of overlap. In
addition, a two-level nested analysis of variance was made on the
overlap index values for the species pairs in each field. The results

are shown in Table 7. Neither fields nor species pairs within fields

show a significant variance component.

28

Table 7. Analysis of variance table for overlap index values on
species pairs within each field.

 

 

Source of variation df SS MS Fs
Fields 1 .0653 .0653 1.39271 ns
Overlapping pairs

within fields 11 .4943 .0449 .5939 ns
Error 215 16.244 .0756
227' I6785tg

 

1!: is based on the synthesis of a new denominator mean square

(.0H10). (See Sokal and Rohlf, 1969, p. 278.)

nalien-significant

29
Frequency of Overlap Use
Another way of viewing the home range utilization by each species
is to determine what percentage of the 60 total observations recorded
on one home range occur within its overlap zone. However, such a
percentage conveys more information when related to the fraction that
the overlap area represents of the total home range area. For example,
if 30%Iof the 60 observations were in the overlap zone and that zone
represented 5% of the home range area, this would indicate a different
utilization pattern than the case in which the overlap represented
95% of the home range area. What I have called an Overlap Proportion
Index was calculated for each set of overlapping home ranges. The

index (a separate one for each species) is

x/6O

 

Y/z
where x = the number of observations out of the 60 (in one observation
set) occurring in the overlap for one species, y = overlap area (in
hectares), and z = total home range area (overlap e non-overlap)
(in hectares) for the same species. This ratio combines into one
measure the percentage of observations in an overlap relative to the
percentage that the overlap represents of the entire home range (Table 8).
The empirically determined extreme values of the index ranged from
O to 85.6 considering both species and both fields. One would expect
on the basis of random location of observations over the entire home
range of a species that the index would range around a value of 1.
(For example, if 30% of the observations were recorded in the overlap,

and the overlap represented 30% of the entire home range, the index

Table 8. ‘Qverlap Proportion Index (I). The values reported are

I - SI (see text).

 

 

Song Sparrow N2 Field Sparrow N2
Field 11 4.320 3' .684 77 4.579 3 1.016 79
Field 111 3.163 ’3 .832 113 3.312 t .524 108

 

1Average values for both species are significantly different from 1
based on 95% confidence intervals on the means (log transformed data).

2The number of overlap sets (N) differ because the incomplete cases
in which less than 60 observations were recorded were not used in
the calculation of the index.

31
value would be 1.) The Overlap PrOportion Indicies, however, are
significantly different from 1 (Table 8), implying that both species
were observed more often within the overlap zones than expected.
In addition, a three-level nested analysis of variance was done on the
overlap proportion indicies of individual Song and Field Sparrows. The
results are presented in Table 9. A significant variance component
exists among fields, but there is no added variance among the species
within fields or among individuals within species.

The pattern, then, is that both species make disproportionately
greater use of the overlap zones, which represent on the average about
25% of the Song Sparrow's home range and 12%»of the Field Sparrow's
home range, relative to their entire home ranges. Yet, in the overlap
areas there is not a high degree of association between the species

(based on the values of the Overlap Index).

Vegetation Structure

The next category of data is that of the vegetational structure
of the home ranges. Based on the analysis of all grid squares in each
field, average values for the Domin scale indicies (see Table 4) of
the vegetational classes were determined for the home range overlap
zones and the exclusive Song and Field Sparrow zones. The structural
classes of height, vegetation clumps, and low patches were not assigned
a Domin value for percent cover. Height was measured as a linear
variable, and clumps and low patches were noted as being either present
or absent in a grid square.

For Field I the approximate percent cover of goldenrodbM-

32

Table 9. Analysis of variance table for the overlap pr0portion
indicies of individual Song and Field Sparrows.

 

Source of Variation df SS HS Fs

 

Fields 1 134.9376 134.9376 20.50441.

Species within 2
fields 2 3.8058 1.9029 .0398 ns

Individuals within
species 16 1098,9616 68.6851 .314? ns

Error 7 12. 218.2431
575%

 

1Fe is based on the synthesis of a new denominator mean square
(6.5809). (See Sokal and Rohlf, 1969. p. 285.)

