Spatial Foraging Patterns in Puget Sound Pigeon Guillemot (Cepphus columba): An Investigation Using Stable Isotopes and Community Science

Abstract

I used stable isotope analysis to infer foraging patterns among Pigeon Guillemot (Cepphus columba) in Puget Sound, WA. I analyzed discarded eggshells collected by community science volunteers from below active Pigeon Guillemot burrows. Samples were collected from 22 colonies on Whidbey Island (within the Admiralty Inlet and Whidbey Basins) and in the Nisqually Reach region (within the South Basin) over a three-month period during the 2019 breeding season. Stable isotope analysis was then performed on the eggshells and eggshell membranes. I found a significant difference in the δ13C values (i.e., 13C/12C) and δ15N values (i.e., 15N/14N) of the samples collected from Admiralty Inlet and Whidbey Basins versus the South Basin, suggesting that the birds in these basins derive their energy and nutrients from slightly different sources. Observations of fish deliveries to burrows by breeding birds were used to infer dietary preferences, which were found to be relatively consistent across basins. Fish proportions in the birds’ diet were compared with fish abundance in the same basins and indicated that Pigeon Guillemot prefer gunnel over other taxa, despite their lower abundance in all basins. The spatial patterns shown by the isotopic information collected in this study indicates that the foraging dynamics of Puget Sound Pigeon Guillemot reflect attributes of the basin in which the colonies are located, such as variations in prey availability or basin biogeochemistry.