Determining the diets of black oystercatchers using stable isotope analysis

A black oystercatcher foraging on a rocky outcrop island.

NPS/Deb Kurtz

The black oystercatcher (Haematopus bachmani) is a charismatic shorebird that resides along marine coastlines in the north Pacific Ocean feeding exclusively in the intertidal on macroinvertebrates like mussels, chitons, and limpets. However, climate change is driving rapid changes in marine systems, particularly in high latitudes. These shifts may impact macroinvertebrate communities that black oystercatchers rely on. Because of these potential changes to the prey base of black oystercatchers, we examined the diets of these birds using stable isotope analysis of carbon (δ¹³C) and nitrogen (δ¹⁵N) in the Gulf of Alaska. Primarily we wanted to answer questions about whether the diets vary over space (are diets different across parts of the Gulf of Alaska?) and time (has their diet changes in the last century?). Results indicate that little variation in the diet exists between individuals across the Gulf of Alaska nor has it changed much over the last 100 years. With a feeding strategy that has changed little over time, we discuss the possibility that changing ocean conditions may alter the abundance of black oystercatcher prey and ultimately have impacts to the black oystercatcher population as a whole.

Stable isotope-determined diets of Black Oystercatchers Haematopus bachmani in the Northern Gulf of Alaska

Abstract

Black Oystercatchers Haematopus bachmani (BLOY) feed on intertidal invertebrates along coasts of the northern Gulf of Alaska and elsewhere. Details of their feeding ecology have only been marginally delineated, however, and as population sizes are small and limited geographically, rapid ecological changes may alter their prey base, placing BLOY resiliency in jeopardy. We examined the diets of BLOYs occupying the coast at three sites in Southeast Alaska using stable isotope analysis of carbon (δ¹³C) and nitrogen (δ¹⁵N) to answer the following three questions: (1) what are the diet proportions of prey groups consumed at different locations along the northern Gulf of Alaska; (2) how do individual diets vary; and (3) how do current diets compare to those of the past? Results indicate that: (a) the diet of individual BLOYs was approximately ~52% mussels Mytilus trossulus or other filter feeders, ~41% limpets Lottia spp. or other kelp and algal grazers, and ~5% dogwinkles Nucella spp. or other secondary consumers; (b) little variation in diet existed between seasons or locality; and (c) diets of adults during summer have varied little over the last 100 years. These findings indicate that BLOYs have a very specialized feeding niche that has not changed substantially over time. We discuss the possibility that changing ocean processes may alter the abundance of filter feeders and ultimately have effects on BLOY success in the study region.

Carney, B., D. Tessler, H. Coletti, J. M. Welker, and D. Causey. 2023. Stable isotope-determined diets of Black Oystercatchers Haematopus bachmani in the Northern Gulf of Alaska. Marine Ornithology 51: 123-135.