2F5 also based on a new denominator mean square (47.8437).
.
Significant at the p < .05 level.

n8Non-significant

33

and covesnml“iow grass is 25% - 33%; for Field II the approximate
percent cover of goldenrod is 50% - 75% and cover of low grass is
20%. All other factors represent cover values of less than 5%. The
average values for all the vegetational structure classes were contrasted
three ways to detect differences: 1) values in the home range overlaps
vs. values in the Song Sparrow's exclusive range; 2) values in the
overlap vs. values in the Field Sparrow's exclusive range; and
3) values in the Song Sparrow's exclusive range vs. values in the Field
Sparrow's exclusive range. Significant differences are based on
98% confidence intervals on the differences between means (Sokal and
Rohlf, 1969). However, since the third contrast is not independent,

0
the associated significance there may not be reliable. The significant
results are diagrammed in Figure 5.

From Figure 5 many structural differences are apparent both
between overlap and non-overlap and between the non-overlaps of both
species. The Song Sparrow (Contrast 1) has an overlap that represents
a greater low grass cover in both fields and a higher percentage of
low patches in Field II. This is contrasted with a non-overlap of
greater chickory, sapling, and goldenrod cover. The Field Sparrow
(Contrast 2) in each field has an overlap with greater goldenrod
cover and vegetation height and a non-overlap with greater sapling
cover. Both fields jointly show further differences for the Field
Sparrow, more so than that for the Song Sparrow. Ths~low grass and
goldsmrod (with greater cover in the overlap areas of Contrasts l and

2) are substrates in which most of the foraging activity for the

season was observed. Contrast 3 of Figure 5 shows that several

Figure 5.

3h

Significant1 vegetation differences for three contrasts of
home range zones. Those vegetation types listed under

the appropriate member of a contrast have a greater value
for cover (or greater percentage or greater height) than
in the opposed member.

fl. “M‘—

 

 

 

 

 

 

Contract 1 Contrast 2
Song over over Field
Sparrow lap lap Sparrow

Field I
chiekory low grass
saplings
shrubs bare ground
goldenrod saplings
height
Field II
goldenrod low grass
low patches
mullein aster
bramble high grass
goldenrod saplings
height wild carrot

35

 

 

 

 

 

 

 

 

 

 

 

 

Contrast 5
Song Field
Sparrow Sparrow
goldenrod low grass
saplings
mullein aster
bramble low grass
height wild carrot
low patches

 

_—

1) Significance based on 98% confidmoe intervals on the differences
betwoen neans (Sokal and Bohlf 1969).
2) Simificence based on test of equality of percentages (p (.05)
(Sokal and Rohlf 1969).

 

36

vegetative classes differ between the two species' non-overlaps.
Considering the two factors of greatest coverage, in Field I goldenrod
cover is higher in the Song Sparrow non-overlap, and in both fields
low grass cover is higher in the Field Sparrow non-overlap._

Another interesting pattern emerges from Figure 5 when Contrasts
1 and 2 are each separately compared to Contrast 3. The low grass
cover in Contrast l, a factor that is greater in the Song Sparrow overlap,
is also greater in the Field Sparrow non-overlap of Contrast 3. If
one assumes for the moment that low grass cover is somehow important
fer both the Song Sparrow and the Field Sparrow, they are potentially
able to co-utilize this resource via the overlap because of its
greater cover throughout the Field Sparrow's exclusive home range.
By the same reasoning, all factors of higher concentration in the
overlap of Contrast 2 for the Field Sparrow (except shrubs) may possibly
be co-utilized relatively better in the overlap, since these same
items are present with greater coverage in the exclusive home range
of the Song Sparrow. Thus, the overall pattern of habitat structure
does indicate many vegetational differences between the overlapping
home ranges of the two species. The significance of each vegetation
type is related to habitat preferences and activity patterns of the

sparrows and will be discussed later.

