Survival of Adult Herring Gulls (Larus argentatus) from a Lake Ontario Colony Over Two Decades of Environmental Change

We estimated annual survival probabilities of color-banded adult Herring Gulls (Larus argentatus) breeding in a colony on the Canadian shore of Lake Ontario, between 1981 and 1984, and between 1993 and 1998. During the study, major changes occurred in prey fish availability, fish-eating bird abundance, and organochlorine contaminant exposure. We used capture-mark-recapture models and an information-theoretic approach to model selection to explore the importance of indices for these environmental variables and the role of year and sex as exploratory effects. In our models, year but not sex acquired strong support suggesting important annual variation in our apparent survival (survival confounded by permanent emigration) and encounter probabilities. The arithmetic mean of our model-averaged apparent survival probabilities provided an estimate of 0.91 (0.02 SE), and four out of the seven estimates were high for the species (≥0.93). Our model-averaged estimates suggested that apparent survival declined after 1995 to a low of 0.79 ± 0.07 (SE). Of the three environmental covariates fitted to survival, only the abundance of Alewife (Alosa pseudoharengus) acquired strong support as an explanatory variable. Annual fluctuations in survival and the decline after 1995 may be a reflection of nutritional stress resulting from the decreased availability of lipid (energy) and high quality protein from this prey species, following the arrival of the exotic Zebra Mussel (Dreissena polymorpha). These nutritional impacts may be exacerbated by some of the PCB-associated sublethal health effects documented in these gulls over the period of this study. Our limited success at isolating other important environmental covariates reinforces the difficulty posed in identifying the critical components of the complex of environmental variables encapsulated in ‘time’.