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SFUDY OF THE RELIABILITY OF

:st OF S‘INGENG
MALE WOODCOCK

Thesis {or fine Um of M. 5.

MICHIGAN STATE UNIVERSITY
Gary Earl Duke

1964

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ABSTRACT

STUDY OF THE RELIABILITY OF CENSUSES
OF SINGING MALE WOODCOCK

by Gary E. Duke

The annual Woodcock Singing Ground Survey of the U. S.
Bureau of Sport Fisheries and Wildlife is widely used as an index to
woodcock abundance. Observers drive along pre-selected roads and
stop at intervals to record the number of male woodcock heard per-
forming their courtship calls and flight songs. This study was under-
taken to determine the nature of factors which may affect this type of
survey.

Counts of “peents” and "flight songs" during many woodcock
courtship performances were made by two minute intervals. They
were made alternately on areas of different densities for two seasons.
Simultaneously cooperators made the standard Singing Ground Survey
each evening through the area in which study observation-points were
located.

Recorded variations in courtship performances showed the time
during the season with the most stable courtship activity. Climatic
factors, unless extreme, had little effect on courtship. The perform-
ance was not significantly affected by moonstage though it appeared to
be initiated by light intensity. The average starting times in relation
to cloud-cover and official sunset were determined. The pre-dawn
performance was found to be unsuitable for singing-ground surveys.

The hearing ability of the person making surveys can significantly alter

Gary E. Duke

survey results. The inclusion of flight songs in the survey tally was
not detrimental to the daily totals obtained by the survey. The stage
of the brood cycle may affect the male performance.

As the density of performing males increased, the level of
activity per bird significantly decreased. Probabilities of hearing

each individual male became lower with increasing density.

STUDY OF THE RELIABILITY OF CENSUSES
OF SINGING MALE WOODCOCK

BY

Gary Earl Duke

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Fisheries and Wildlife

1964

i: £64m.

ACKNOWLEDGEMENTS

This study of the reliability of woodcock censuses was a co-
operative project of the Department of Fisheries and Wildlife at
Michigan State University, the United States Bureau of Sport Fisheries
and Wildlife, and the Game Division of the Michigan Department of
Conservation.

I wish to express my appreciation to Dr. George A. Petrides,
Professor, Department of Fisheries and Wildlife, for his advice in
planning and methods, constructive criticism of the project, and for
his editing of the manuscript. Dr. Leslie W. Gysel, also of that
Department, provided advice and assistance in making field measure-
ments of physical factors. Drs. Gysel and George J. Wallace of the
Department of Zoology, both critically read the manuscript.

Dr. G. A. Ammann of the Michigan Conservation Department
gave considerable assistance in the field. His advice, encouragement,
and interest throughout the project are gratefully acknowledged. Other
members of the Conservation Department made woodcock brood
searches and made their results available to me.

Messrs. John Arnsman, Charles Eakle, John Fletcher, John
Foster, Joe Johnson, Robert Kart, Michael Peterson, Carl Polomski,
Roger Priest, and Nick Steen, Michigan State University students,
served as cooperators in making Singing Ground Survey counts. They
each devoted several evenings during the season to this work and also
participated in hearing tests. Their help is greatly appreciated.

Dr. Fant W. Martin of Patuxent Wildlife Research Center of the
U. S. Fish and Wildlife Service offered valuable advice and super—

vision throughout the research and during the writing of the dissertation.

My wife, Maryann, deserves much credit for her typing skills,
her help in the field, and for constant encouragement.

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ii

TABLE OF CONTENTS

INTRODUCTION . . . . . . . .................

THE STUDY AREAS ........ . . . .......

A QUANTITATIVE DESCRIPTION OF THE COURTSHIP

PERFORMANCE . . . . . ...............

SOURCES OF VARIATION IN THE WOODCOCK

PERFORMANCE ....................

Seasonal Variations in Activity and the Stable Period .

Effect of Density of Performing Males on Singing
Activity. . . ...............

Effects of Physical Factors. . . ..... . .

Relationship of Brood Activity to Male Courtship
Activity ..... . . . ............

SOURCES OF VARIATION IN THE SINGING GROUND

SURVEY........ .................

Hearing Tests. . . . ..............
Effect of Including the Flight Song in the Singing
Ground Survey ................

PROBABILITY THAT THE SURVEY WILL TALLY ALL
PERFORMING MALES PRESENT . . . . .....

CONCLUSIONS AND RECOMMENDATIONS . ......

SUMMARY . . . . . ..........

iii

10

10

11
2.5

32

34

34

37

38

46

47

49

TABLE

LIST OF TABLES

A comparison of activities of male woodcock on
singing grounds occupied by different densities of
birds.

Results of significance tests to determine whether
differences in peenting activity indicate dominance
among male woodcock who share a singing ground.

Correlation coefficients between physical factors
and three measure of woodcock courtship activity .

Mean and standard deviation in minutes from sun-
set to start of the dusk woodcock courtship
performance.

Mean and standard deviation in minutes before
sunrise to start of the pre-dawn woodcock court-
ship performance .

The probability of hearing each individual bird
under various conditions, during the stable period
after waiting six minutes then counting for thirty
minutes .

iv

Page

22

24

27

30

31

41

FIGURE

LIST OF FIGURES

Page
The woodcock listening posts. . . . . . . . . . . . . 5
Woodcock peenting and flight song activity per two-
minute period during the performance . . . . . . . . 9
Woodcock pre-dawn length of performance, 1963 . . l3
Woodcock length of performance at dusk, 1963 . . . 15
Daily woodcock Singing Ground Survey totals, 1963 . 17
Woodcock length of performance at dusk, 1964. . . . 19

Daily woodcock Singing Ground Survey totals, 1964 . 21

INTRODUCTION

Each spring throughout the eastern United States and south-
eastern Canada, several hundred peOple are involved in singing ground
surveys to determine the annual relative abundance of the American

woodcock (Philohela minor). The survey findings are utilized in regu-

 

lating woodcock hunting. The woodcock, a migratory game bird of the
shorebird family, Scolopacidae, inhabits uplands. Its courtship per-
formance and calls are given during two periods each day throughout
the mating season and make the survey possible.