37
Activity and Substrate Use
As mentioned previously, 60 location observations on a designated
home range established what was called an observation set. The activity
in which a bird was engaged and the substrate type on which the activity
was recorded at each of these 60 points were also noted. It was of
interest to determine if the frequency of a particular activity or
the frequency of utilization of a particular substrate differed either
between the zones created by home range overlaps or between species
within one of these zones. As mentioned previously, it is important
to consider the ratio of overlap area to non-overlap area when drawing
conclusions about differing observation frequencies. For example,
assume that the frequency of singing by one species was greater in its
non-overlap relative to the overlap area. Interpretation of this
situation would differ depending on whether the non-overlap was ten
times the size of the overlap or whether the two areas were equal in
size. It is, therefore, necessary to weight the observed frequencies
with respect to area in order to obtain real contrasts. Given an
activity, its frequency (the number of times recorded out of 60) in
both the overlap and non-overlap portions of a home range was calculated.
These frequencies were then re-calculated based upon the ratio of
non-overlap area to overlap area. The resulting difference between
these values was found for every home range of each species. Using the
Wilcoxon matched-pairs signed-ranks test (Nie‘gthglf, 1975), it could
be determined whether or not the activity occurred with significantly
greater frequency in either of the two areas. This type of analysis

was used on all activities and substrates listed in Table 2 in order

38
to find differences between overlap and non-overlap zones for each
species. A similar procedure was used to find differences in activity
frequencies and substrate utilization frequencies between the two
species in the area of overlap itself. The significant results (out
of the total of activities and substrates listed in Table 2) are
diagrammed in Figure 6.

The most striking fact to be seen in Figure 6 is in Contrast l,
where the majority of all activities and associated substrates were
recorded more often within the Song Sparrow overlap zone than in its
non-overlap zone in each field. The pattern in both fields is
remarkably similar. The activity categories listed also are among the
more ecologically important activities of the breeding season. The
disproportionate amount of time spent within the overlap (see Overlap
Proportion Index) is accounted for on the basis of a wide range of
behaviors. Contrast 2 indicates a different situation for the Field
Sparrow. In Field I only two activities, foraging and perching, were
more frequent in the overlap, while perch-singing was more frequent
in the non-overlap. Host major substrate types were used more often
in.the overlap zone. In Field II more activities but less associated
substrates were feund to be more frequent in the overlap zone. The
Field Sparrow's disproportionate use of the overlap is, then, divided
among these activity types. Perched with food and posts were the
only activity and substrate types occurring and used more often in
the non-overlap zone. It is interesting to note that within each
field all activities (eXcept feeding young) and substrates that are

more frequent in the Field Sparrow overlap are also more frequent in

Figure 6.

39

Significant activity and substrate differences (adjusted
fer area - see text). Those types listed under the
appropriate member of a contrast were observed more often
than in the opposed member. (The differences are based
on the Wilcoxon matched-pairs signed-ranks test with

p<.05.)

 

 

 

 

no

 

 

 

 

 

 

Contrast 1 Contract 2
Sang over over Field
Sparrcw‘ lap >lap Sparrow

J
1
Field I I
singing foraging perchpsing
foraging perching
perching
perohpsing
‘3 aggression-
33 display
3 ;preaning
perchpslann t
perchpfood
feeding young t
O grsmnoid graninoid
ii fcrb forb
1; shrub shrub
A tree tree
5: post
Field 11
singing singing * perehpfood t
foraging foraging
perching perching
V perch-sing perch-sing
33 flying feeding young s
:3 preening
perchsfood
sir graminoid post
3 graminoid forb
I: forb
+3 shrub
I
a: tree
i: post

 

nest associated

Ginsu:
foraging
perohpsing
aggression-
display a
perchsalann

forb
shrub
tree

post

“lens
perchpsing
perchnalann *
perchpfood

fbrb
shrub *

post

 

41

the Song Sparrow overlap. However, when the Song and Field Sparrow are
compared to one another within this overlap zone (Contrast 3), it is
clear that the Song Sparrow predominates with respect to several
activities and substrates. With the exception of nest associated
behavior, the Field Sparrow neither engages in any activity nor uses
any substrate more often than does the Song Sparrow.

The various analyses used above reveal several sets of differences
between the Song and Field Sparrows. The Field Sparrow's home range
is from 1.7 to 2.5 times as large as that of the Song Sparrow (Fields II
and I respectively). The overlap zone comprises 25% of the Song
Sparrow's home range area and about 12% (on the average) of the Field
Sparrow's home range area. In each of the two fields, Overlap and
Overlap Proportion Indicies indicated that both species spent a dis-
proportionate amount of time within the overlap zones, yet maintained
a high degree of spatial separation there. Overlap and non-overlap
zones for each species were shown to differ in at least several classes
of vegetation structure. The two species differed further in their
utilization of the home range zones. Host of the activities and
associated substrates were observed more frequently in the overlap
area relative to the non-overlap area in the Song Sparrow. The Field
Sparrow also showed activities and substrates which were more frequent
in the overlap, but fewer categories were involved (especially in
Field I). When contrasts were made between the two species within the
overlap zone, the Song Sparrow predominated in terms of both activity

and substrate usage patterns.

