An ecological-physiology perspective on seabird responses to contemporary and historic environmental change

Abstract

The chapters included in this dissertation implement an ecological-physiology approach to understanding how long-lived marine organisms, using seabirds as a model, respond to changes in the environment. Many seabird populations are governed by bottom-up processes, yet efforts to connect prey dynamics and parameters such as breeding performance often yield mixed results. Here I examined how individual foraging behavior and nutritional status change at the inter-annual, decadal, and multi-decadal scale. I validated that the concentration of the avian stress hormone in seabird feathers is indicative of their exposure to nutritional stress. I then used this technique to show that young seabirds (Rhinoceros auklets, Cerorhinca monocerata) that experience variable foraging conditions during their prolonged nestling period incurred higher nutritional stress when provisioned with prey that was relatively low in energy content. On the other hand, when examining adult foraging behavior, a signal of environmental variability was lost in the noise of changing diets. Foraging behavior of adults appeared to be highly flexible and less informative in regard to detecting an environmental change. I used stable isotope analysis to re-construct the isotopic niche dynamics (where and at what trophic level seabirds were obtaining prey) and partitioning of food resources for three abundant seabirds (common and thick-billed murres, Uria aalge, and U. lomvia, respectively; and black-legged kittiwakes, Rissa tridactyla) breeding in the southeastern Bering Sea under cold and warm states of the ecosystem. Access to diverse habitat reversed how seabirds partitioned prey during food shortages: seabirds with access to multiple habitats contracted their isotopic niche during food-limited conditions in contrast to the expansion of the isotopic niche observed for seabirds with access to only one type of habitat. Finally, I measured nutritional stress and stable isotope signatures (carbon and nitrogen) in contemporary and historic red-legged kittiwake (Rissa brevirostris) feather samples to examine how birds breeding on St. George Island have responded to changes in summer and winter conditions in the Bering Sea over time. Red-legged kittiwakes were less nutritionally stressed during warm summers and winters. It is not clear, however, whether all seabirds would do well if the Bering Sea were to break with its pattern of oscillating between warm and cold conditions. Prey for these birds may either be negatively affected by continuously warm conditions (murres and black-legged kittiwakes feeding on juvenile pollock, Gadus chalcogrammus) or the conditions that are most beneficial to the prey are not known (red-legged kittiwakes feeding on myctophids). With this work I suggest that measuring nutritional stress in feathers and using stable isotope analysis to characterize foraging niches may document more dynamic responses to changes in the environment than population level parameters such as breeding performance. To do so, however, requires a better understanding of the relationship between these individual-level responses and fitness.