Survey observations (U. S. Fish and Wildlife Service, 1963) are
made in preselected woodcock habitat. A point where woodcock are
known to perform is selected as the start of the survey and the census
begins when the first male is heard. The observer travels by car over
a planned route for 35 minutes at dusk. Stops are made at no less than
0.4 mile intervals, since it is believed that woodcock may be heard
for 0. 2 miles, but may be made at greater intervals to avoid unsuitable
conditions such as busy road intersections. At each stop the number of
male woodcock heard performing during a two-minute period is re-
corded. The count is carried out under standardized procedures and
is intended to yield an index to population size. Summarized counts
serve to determine population trends; no estimate of the total woodcock
population is expected.

This study was conducted to determine what factors may influence
the male mating performance and thus affect the population index as

determined by the survey.

The bird's courtship performance occurs just prior to dawn
and just after sunset. It takes place on a relatively open area called
the singing ground. Most of each performance is on the ground where
a buzzing ”peentH call is given at regular brief intervals. "Flight
songs" also occur during aerial displays which involve a rapid ascent
to a height of two or three hundred feet, several circling maneuvers,
and a descent in zig-zag swoops to the singing ground again. Character-
istically two sounds are emitted during the courtship flight: a mechanical
twittering made by the rapid beating of the wings and a vocal chirping
given usually during the descent (Mendall and Aldous, 1943).

Mendall and Aldous (1943) studied the ecology of the woodcock
and initiated the survey. Sheldon (1953) analyzed the singing ground
survey, made observations of single birds, and appraised trapping data
in efforts to improve the survey. Goudy (1960) studied the effects of
various factors on the survey results and attempted again to refine the
methods used.

In order to appraise further the sources of error in the survey
method, my study was undertaken to determine (1) the most desirable
daily and seasonal time for useful surveys; (2) the most desirable
climatic conditions for useful surveys; (3) the.effects of climatic and
physical factors and population density on the courtship performance;
and (4) the effects of observers' hearing ability and the inclusion of

flight songs on the survey results.

THE STUDY AREAS

The study was conducted at Rose Lake Wildlife Experiment
Station of the Michigan Department of Conservation, located about
12 miles northeast of Lansing, in Clinton and Shiawassee Counties.

Listening points in 1963 were established in four predominately
grassy fields (Figure 1). Two fields possessed scattered brush and
two were fairly clear of woody vegetation. In both 1963 and 1964,
a single male used field number one, fields two and four had two
singing males each, and in field three, three birds could be heard
from one listening point. Field number two was not used in 1964.
The number of performing males that I could hear from one listening
point was accepted as the "density" of males there. The presence of
a male was ordinarily established when I heard at least a part of its
courtship performance.

Field number one was the smallest, about 2. 5 acres. It was
in an early stage of old field succession with grasses predominant and

scattered aspen seedlings and dogwood (Cornus spp.) shrubs. It was

 

bordered by cultivated land. Fields two and four were about the same
size, approximately 7. 5 acres each. Field two was currently pastured
and was also bordered by cultivated land. Field four was edged by
water and woodland and was in a moderately advanced stage of old field
succession where low vegetation was predominately grasses and woody
vegetation was predominately aspen from seedlings to pole sizes.
Field number three, a pasture, was the largest of the study areas,
about 75 acres.

All fields were flat in general and moderately well drained. Rose

Lake soils are from medium to poor fertility.

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A QUANTITATIVE DESCRIPTION OF THE
COURTSHIP PERFORMANCE

An examination of 50 birds shot while peenting (Fant W. Martin,
personal conversation, July 1963) revealed that all were males.
Apparently, only males make this call.

Peents and flight songs were counted during two-minute intervals
during each performance in 1963 and for both one and two-minute inter-
vals in 1964. Three types of counts were made during each perform-
ance: t e one, collective counts of all peents and flight songs of all
birds present; type two, counts of peents and flight songs of a single
bird of those present; type three, counts of peents and flight songs of
one particular second bird, if more than one was present. These
counts allowed a quantitative description of the performance. Also,
these counts and a Singing Ground Survey taken simultaneously each
evening by student cooperators made four measures of woodcock court-
ship activity available. These were: the number of peents and flights
per bird per two-minute period, the length of each daily performance,
the daily Singing Ground Survey totals, and the consistency of activity
throughout the performance.

A special set of observations, made as time permitted during
the counts described above, showed there was an average peenting
frequency of 21 peents per minute per bird (40 one -minute counts).
Peenting frequency was greatest during the middle of the performance
(Figure 2). Fifty flights were timed and their average duration was 56
seconds. Courtship flights and flight songs occurred predominately
during the first half of the performance (Figure 2). During the time

when the most flights occurred, there was an average of about 28

6

peents between flights (35 counts).

These averages were determined at the dusk performance when
one bird was present. The same pattern of peenting and flighting
activity prevailed when more than one bird was present within my hear-
ing range, but the activity did not increase proportionately with density

(see beyond).

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SOURCES OF VARIATION IN THE
WOODCOCK PERFORMANCE

Seasonal Variations in Activity and the
Stable Period

 

 

Repeated visits to the study area were necessary in early spring,
and even after the first male was heard, several evenings were re-
quired to select listening points where there were different densities
of singing males. In 1963, the first male was heard on March 18
and observations were recorded from March 25 to June 7. In 1964,
the first bird was heard on March 14 but recorded observations were
delayed until April 3 by snowfall and very cold weather. Observations
then were made through June 3.

Early in the 1964 season, courtship activity was limited somewhat
by adverse weather conditions, but I observed fairly normal activity
even during a snowfall. Early-season activity on the singing ground
normally is characterized by daily variations in the number of singing
males present, apparently due to the presence of transients (Figure 5
and 7), and by frequent performances evidently to demarcate territories.
The territorial performance involves a very low flight by a territory-
holder over an intruder. It is accompanied by a "cackling” or growling
call given above the intruder's head. On one occasion, five of these
flights were made in one evening by a bird dislodging an intruder.

The disputes usually ended with both birds flying together out of my sight,
the territory-holder cackling at frequent intervals all the way. On one
occasion, an apparently pugnacious or confused male dived and cackled
at me. This aggressive behavior did not last beyond the first week of

April in either season of the study.