DISCUSSION

Home Range Sizes

The home range sizes for the Song Sparrow in Fields I and II
were .161 and .326 hectares respectively. No reference could be
found in the literature on Song Sparrow home range sizes. However,
several studies report areas in terms of territory size. Recalling
the general similarity between the Song Sparrow's type-A territory and
its home range, it may be possible to compare the areas of these two
different systems. In habitats of weeds, shrubs, and trees in Ohio,
Nice (1937) reported a mean territory size of .271 hectare (range .202
to .607) for an area well filled with Song Sparrows. Along Minnesota
lake shores, territories averaged .202 hectare (Southers, 1960). Under
the specialized conditions found on small islands, Song Sparrow territory
sizes may reach a minimum of .016 hectare (Beer _e_t_ g, 1956). In
mixed island and mainland sites in the Pacific Northwest, mean territory
sizes of .029 to .328 hectare have been measured (Yeaton and Cody, 1974).
The aVerage areas found above are within the size ranges presented in
the literature. The home range sizes of the Field Sparrow were .h08
and .569 hectares in Fields I and II respectively. With the qualifica-
tions noted above, these values are slightly below the .809 to 2.h28
hectare territory sizes reported for old field and bushy fencerow

habitats by Walkinshaw (Bent, 1968).

#2

43

The Song Sparrow's home range size in Field II is about twice
as large as its size in Field I. Similarly, the Field Sparrow‘s home
range is about l.h times larger in Field II. While no detailed between-
field analyses were made, it is possible that the general differences
in vegetation structure for the two fields could have had some effect
(direct or indirect) on the size differences. Field I contained a large
central area of shrubs and trees while Field II appeared uniformly
covered with low herbaceous vegetation. A loose correlation may exist,
then, between gross scale vegetation uniformity and.large home range
size. The densities of the two sparrows (which will be discussed later
in another context) could also explain the home range size differences
but the direction of causation is uncertain. In Field II both species
had larger home ranges and lesser densities than in Field I. The
densities in males per hectare were 1.2% and 0.99 respectively for
Song and Field Sparrows in Field II. The values in Field I were 1.98

and 1.#8 per hectare respectively (see below).

Structural Characters

From studies that have been made on habitat selection and
territorial structure, it is currently thought that some birds rely
on the overall pattern of vegetation structure, the physiognomic
features of an area, in determining its suitability for breeding
(Wiens, 1969). Thus, the individual species of plants may not be
important in the initial selection of’a home range as long as the
components of the habitat are of the prOper type (Cody, 1968; MacArthur

and icArthur, 1961, MacArthur at 51., 1962; Beaver, 1976). yoreover,

4h

if birds select their habitats through such structural cues, their
response to and utilization of the habitat should be related in various
ways to this structure (Wiens, 1969). This concept has been extended
here to the analysis of overlapping home ranges. An effort was made
to record all plant species and substrate characteristics in each home
range, but certain species were subsequently eliminated from the data
since it was Judged that they played very little role in the physiognomy
and utilization of the habitats. While no claim is made to know the
exact factors in this study that are selectively perceived by the
birds, the contributors that were deemed important to home range structure
in Fields I and II have been listed in Table 3. Grasses andhgpldenrods
‘coveredimost of both fields, and together with trees, whmnbs, and
saplings. gave each'field'its'chamacteristic appearance. The other
plants stood out in importance because they formed noticeably distinct
patterns, and served as perch sites and protective cover for both
species of birds. Vegetation height, the percentage of bare ground,
and the presence of clumps and low patches also were considered
important in the overall physiognomy, and are closely associated with
foraging‘activity. Fro-{Figure 5 the structural make-up of the home
ranges as well as many differences between overlap and non-overlap
zones and between the non-overlaps of both species are apparent. It
must be noted that there is no way to test here whether those
statistically significant differences are of the magnitude that would
constitute real differences for the birds.

