Recent evidence indicates that over the last 150 years, humans may have impacted seabird populations through modification of their marine food resources. Unfortunately, the high mobility and large pelagic ranges of many seabirds has resulted in a dearth of information concerning even their basic feeding habits. Here, I use stable isotope analysis to investigate the modern and ancient foraging ecology of an endangered seabird, the Hawaiian petrel (Pterodroma sandwichensis). Stable isotopic composition of Hawaiian petrel tissues (δ13C and δ15N values) reflects trophic level and foraging location and can therefore be used to describe patterns of foraging segregation or long-term temporal variation within the species. Chapter 1 investigates isotopic variation within individual flight feathers, with the goal of designing minimally-invasive and ecologically informative sampling strategies. δ13C values increased from tip to base in all 52 feathers within the study, including 42 remiges from the Hawaiian petrel and 10 from the Newell's Shearwater (Puffinus auricularis newelli). Such a consistent trend, observable among different species and age classes, is unlikely to result from shifts in diet or foraging location during feather synthesis. Considerable variation of δ15N values was also present within feathers (average range of 1.3 / within Hawaiian petrel remiges). A sampling protocol is proposed that requires only 1.0 mg of feather and minimal preparation time. Because it leaves the feather nearly intact, this protocol will likely facilitate obtaining isotope values from remiges of live birds and museum specimens. Chapter 2 explores ecological variability among modern Hawaiian petrel populations. δ13C and δ 15N values of feathers demonstrate segregation in foraging location during both the breeding and non-breeding seasons for petrels nesting on Kauai and Hawaii. Genetic analyses based on the mitochondrial Cytochrome b gene also reveal strong differentiation: coalescent-based analyses estimate < 1 migration event per 1,000 generations. Finally, feathers from multiple age groups and islands show unexpected divergences in δD that cannot be related to variation in source water. Overall, these data demonstrate foraging and genetic divergence between proximately nesting seabird populations. This divergence occurs despite high species mobility and a lack of physical barriers between nesting sites.Chapter 3 investigates Hawaiian petrel foraging habits and inter-colony segregation over the course of approximately 2,000 years. The most pervasive temporal trend is a 1.4-2.6 / decrease in average δ15N values, which likely reflects declining trophic level over the past 300-1,000 years. Isotopic chronologies also document ca. 2,000 years of foraging segregation between Hawaiian petrel colonies, observed as a long-standing divergence in average δ15N values. The degree of foraging segregation between petrel colonies diminishes through time and correlates well with genetic population structure. Shifting foraging habits of the Hawaiian petrel may reflect relatively widespread trophic alterations in the pelagic realm of the North Pacific. Such changes in foraging are concerning, given their implications for reproductive success and genetic diversity.
Read