10

11

With the passing of migrants, courtship activity seemed to become
fairly regular until mid-May when activity waned. This pattern of
activity allows the determination of a period which has stability and a

high level of courting activity. This I have called the stable period and

 

unless otherwise stated all results which follow are based on observations
recorded only during this time (Figures 3, 4, 5, 6, 7). The stable period
extended from April 15 to May 16 in 1963 and from April 12 to May 14 in
1964. Confidence limits based on daily performance length and daily
singing ground survey totals were used to determine the extent of this
period in 1963. A similar but less involved technique was used with the
1964 data. Counts of "peents" and "flight songs" as measures of activity
could not be used in establishing the stable period since, as will be shown
later, peenting and flight song activity varies with the density of perform-
ing males.

The description of this stable period and its chronological position
in the season showed general agreement with the findings of both Sheldon
(1953) and Goudy (1960). Sheldon observed that in the period prior to
April 15 there was much variability probably due to passing migrants;
and that the period after May 10 probably was variable because of
diminution of courtship activity. Goudy recommended a "central period”

of April 20 to May 10 for making surveys.

Effect of Density of Performing Males on
Singing Activitj

 

 

Tests showed that during the stable period both peenting and flight-
ing activity per bird became less per two-minute period as the density
of performing males increased (Table l). Peenting was highly signifi-
cantly greater for all lone birds than for others in 1963. There were
significant differences in two of three of the 1964 comparisons. The

exception was that there was no significant difference in peenting activity

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Table l. A comparison of activities of male woodcock on singing
grounds occupied by different densities of birds.
Rose Lake Wildlife Experiment Station, East Lansing,
Michigan. March-June, 1963 and 1964.

 

 

 

 

Activities Comparison of Comparison of Comparison of
one-density to one-density to two-density to
two-density three-density three-density

Peenting

1963, Dusk t=5.6232, lZd.f.* t=9.3158, 10d.f.* t=2.8961, 12d. .

Dawn t=3.2349, lZd.f.* t=3.8450, 10d.f.* t=3.349 , 10d. .

1964, Dusk t=0.0001, 17d.f. t=l.529, 10d.f.# t=Z.368 , 13d. .

Flight Calls

1963, Dusk t=l.6771, 12d.f.# t=l.6158, 10d. .# t=0.1890, 12d. .

Dawn =0.9015, 12d.f. t=1 # t=0.9434, 10d. .

f
.3994,10d.f
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*= Highly significant difference, at the one percent level
6: Significant difference, at the five percent level
#= Significantly greater, at the ten percent level

d.f. = Degrees of freedom

t==2.197 , 13d. .

23.

when one and two birds were present. The density difference could
have been in part due to my inability to hear every "peent" of all birds
at the higher density. This would not account for all the difference,
however, since there was a decrease in the frequency of flight songs
with increased density as well, and flight songs can be counted with
certainty.

This density response also supports the idea that it is not simply
hearing ability which lowers probabilities of counting birds at higher
population densities (see beyond). Similarily, some of the difference
between the two years may be due to my increased experience. However,
the birds at the two-density listening post (i. e., two birds present)
were separated by a line of brush and were further apart in 1964 than
in 1963, so that there may have actually been a decreased effective
density. Although activity decreased as density increased there should
be little effect on the probabilities of hearing each bird (see beyond)
since one "peent" in two minutes is enough to determine the presence
of a bird. However, since fewer "peents" were uttered, the periods of
silence are longer with consequent lowering of the likelihood of hearing
each bird in every two-minute period.

Since singing activity per bird decreased as density increased,
it was thought that this might be due to the dominance of one or more
individuals on a field occupied by several performing males. Field
records were studied to determine what differences in peenting activity
occurred when more than one bird was present. Eight cases of dominance
seemed apparent (Table 2), if the following is an acceptable criterium.
Dominant birds were assumed to be those which displayed during the
stable period at least two peents more per two minute period than other
birds in the group, without marked reduction in flight performances.

Only four of the eight cases tested showed significant differences

in peenting activity (Table 2). Thus, of 68 acceptable performances

24

Table 2. Results of significance tests to determine whether differences
in peenting activity indicate dominance among male woodcock
who share a singing ground. Rose Lake Wildlife Experiment
Station, East Lansing, Michigan. March-June, 1963 and 1964.

 

 

 

Number of Test Results from
birds present average peenting frequencies
2 sample sizes 7,7; means 16.1, 18.7
t = 0.489
2 # sample sizes 9, 9; means 18.8, 15.7
t = 0.7908
2 sample sizes 6,6; means 23.1, 14.7

t = 2.3416*

3 # sample sizes 6,6; means 12.7, 22.8
t = 3.485*

3 sample sizes 8, 5; means 22.8, 18.0
t = 1.563

2 sample sizes 6,6; means 23.7, 19.5
t = 0.7183

2 sample sizes 7,7; means 24.6, 14.6

t = 1.4708*

2 # sample sizes 8,6; means 20.3, 31.3
t =1.5674*

 

.,
ll

Significantly more peents uttered by one bird
Combat flights observed

=3: ‘5-
ll

25

observed during both seasons, both dawn and dusk in 1963, and during
the stable period, only 5. 8 percent actually displayed dominance. In
two of the four cases showing this dominance effect, combat flights
occurred. They also occurred in one case where there was no
dominance. If a dominant individual were present, one would not expect
combat to occur; whereas it would be expected from those individuals
attempting to establish dominance.

Perhaps the dominant individual is the one which does not share
a singing ground. Lone males do more singing than males which share
a singing field but this may be merely a result of efforts to attract
females in the absence of close neighbors which help to bring them in.

Dominance, if it exists, is not considered to be important in
affecting the woodcock Singing Ground Survey. Sheldon ( 1954) mentions
"a surprising amount of intraspecific tolerance" and varying degrees of
territorialism among birds. I observed that even when birds were

widely spaced on the ground, their flight paths frequently overlapped.

Effects of Physical Factors

 

According to Sheldon ( 1953) "analyses suggested no correlation
between weather conditions and count unless the former was extreme
or there was bright moonlight. " With the exception that I found no sig-
nificant relationship between activity and moon stage, this quotation
aptly summarizes my findings too.

Physical factors were determined for each performance.
Temperature, relative humidity, wind velocity, and cloud cover were
measured every half hour throughout each performance. Other factors
Were recorded daily or measured prior to the start of the performance.

