Implications of Bald Eagle (Haliaeetus leucocephalus) Predation on the Goose Island Glaucous-winged Gull (Larus glaucescens) Nesting Colony

Abstract

In Washington state, rebounding Bald Eagle (Haliaeetus leucocephalus) populations are having both direct and indirect effects on the reproductive success of seabirds like Glaucous-winged Gulls (Larus glaucescens). In an attempt to quantify the effects of eagles on seabird nesting colonies and their reproductive success, we conducted a behavioral survey of the Glaucous-winged Gull nesting colony of Goose Island in the San Juan Islands. During a five-day-long field survey, we used interval sampling to establish the percentage of gulls engaging in preening, vigilance, and inactive behaviors in relation to the number of eagles in proximity to the colony. We also tracked eagle attack rates and successes, to see whether there was variability in prey preference. We found that there was little variability in the average percentage of gulls vigilant compared to the number of eagles present. However, the proportion of inactive gulls remained relatively high throughout all days surveyed. We also observed that during periods of high wind, eagles were less likely to be present and gulls were more likely to participate in flying or foraging behaviors. The attack rate seemingly decreased over the study period, suggesting that rates of eagle predation on nesting colonies decrease toward the end of the hatching season. These results suggest that towards the end of the nesting period, gulls may be less at risk for the indirect effects of eagle predation. However, more research is needed over a full nesting period to establish an accurate relationship between gull behavior and eagle presence, which can be used to understand the potential reasons for the decline in other seabird colonies.