Separate studies of the Song Sparrow in Ohio, San Francisco, and

British Columbia agree upon the species basic habitat requirements:

45

concealment and shelter, perches, singing posts, accessible ground,
good light conditions under cover (dense but low vegetation or
high thin canopy), and proximity t6 water (Bite, 1937; Marshall, 1948;
Tompa, 1969). Nice states that trees, shrubs, and large weeds offer
singing posts and look-outs for the male, while shrubs, grasses, and
weeds provide both sexes with food, protection, and nest sites. All
these requirements are met in each field of this study. The greater
low grass cover in the overlap of Contrast 1 (Figure 5) may provide
added protection for the Song Sparrow and account for its disproporo
tionate occurrence in its overlap zones. In addition, low grass
Slifilflcnted.aumajer foraging substrate. The greater goldenrod cover
and vegetation height in the Song Sparrow non-overlap sons (Contrast 3)
(also Contrast 1 for goldenrod) also indicates a potential for con-
cealment. As compared to the overlap, the Song Sparrow's non-overlap
with higher chickory and sapling cover values may offer relatively
more perching, singing, and look-out posts. Moreover, those categories
of structure which do not differ within contrasts may also provide
many of the basic needs for the species.

The literature is insufficient in its description of basic
habitat requirements for the Field Sparrow. However, studies done
in Earth Carolina, Texas, and”Hichigan report this species breeding
in farmlands, over-grown pastures, and abandoned fields all containing
grasses, forbs, and scattered shrubs and trees (Pulliam and Enders, 1971;
Fretwell, 1959; Allair and Fisher, 1975: Evans, 1961+; Walkinshaw, 1968).
Nice (1937) and Pulliam and finders found both Field and Song Sparrows

to be locally sympatric, an apparently common situation in their

46

broadly overlapping ranges. Evidence such as this indicates that
the two species may share many of their habitat requirements, with
the exception that the Field Sparrow favors more of the open field
type of habitat with scattered woody vegetation. Fbr example, consider
the contrast between the Field Sparrow overlap and non-overlap for
Field I (Figure 5). Shrubs and goldenrod cover are higher in the
overlap, and both offer very good concealment (as well as a good
foraging location). The non—overlap, on the other hand, contains more
bare ground and sapling cover, two structural factors which are
themselves scattered in their distribution over the field. This pattern
then, seems to fit this rough habitat preference profile, and may
account for the relatively large home range size of the Field Sparrow
and the smaller percent of overlap with the Song Sparrow. A similar
situation can be seen in Field II, where goldenrod cover and vegetation
height in the overlap also offer the potential for increased concealment.

The home ranges and overlaps of each species differ in one or
more aspects of habitat structure. Nol,in order to generalize about
the ecological relationships between the species, these structural
characters of the habitat should be considered in association with
the ways that they are used by their occupants. Features of the
habitat such.as those discussed above nay affect the behavior of the
birds or may determine differences in the supply of food available

(Stenger and Falls, 1959). These possibilities are discussed below.

#7

Activity and Associated Substrates

A substantial potential for coexistence of the Song and Field
Sparrow is manifest in the differences of home range utilization
presented in Figure 6. Categories differ greatly between overlap and
non-overlap for each species, especially in the Song Sparrow. Heavy
use is made of the overlap zone, but according to Contrast 3, the two
species may be able to reduce competition in the overlap zone through
differential usage.

The obvious activity category to consider when discussing
coexistence is that of aggression.and/cr display. It is worth noting
that the aggressive component of this activity category was in almost
all cases the supplanting of the Field Sparrow by the Song Sparrow.
Song Sparrows generally dominate most species that venture into their
territory (Rice, 1937; Tbspa, 1964), while Field Sparrows are rarely
aggressive toward other species (Bent, 1968). In fact, Nice states

that the Field Sparrows were driven off by the Song Sparrows with special
vigor, but they merely avoided the attacks and continued to nest in

the midst of the Song Sparrows. This aggression and submission, rare

as it was, might have served to spatially segregate the species within

the overlap (especially in Field I - Contrast 3). not only, then, is

there a low degree of association between the species in the overlap,

but separation is also evident in their activities and use of substrates.
The relationship between vegetation structure and utilization

records is not entirely clear. If Figures 5 and 6 are compared, only

a few related patterns can be seen.’ For example, in both fields the

Song Sparrow spent lore time foraging and more time on graminoid

43
vegetation in its overlap relative to its nonpoverlap; and low grass,
graminoid and a prime foraging substrate, was indeed more concentrated
there. Similarly, in each field the Field Sparrow foraged more often
and used forbs.more frequently in the overlap relative to its non-overlap;
goldenrod, a forh and also a principle foraging substrate was more
concentrated there. Other than these loose correlations, no consistent
relational pattern can be seen. Several factors may be responsible:

1) most activities can take place in a variety of substrates; 2) some
substrates might in fact inhibit certain activities; and 3) the two
species may do different things on the same substrate. Also, some
inconsistencies may arise by Jointly analyzing the two fields which
differ sosewhat in their vegetational composition (although certain
patterns are evident in both fields). Nevertheless, there are some
structural and utilization differences between the species. In the
study of grassland birds, Wiens (1969) concluded "absolute segregation
is not required to permit the coexistence of two or more species since
even slight differenses: in the frequency of utilization of a habitat
component, or subtle differences in the degree of occupancy of a
vegetation type, may be sufficient to allow continuing co-occupancy
ofthe habitat especially if resources are abundant."

It has been stated that animals partition environsental resources
in three basic ways: spatially, trophically, and temporally (Pianka,
1973). Spatial factors have already been considered, but no statistical
analysis of trophic factors were nade in this study. The gross
observational data collected did suggest that both Field and Song

Sparrows took similar food items, grass seeds and Lepidopteran larvae.

“9
The Field Sparrow appeared in general to be more visible and "acrobatic"
during foraging than the Song Sparrow, taking seeds and small insects
fros plants while hanging in various positions. The smaller size and
average weight of the Field Sparrow could explain'some of this apparent
agility. On the importance of the trophic factor, Baker and Baker (1973)
state "little doubt renains that at least for most birds food resources
are the ultimate limiting constraint on population size," and this
sentiment is echoed by other investigators (Lack, 1971: Edington and
Edington, 1972, Cody, 1974). If food is in short supply locally,
competition between the species could be reduced through: foraging
behavior, kinds of food eaten, and differences in size and shape of the
food eaten (Allair and Fisher, 1975). Pulliam and Enders (1971) detected
slight differences between Field and Song Sparrows in each of the
above three categories during the winter sonths despite a high degree
of overlap. Also, in a study of Field, Chipping, and.Bachman's Sparrows
in eastern Texas, Allair and Fisher (1975) found the Field Sparrow
using higher perches to obtain seeds than did the other species. Thus,
the potential for coexistence is present. 0n the other hand, Evans
(196%) studied Field, Chipping, and Vesper Sparrows on an old field
in southeastern“Nichigan and found no evidence of significant differences
in the kinds of food eaten, nor any indication that food was in short
supply during the breeding season. He concluded that the three species
bred synpatrically without competitibn for food. This could well be
the situation for the Song and Field Sparrows of this study. However,
even under these circumstances it is possible that the species differ

in their ability to locate the same food types.

50

The temporal factor in resource partitioning was not given
detailed analysis in this study. Spatial and behavioral observations
were not taken sisultaneously for each of the two species, and it was
assumed that the patterns observed were the results of continuous and
widely similar types of activity over the season. Indeed, in his
study of several species of grassland birds, Hiens (1969) found them
to be broadly overlapping in their seasonal use of the area, and saw
no evidence of any temporal segregation.

The response of the Song and Field Sparrows to habitat structure
may depend upon both their population densities and the number and
kinds of other bird species present as the potential resource competition
becomes more severe or less severe (Viens, 1969). gist (395?) records
San'lparrow densities of free 1.5.1” per hectare to 2.7 hiss per
hectare in.a habitat of trees, shrubs, weeds, and gardens. The present
study found 2.0 males per hectare on Field I and 1.2 males per hectare
on Field II. Fretwell (1969) records Field Sparrow densities of fro-
l.“ males per hectare to 3.7 sales per hectare in pastures and fields
of Broomsedge and pine plantings. Bent (1968) gives densities in old
fields of’O.17 to 1.95 males per hectare. This study found 1.5 males
per hectare in Field I and 1.0 males per hectare in Field II. There
is no way to know at this time whether the densities in Fields I and II
are low enough to serve as significant coexistence aechanisms, but
it appears that they are comparable to the general range of densities
previously published.