Temperature and relative humidity were measured with a sling
psychrometer. Wind velocity was determined using both a Dwyer Wind

Meter and the Beaufort wind scale. Percentage cloud cover was

26

estimated visually. Vegetation moisture was determined by the weight
differences of blotters in 1963 and of air-dry plaster of paris slabs in
1964. These blotters and slabs were weighed before and after two
minutes of contact with the grass at preselected random points on a

field used as a singing ground. The blotters were held down by weighted
plywood covered with i— inch wire mesh. The slabs required no weights.
Barometric pressures, moonstage, and times of sunrise and sunset
were obtained from local U. S. Weather Bureau reports. Light intensity
measurements were made with an Olden light meter.

. Phenological calendars which I maintained throughout both seasons
were similar but not identical for the two years. The appearance of
first buds and blossoms of common native plants was about eight days
later in 1964 than in 1963 at the beginning of the season. By mid-April
this difference in blooming dates for certain species for the two years
was reduced to about two days. The early season differences were
undoubtedly due to cold weather in 1964.

Correlation coefficient tests were made between some of the
physical factors measured and three measures of courtship activity,

(1) peenting frequency, (2) daily Singing Ground Survey totals, and

(3) performance lengths at dusk. Due to the density effect, only the
activity of the one-density bird for 1963 and only that of one and two bird
density groups combined for 1964 could be used for these correlation
tests. The results of these tests, with two exceptions, showed no sig-
nificant correlations between climatic factors and those measures of
activity compared (Table 3).

The two exceptions were a negative correlation between wind
velocity and peenting activity and a positive correlation between survey
totals and temperature. As will be shown later, hearing efficiency is
affected by wind velocity. Therefore, the significant negative correlation

between peenting activity and wind velocity may be due to hearing

27

 

 

 

 

 

 

Table 3. Correlation coefficients between physical factors and three
measures of woodcock courtship activity. Rose Lake Wild-
life Experiment Station, East Lansing, Michigan. March-
June, 1963 and 1964.

Physical Measures of activity Measures of activity

Factors 1963 1964

Daily Peents Perform- Daily Peents Perform-
survey per two ance survey per two anée
totals minutes length totals minutes length

Temperature +. 1534 -.0775 +.0207 *+.5231 -.2083 -.3373

Relative -

humidity .1541 +.2924 +.1793 +.2214 -.05l6 +.3019

Wind

velocity .2113 *-.9563 —.2383 +.3073 -.3676 +.3348

Cloud

cover .1234 -.6059 -.2408 +.0323 -.1925 +.1585

Vegetative

moisture .2529 +.34l4 -.1825 +.4708 -.0539 +. 1549

Barometric

pressure .1411 +.5903 +.0486 -.l421 -.2536 -.0355

 

*2

five perc ent level.

a correlation coefficient that is significantly different from zero

at the

28

difficulty rather than to a significant change in male courtship activity.
Also, observations of peenting activity would be more severely
affected by increases in wind velocity than determining performance
length or obtaining an accurate singing ground survey tally.

The positive correlation between daily survey totals and tempera-
ture may also be due to human sensitivity. Colder temperatures
probably lower the efficiency of the counter. Also, warmer temperatures
occur later in the season and the student cooperators had gained experi-
ence, thus probably raising their efficiency. Lastly, temperatures
probably do have a slight effect on the birds, with warmer temperatures
stimulating activity. This is indicated by the probability data too (see
beyond).

As mentioned above, no significant effects on activity were found
to be due to the stage of the moon. Instructions for the 1963 woodcock
Singing Ground Survey directed that “counts should not be made during
the period of two days before a full moon and one day afterward. "

In 1963, data were not sufficient to allow conclusive analysis of
the effect of moon stage. However, in 1964, a special set of observa-
tions was made to determine the effects of the full moon. One particular
bird was observed for three days before, and three days during the full
moon. This test allowed comparison of performance length, daily survey
totals, peenting and flighting activities, and probability of hearing each
bird for performances during and before the full moon period. No sig-
nificant differences were found between any of these activities for the
two periods. The probability of hearing each individual bird during the
full moon was lower than that for the period preceding the full moon,
however, due to earlier cessation of the performance. The probability
of hearing each individual bird in the period preceding the full moon was
0. 933. During the full moon period this probability was 0.867. (The

computation of these probabilities is explained beyond). Three of the

29

performances during this special test were slightly less than 36 minutes
long, but this is not characteristic of the full moon period (see Figures
4 and 6). The results of comparisons between activities observed in

this test before and during the full moon period are:

performance peenting activity flighting activity daily survey
lengths per 2 minutes per 2 minutes totals
t=.5331 t=1.224 t==.9937 t=.6795

Since moonlight did not seem to affect the performance (see also,
Table 6), it was desirable to know to what extent the performance was
influenced by light intensity. Light intensity was measured at the begin-
ning and ending of each performance. The performance was determined
to have started with the first "peent" or ”flight song" given on the singing
ground (not in diurnal covert). The light measurement was made in a
standard way each day and readings were taken in an open area where
light conditions would have been similar to those on a singing ground.
Average light intensity at starting time was zero foot candles at the pre-
dawn performance. This average was 2.1 foot candles at dusk with a
standard deviation of 2.4 foot candles. Light readings at the end of all
performances were zero. No attempt was made to determine the part of
the spectrum influencing the starting time. Low variability in starting-
time light readings indicated that light intensity initiates the dusk per-
formance. Minor variation may have been due to the position of the bird
and the amount of light reaching it.

Further evidence of a phototropic response is found in the relation-
ship of starting time under varying amounts of cloud cover to official
sunset time (Table 4). As cloud cover increases, starting times become
earlier in the evening. This is as would be expected if a particular light
intensity is necessary to initiate the male performance. However, as

cloud cover increased, so did the variability of the starting time.

30

Table 4. Mean and standard deviation in minutes from sunset to start
of the dusk woodcock courtship performance. Rose Lake
Wildlife Experiment Station, East Lansing, Michigan.
March-June, 1963 and 1964.

 

 

 

Mean for
Cloud Mean for Standard Sample stable Standard Sample
cover season deviation size period deviation size
1963
0-25% 18.000 3.835 18 16.125 3.682 8
26-50% 17.889 4.649 9 17.000 3 873 5
51-75% 11.750 7.111 7 16.333 7.637 3
76-100% 9 412 9.636 17 10.000 _ 12 501 8
90-100% 9.333 9.582 15 10.000 12.501 8
1964
0-25% 15.235 5.506 ' 17 16.714 3.684 7
26-50% ...... 4 ..... 0 14.500 4.679 6
51-75% 15.300 5.857 5 15.500 9.192 2
76-100% 6.000 6.640 12 6.750 7.747 8
90-100% 7.200 6.300 10 7.167 9.497 6

 

* = Conditions of 26-50% cloud cover occurred only during the stable
period in 1964.

