Population, reproduction, and foraging of pigeon guillemots at Naked Island, Alaska, before and after the Exxon Valdez Oil Spill

After the 1989 Exxon Valdez oil spill in Prince William Sound, Alaska, we studied pigeon guillemots Cepphus columba breeding just 30 km from the grounding site. The postspill population was 43% smaller than the prespill population, but we could not attribute the entire decline to the spill because a decline in this guillemot population may have predated the spill. However, relative declines in the population were greater along oiled shorelines, suggesting that the spill was responsible for some of the decline. Reproduction appeared largely unaffected, but the cryptic nature of guillemot nests undoubtedly reduced our ability to detect failed nests. Nesting success was lower, but the apparent cause-greater losses of chicks to predators-was not obviously related to the spill. Fledging weight and growth rates of chicks and the rate at which adults delivered food to their chicks were not lower after the spill. The most likely explanation for the few effects observed is that oil was present on the surface waters of the study area for a relatively short period before the guillemots returned to begin their annual reproductive activities.