Other species can also have an effect on the local coexistence

of the Song and Field Sparrow. They may compete for the same limited

51
resources, may affect the size of home ranges (Ieaton and Cody, l97h),
or may act as predators keeping densities low (Connell, 1970).
Table 10 lists those species which were seen regularly on or near the
home ranges of the Song and Field Sparrows. The two sparrows, however,
dominated both fields in terms of numbers of breeding pairs. Moreover,
none of those species listed that nest on or near the ground did so
within the home ranges of the two sparrows. The species composition
here is not unusual, but the potential still exists for partial overlap
in diet among some of the species (an important factor if food were
found to be in short supply). so descriptions of Song and Field Sparrow
home ranges in habitats lacking other bird species could be found in
the literature. The two obvious interactions with other species were
those of nest predation and parasitism. On at least three occasions
eggs or nestlings were taken fros nests of both species presusably
by avian predators. Also, Cowbird parasitism was widespread, and both
sparrows were seen feeding the young of this species. The two sparrows
are typically parasitized by the Cowbird, but the Song Sparrow appears
in general to be the favored host (Rice, 1937). Obviously the
Cowbird can affect the sparrows' pepulations, but the extent of this

effect and its relation to coexistence patterns needs further study.

Table 10. Avian species seen on or near the home ranges of the
Song and Field Sparrows. Observations from both fields
are combined.

Cardinal (Cardinalis cardinalis)

Indigo Bunting (Passerina 911212)
American Goldfinch (m tristis)
Eastern Meadowlark (Sturnella m.)
Red-winged Blackbird (Agelaius phoeniceus)
Co-on Crackle (Quiscalus guiscula)
Brown-headed Cowbird (Holothrus 539;)
Northern Oriole (Icterus gglbula)
Harbling Vireo (£133 £123)

Starling (Sturnus vulflis)

American Robin (_Tu_1_'_¢_i_u_§ nigatorius)

House Wren (Trogl‘ oytes 93293)

Blue Jay (Cyanocitta cristata)

Downy Woodpecker (Dendrocopos pubescens)
Co-on Flicker (Colaptes 3M)
Yellow-billed Cuckoo (Coccyzus americanus)

SUMMARY

Coexistence is the persistence of two or sore species in the
same habitat, and such coexistence is achieved by the evolution of
some mini-a1 degree of difference in resource use (Cody, 197k).

This study has established, first, the gross pattern of resource
partitioning through the measurement of structural components in
overlapping Song and Field Sparrow home ranges. Differences in from
3 to 8 vegetation classes (out of’a total of 11 seasured) were found
between areas of hose range overlap and non-overlap in both species.
More structural differences were apparent in the contrast between the
Field Sparrow overlap and non-overlap zones than that of the Song
Sparrow. Second, the spatial and behavioral relationship to the
habitat structure demonstrated several areas of separation in both
occupancy and utilization between the species. Overlap Proportion
Indicies greater than 1 indicated that both species made disproportionate
use of the overlap zones. Moreover, Overlap Indicies of from 0.32

to 0.}? demonstrated small degrees ofsspatial'association between the
species in this overlap zone. Many of the Song and Field Sparrow's
activities and associated substrates were recorded with different
frequencies in their zones of overlap relative to the non-overlap
zones. within the overlap areas, the two sparrows differed in the

frequencies of several important activities. Such separation along

53

 

l i l I III

54
more than one niche dimension has been found in similar studies of

grassland birds (Wiens, 1969) and lizards (Pianka, 1971:).

 

' II III ‘ (I A!

SUGGESTIONStFOR FURTHER RESEARCH

It would clearly be interesting to analyze additional over-
lapping home ranges of these species over several seasons and in other
geographic areas. Spatial distributions of the sparrows plotted
continuously and simultaneously for each overlap night show associations
not discovered in the present study. More direct measures of resource
partitioning and cospetition could be sade through trophic analyses
and removal experisents if aatched sets of local overlap zones could

be found.

55

\ I I I l til.‘ I

LITERATURE CITED

 

and! :lhlllq

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