31

Data on pre-dawn starting times are interesting but inconsistent
and do not show a clear-cut relationship to light intensity (Table 5).
Normally there is complete darkness at the start of this performance.
Pre-dawn starting times are related to sunrise but not to cloud cover.

The greatest inconsistancy in pre-dawn starting times was shown
by an abrupt change occurring about May 1, 1963. The time range from
start of the performance to sunrise before this date was 42-58 minutes.
After this date this range became 56-85 minutes. The averages of the
minutes before sunrise under different amounts of cloud cover for these

two periods are:

Cloud Cover Mean before May 1 Mean after May 1
0-25% 50.27 73.50
26-50% 46.50 66.00
51-75% 58.00 75.50
76-100% 50.25 69.57
90-100% 50.00 70.60

This difference cannot be explained. The first week in May of
1963 was approximately the center of the stable period and appears to

be a significant time in the seasonal cycle for that year (see next section).

Table 5. Mean and standard deviation in minutes before sunrise to
start of the pre-dawn woodcock courtship performance.
Rose Lake Wildlife Experiment Station, East Lansing,
Michigan. March-June, 1963.

 

 

 

Cloud Cover Mean for Season Standard Deviation Sample Size
0-25% 58.47 12.684 17
26-50% 58.20 13.516 5
51-75% 69.67 12.583 3
76-100% 59.27 11.126 15

90-100% 59.36 11.944 11

 

32

Previous results have shown sizeable differences in activity
between morning and evening performances. This led me to a statistical
comparison of aspects of the performance for these two times of the day.

These comparisons showed that: (1) there are significantly fewer
birds heard at dawn than at dusk (at the six percent level, Z 2: l. 643);
(2) the dawn performance is significantly shorter than the dusk per-
formance (at the one percent level, Z- '-‘ 3.670); (3) there are significantly
more "peents" per bird per two minute period at dusk (at the five per-
cent level, t 2: 2.1296, 11 degrees of freedom); and (4) there are signifi-
cantly more flights per bird per two minute period at dusk (at the ten
percent level, t = 1.543, 11 degrees of freedom).

The first two tests were made using data for the entire season,
the second two were made with data from the stable period when there
was evidence that only one bird was present.

Evening performance length averaged 43 minutes in 1963 and 41
minutes in 1964. The average length of the morning performance during
the peak period was 27. 1 minutes. This, coupled with the results pre-
sented above shows the performance at dawn is less desirable as a time
for singing ground surveys. Another factor also contributes to this
conclusion: interference due to wind and/or rain is more noticeable in
the morning performance because birds start in the dark and are more
difficult to locate by sight as well as by sound. Pre-dawn observations

were not made during 1964 because of the above findings.

The Relationship of Brood Activity to
Male Courtship Activity

 

 

Brood searches were conducted each year by Michigan Conservation
Department personnel in the areas of my study. In 1963 nine broods were
located and the following approximations of the hatching dates were made

(Dr. G. A. Ammann, conversation, 1963):

33

April 21 or earlier April 24
April 22 April 24
April 23 or 24 May 2
April 23 May 15
April 23

Three-fourths of these hatchings occurred just before the first
week of May, the week which represented the approximate peak in male
activity in 1963 (Figures 3, 4, and 5).. The last hatching date probably
represents a re-nesting effort.

These results agree well with those of Sheldon (1953) who found,
with the exception of the first week in April, that the first week in May
yielded the highest roadside counts and the best results in trapping;
and that this week occurred “just after the height of the hatch in
Massachusetts coverts. " However, Blankenship (1957) found that the
major nesting season in southern Michigan occurred during the last two
weeks of April and the first two weeks of May.

Hatching in my study areas in 1964 agreed more with Blankenship's
findings in that the dates of hatchings were spread out during the last of
April and the first of May. These dates are as follows (Dr. G. A.

Ammann, conversation, 1964):

April 14 April 30
April 19 May 2
April 21 May 10
April 25 May 12
April 27 May 12
April 28 May 22
April 29

This spread may have been due to late March snowfall and cold
weather in early April. The significant point however, is that no real
peak in male courtship activity appeared in the 1964 season (Figures
6 and 7). Thus, there is indication that male activity may increase with

the coincident appearance of many broods.

SOURCES OF VARIATION IN THE
SINGING GROUND SURVEY

Hearing Tests

 

The woodcock peent is well within the range of normal human
hearing. "The energy of the peent is concentrated between 4. 7 and
6. 0 Kc. and almost all energy occurs between 2.6 and 6. 7 Kc. "

(Stein, 1963). ”The audiogram of man shows maximum sensitivity in
the range from 800 to 2500 c.p. s. with the upper limit around 16, 000
c.p. s. " (Prosser and Brown, 1961).

Two hearing tests were made in the field during each season of
this study. Weather, habitat, and aural interference conditions were
different for each of the tests in one season. Conditions in the second
season were similar to those of corresponding tests in the previous
season. I conducted the field tests using the student singing ground
survey cooperators. Each test was made by approaching the test bird
(which I knew to be singing) from a distance of 500 yards. The approach
was made by 50 yard advances along a route tangent to the bird's singing
ground. The persons being tested were given a record sheet and in-
structed to note at each stop either a zero or the number of birds heard.
Also, they were to record whether each bird was heard by peenting or
by flight song, and they were not to reveal their findings to the other
testees. A listening period of 30 seconds was used at each stop. Since
the position of the bird was known in relation to each stop, hearing
distances were computed by triangulation.

The first test each season required that the bird be heard through

a beech-maple forest for the first half of the test. Aural interference

34

35

was at a minimum for this test with wind velocity averaging from 3-4
m. p.h. and there was no frog or automobile interference. Temperatures
were similar in both years, ranging from 550-590 F.

In 1963, the test bird was first heard by one man at 257 yards,
three other people heard it at 209 yards, and one person heard it at 118
yards. In this test in 1964, five men first heard the bird at 102 yards,
and the other two men heard it at 82 yards. Thirty seconds of silence
by the test bird probably caused the lower results in 1964.

Field test number two involved a singing ground on which there
were two birds in 1963 and three in 1964. The area was a grassy field
with scattered shrubs which did not hamper hearing. There was some
interference from both frogs and automobiles and the average wind
velocities each year were six and nine m. p. h. Temperatures ranged
from 500-600 F. for these tests. The maximum hearing distance for

the two years at which the men being tested could hear each bird were:

 

 

 

 

 

 

First Bird Second Bird Third Bird
Hearing No. of Hearing No. of Hearing No. of
distance testees distance testees distance testees
1963 162 yds. 1 82 yds 5* -------------
116 yds. 4*
1964 71 yds. 5* 170 yds. l 117 yds. 4*
50 yds. l 129 yds. 4* 78 yds. 1
not heard 1 not heard 1

 

* Hearing distances which applied to me.

The results of the second tests, particularly for the 1963 test,
showed the adverse effect of aural interference. The rather short hear-
ing distances for the second bird of the 1963 test and for the first bird

of the 1964 test were due to interference from frog calls.

36

None of the hearing distances recorded were over 0.15 miles
(257 yards). This is less than the . 2 miles distance observed in the
standard FWS Singing Ground Survey. The FWS recommends that
survey stops be no less than 0.4 miles apart to avoid errors due to
counting the same bird at two consecutive stops. A 0.4 mile interval
is probably better than a 0. 3 mile interval to allow a margin of safety
from this error.

A hearing examination given to the cooperators by the Michigan
State University Speech and Hearing Clinic strongly validated the results
of the field hearing tests. Those individuals with low scores in the field
tests in 1964 had below-normal hearing in the frequency range of the
woodcock "peent. " Individuals with the higher scores on the field tests
had normal hearing. Two individuals who had below-normal hearing
test results had averaged daily singing ground survey totals which were
significantly lower at the one percent level (t = 2. 71) than the averages
of those individuals with normal hearing (see also Figure 7). Because
of this difference the surveys made by the two men with lower hearing
ability were not used in the analysis of data.

On the basis of my findings it would seem worth-while to provide
a hearing test for prospective participants in Singing Ground Surveys.
The field tests described above are simple and can be conveniently
accomplished. A recording of the woodcock "peent" would probably work
as well as a performing male and would allow the test to be made at any
suitable time or place. Hearing distances determined for cooperators
with normal hearing may be used in establishing adequate hearing ability
under various conditions. Distances of 102-257 yards were obtained
for testees with normal hearing under conditions of low interference.
Distances of 71 -170 yards were obtained for testees with normal hear-

ing under conditions of moderate wind, auto, and frog interference.

37

The elimination of participants with below-normal hearing would
reduce the chances of a low woodcock population estimate due to this

€I‘I‘OI'.

 

Effect of Including the Flight Song in the
Singing Ground Survey '

 

Since Sheldon (1953) advised that ”only peenting birds should be
counted, “ an investigation was made to determine in what manner and
to what extent the inclusion of flight songs affects the survey.

Singing Ground Survey cooperators recorded the number of birds
heard by peenting, the number heard by flight song, and the total
number of birds heard at each stop during both seasons for a total of
104 Singing Ground Surveys. A comparison of daily survey totals when
the flight song was included with totals when it was not, showed no signifi-
cant difference between the totals from either method for the first
season (t = . 96164). A similar comparison for the data of the second
season showed that the counts were significantly higher due to the in-
clusion of the flight song but only at the ten percent level (t = l. 3387).
Since the inclusion of flight songs tends to increase the quantity of
survey data it probably should be included. The flight song is helpful
too, in locating birds. But since less than ten percent of the birds are
located by flight song alone, flights cannot be depended upon to the

exclusion of peenting .

PROBABILITY THAT THE SURVEY WILL TALLY
ALL PERFORMING MALES PRESENT

Sufficient data were collected to account for all birds present
during a performance and during any two-minute period. Thus it was
possible, using type one counts (see above), to calculate the prob-

ability of hearing each individual male woodcock present at any one

 

listening post as follows:

Total number of birds heard in all two-minute periods
Number of birds present x Number of periods

 

In other words, the number of birds heard calling is divided by the
number expected to call, yielding the proportion of the expected birds
which call. The expected number is my density rating for the perform-
ance, as described earlier. I determined the density for each per-
formance at the time of observation. Although it was ordinarily constant
at each listening point during the stable period, changes in the birds
heard calling did occur for a single performance, more often due to
birds leaving my hearing range than to the occurrence of additional
birds in an area.

My hearing ability was shown by all tests to be average and the
probability of hearing each individual male within my hearing range
during any part of the season or performance regardless of external

conditions was calculated to be:

Dawn (1963) Dusk (1963,1964)

When 3 birds were present 0. 756 0. 742
When 2 birds were present 0.802 0.863
When 1 bird was present 0. 976 0. 977

38

39

These probabilities were based on 152 two-minute periods at dawn
and 570 two-minute periods at dusk.

Obviously all birds will not start or cease their performance at
precisely the same time. Therefore, the probability of hearing all
birds in the area is greater if time is allowed at the start of the per-
formance for all males to begin and if counting is stopped early enough
to avoid the irregularities of stopping times.

As shown earlier, evening performance lengths during the stable
period averaged about 42 minutes. When six minutes at either end of
the performance are allowed for irregular starting and stopping times
(of the birds), 30 minutes remain in which a survey may be made.

A survey system is which the counter waits six minutes after hearing
the first bird and then counts for 30 minutes would provide a higher
probability of hearing each individual male.

Probabilities were found to be highest under this system when
computed from my evening observations. However, since the pre-dawn
performance averaged only 27 minutes in length, probabilities of hear-
ing each individual bird are lower with this system. These lower
probabilities contributed to the earlier conclusion that the pre-dawn
performance is unsuitable for making surveys.

The probabilities of hearing each individual male are higher also,
if only data from the stable period are used in their computation.
Employing the stable period, the six minute wait, and the subsequent

30 minute counting period as standard conditions, the probabilities of

 

hearing each individual bird at various densities were:

Dawn (1963) Dusk (1963, 1964)

When 3 birds were present 0.680 0.867
When 2 birds were present 0.700 O. 911

When 1 bird was present 0.657 0. 943

40

These probabilities were based on 90 two-minute periods at dawn and
225 two-minute periods at dusk.

The probabilities of hearing each individual bird under standard
conditions and for various physical conditions were computed for the
several bird densities encountered (Table 6). These probabilities allow

the selection in some cases of optimum conditions for hearing each

individual bird.

 

Considering sample size as well as the probability of hearing
each bird, temperatures 400-60o F. , relative hurnidities 51-75%, and
wind velocity 0-4 m. p.h. , represent optima when probabilities were
conclusive as to optimum conditions. Lowered probabilities of hearing
each bird when temperature was over 600F. probably were associated
more with waning activity during the later part of the season (when
higher temperatures occur) than with temperature alone. Similarly,
the lower probabilities at temperatures under 4OOF. may be in part
due to irregular early season activity.

Probabilities of hearing each individual bird during one-minute
and three-minute periods were also computed. Under standard con-

ditions these probabilities (without regard to weather conditions) are:

One-minute Three-minute
period period
When 3 birds were present 0. 933 1. 000
When 2 birds were present 0. 985 1. 000
When 1 bird was present 0. 943 1. 000

This set of probabilities is from the 1964 data only since the field
recording sheet was not arranged to gather data on a one-minute basis
in 1963.

These probabilities are deceptively good. In comparing the one-
minute period to the two-minute period, the sample size was doubled but

the number of periods in which birds were not accounted for was not

41

 

 

 

 

 

Table 6. The probability of hearing each individual bird under various contitions,
during the stable period after waiting six minutes then counting for
thirty minutes (dusk). Rose Lake Wildlife Experiment Station,

East Lansing, Michigan. March-June, 1963 and 1964.
3 Birds Present 2 Birds Present 1 Bird Present
Number of Prob- Number of Prob- Number of Prob-
2-minute ability 2-minute ability 2-minute ability
samples samples samples

Tempeiéature

20-40 15 0.800 20 0.550 5 1.000
41-50O —- ----- 70 0.900 30 1.0004
51.600 105 0.895* 90 0.9334 20 1 000
60 or more 45 0.822 10 1.000 15 0.733

Relative hum.

0-25% -- ..... -- ----- -- -----
26-50% -- ----- 4O 0. 925 20 0.800
51-75% 75 0.891* 120 0.925* 45 1.0004
76-100% 90 0.833 30 0.700 5 1 000

Wind velocity

0-4 mph 90 0.878* 110 0.945* 35 1.00096
5-8 mph 45 0.844 50 0.900 20 1.000
9 or more 30 0.867 30 0.667 15 0.733
Moon stage
0-8 days 15 0.867 80 0.888 25 1.000
9-11 days 30 0.900 10 1.000 15 1.000
12-15 days 45 0.889 20 1.000 5 1.000
16-20 days 15 1.000 30 0.800 5 1.000
21-29 days 60 0.800 50 0.880 20 0.800
Vegetative moisture
0 gm. 60 0.800 80 0.875 40 0.900
more 60 0. 917 50 0.820 15 1. 000
Cloud cover
0-25% 60 0.883 70 0.957 20 1.000
26-50% 30 1.000 50 0.840 20 1.000
51-75% 15 0 733 -- ----- 5 1 000
76-100% 60 0.817 70 0.857 25 0.840
90-100% 45 0 844 60 0.833 15 1 000
Barometric Pressure
29.50-29.80 60 0.867 20 0.700 15 1.000
29.81-30.00 30 0.867 40 0.925 15 0.733
30.01-30.20 45 0.844 70 0.843 15 1.000
£0.21 more 15 0.933 40 0.975 10 1.000

 

* = Optimum conditions.

42

doubled. However, there were more periods when birds weren't
accounted for on a one-minute basis than on a two-minute basis.

Thus, for small samples, probabilities would be lower. Also, there
probably would have been a greater number of one-minute periods
when birds were not accounted for in my records had I not been present
throughout the performance and been aware of the number of birds
present. In other words, one minute is not enough time to locate and
account for individual birds when one is not already familiar with the
number of birds actually present.

Using a three-minute period appears to provide the perfect solu-
tion to hearing each individual bird that is present. However, the
three-minute period is undesirable because less area can be covered
on a survey, particularly where roads are rough and where the 30
minute survey period is used.

In 1963 and especially in 1964, efforts were made to determine
what caused probabilities to be less than one. Since probability of hear-
ing each bird was best at a one-bird density, the most apparent reason
seemed to be that the birds were active but had moved from my hearing
range at the two and three bird densities. Previous studies (Mendall and
Aldous, 1943, and Sheldon, 1953) have shown that males may change
their singing grounds throughout the season or even during a performance.

It should be made clear that I was closer to the bird when one male
was present than to any of the birds when two or three birds were
present. This was due to the spacing of the birds caused by their
territorialism. At the single bird area, minor movements in singing
position were observed (by the bird walking or running as well as by
changes after a flight) but they usually didn't take the bird out of my hear-
ing range. These same minor movements on areas of greater density
could have moved the bird out of my range and they would have been

difficult to distinguish from periods of silence. When movement

43

occurred by flight or if the bird peented as it moved away I could
account for periods of silence.

Field records from 1963 indicated that in one-third of the cases
where birds were not accounted for they were not heard after a flight
song. These were cases where birds were not accounted for during
one or more two-minute periods (of the 30 minute counting period) when
they should have been heard. In 20. 0 percent of these cases, some
birds were not heard in all two-minute periods because they started
later than other birds at the listening points. The birds were not heard
2. 5 percent of the time simply because they were silent. In 44. 2 per-
cent of the cases I could not be certain why birds were not heard.

In 1964, birds were not heard in all periods 42.8 percent of the
time because they moved out of my hearing range. Another 42. 8 per-
cent of the time birds were not heard because they ceased performing
before the end of the 30-minute counting period. In 14.4 percent of
the cases birds weren't heard because they started their performance
later than six minutes after the first bird had begun. Thus, approximately
40 percent of the cause of lowered probabilities is due to movement of
the birds.

During the stable periods of both seasons only ten cases of males
moving into my hearing range occurred in over 600 acceptable two-
minute periods. So, movement into my hearing range did not balance
with the movement away from it. However, habitat limitations could
have accounted for this. My one and two-bird areas were probably cap-
able of supporting only the number of males I found there with room left
for minor movements but no room for additional males. My three-bird
area was so large that performing males were widely spread and minor
movements would not be enough to move them into my hearing range.
Movements into my range were accounted for by flight song in nine of the

above ten cases; in other words, these were "overflights. "

44

It is unfortunate that movement is not the only cause of lowered
hearing probability. Movement towards a survey route is probably as
likely as movement away from it and birds who move and still peent
would not affect the reliability of widespread censuses. However, it is
still possible to compute a correction factor on the probabilities for
use in Singing Ground Surveys to consider those periods when males
are not accounted for due to things other than movement. First, it
must be assumed that the proportion of movement is the same for all
birds. That is, 40 percent of the cause of not hearing any bird is
because minor movements had taken it from the observers hearing range.
Also, it must be assumed that the direction of movement is random.

I did not find the first assumption to be true on my one-density area,

but I sat closer to the bird in that case and movement didn‘t contribute
to my not hearing the bird in all periods. If these assumptions are true,
then movement alone should not prevent a bird from being included in

a Singing Ground Survey tally. Thus, the loss to the probability due to
movement, now may be regained.

No cases were observed where two of the three birds present
were not accounted for simultaneously during the 30 minute counting
period. Thus, it should be possible to correct Singing Ground Survey
tallies of zero for the possibility of one male being present, tallies of
one for the possibility that two we‘re'present,vand tallies of two for the
possibility that three were present. 1

First, the probabilities of hearing each individual bird can be
corrected to include 40 percent absence from the hearing range due to

movement, thus:
P + (l-P) ( .4) = P corrected for movement.

Additionally, the counters tally for birds present but not heard can be

corrected as:

45

X corrected = X + (1 - P corrected)
X = the counters estimate of density at any one listening point.

P = the probability of hearing each individual bird for X + 1 bird
density. (See the probabilities of hearing each individual
bird under standard conditions).

Putting the two formulas together:
X corrected = X + {1 - [P + (l - P) ( .4)]}
For example, if a counters tally is one:
X corrected = 1+ {1 - [ .911 + (1 - .911)( .4)]} = 1.0534

Usually no more than two birds are heard at a single listening
point. Therefore a corrected tally for zero birds would be 0.0342
birds, and a corrected tally for two birds would be 2. 0798 birds.
Although the correction factor is small for any one survey count, the
corrected value may be important where many surveys are considered
collectively.

The probability results indicate most clearly that the stable period
is a better surveying time than other portions of the season and show in
some cases which physical conditions produce the best survey results.
Probability findings agree with the findings of Goudy (1960) who recom-
mended that surveys not be made when the roadside air temperature is
less than 400 F. and wind velocity is over 15 m.p.h. Thus, when observ-
ing standard and optimum conditions (see above) the survey is a truer

representation of the number of males present on the survey route.

CONCLUSIONS AND RECOMMENDATIONS

With the exception of light intensity, male woodcock are little

affected by physical conditions except in extremes. They are most

affected by the time of season and density of performing males on an

area.

Thus, Singing Ground Surveys can yield a truer appraisal of

population levels if the following recommendations are adopted:

1.

Test cooperating observers, selecting only those whose
ability to hear woodcock (see text), or sounds between 4. 7
and 6.0 K.C. is normal.

. Confine the surveys to any time during the period of great-

est uniformity in courtship activity which in central Southern
Michigan is April 15 to May 15.

. o
. Make surveys on calm evenings With temperatures 40-60

and the relative humidity 51 -75 percent.

. Restrict the surveys to the dusk performance, avoiding pre-

dawn.

. If a male is not heard at the starting point of the survey

route, starting time should be 22 minutes after official
sunset when 0-75 percent cloud cover exists, and 15
minutes after sunset with 76-100 percent cloud cover.

. If a male is present at the starting point, wait six minutes

after hearing the first call, then make the survey during the
next 30 minutes, utilizing two-minute listening periods.

. Use both peents and flight songs in counting the number of

birds heard at a listening point on the survey route.

46

SUMMARY

The annual woodcock Singing Ground Survey of the U. S. Bureau
of Sport Fisheries and Wildlife is widely used as an index to woodcock
abundance. Observers drive a preselected route and stop at intervals
to record the number of male woodCock heard performing their court-
ship calls and flight songs. This study was undertaken to determine
the nature of factors which may affect this type of survey.

The male courtship performance occurs before dawn and again at
dusk on a relatively open field called the singing ground. Most of the
performance occurs on the ground where the peent call is given.
lnterspersed throughout the performance are flights which involve ascent
to 200 to 300 feet, circling and descending to the ground. A rapid
mechanical twittering made by the wings and a vocal chirping are given
during the flight.

I made counts for two seasons of peents and flight songs by two-
minute intervals on areas occupied by several different densities of
performing males. Physical conditions were measured and recorded
for each performance. Cooperators made the standard Singing Ground
Surveys each evening through the areas in which my observation points
were located.

Woodcock surveys evidently can be most affected by the time dur-
ing the season when counts are made and by the hearing ability of the
counter. Climatic conditions were not found to be important in influencing
the survey except for extremes. However, the probability of hearing
each individual bird is best when wind velocity is below four m. p. h. ,

. . . 0 .
air temperature during the count is between 40 and 60 F. and relative

47

48

humidity is 51 to 75 percent. The survey results are not lowered by
counting flight songs as well as peents, nor are they altered significantly
during the occurrence of the full moon. Woodcock courtship activity is
affected by the density of performing males on an area and by the stage
of the brood cycle, but surveys are not seriously affected by either
factor. The pre-dawn performance is much less desirable as a survey

period than the dusk performance.

LITERATURE CITED

Blankenship, Lytle H. 1957. Investigations of the American
woodcock in Michigan. Michigan Conservation Department,
Game Division, Final report number 2123, 217 pp.

Goudy, William H. 1960. Factors affecting woodcock spring popu -
lation indexes in southern Michigan. Michigan Conservation
Department, Game Division, report 2281, 44 pp.

Mendall, Howard L. and Clarence M. Aldous. 1943. The ecology
and management of the American woodcock. Maine Coop. Wildl.
Research Unit, Orono, Maine. 201 pp.

Prosser, C. Ladd and Frank A. Brown Jr. 1961. Comparative
animal physiology. W. B. Saunders Co. Philadelphia. 688 pp.

Sheldon, William G. 1953. Woodcock studies in Massachusetts.
Trans. No. Am. Wildl. Conf. 18:369-377.

Stein, Robert C. 1963. Personal communication to Cornell Laboratory
of Ornithology from the author. Dated August 14, 1963.

U. S. Fish and Wildlife Service. 1963. Instructions for the Singing
Ground Survey, 1963. Mimeographed, 4 pp.

49

 

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