| Summary: | Ecological studies characterising population trends over decades or centuries can help to describe the range of variability in a study system, with well-studied species being strong candidates for providing the long-term data required for retrospective studies. Seabirds represent useful real-time monitors of marine systems and may also play this role in studies characterizing historical ecological change. The glaucous-winged gull L. glaucescens is a generalist marine bird occurring in the Salish Sea, an urbanized coastal area of southwestern British Columbia and northwestern Washington, where it has been studied or collected since the mid-1800s. Its twentieth-century populations experienced dramatic growth followed by a steep decline, with recent trajectories unclear. I used multiple methods to characterise long-term trends in gull number, diet, and egg production, and to test hypotheses about causes of population change. My approach combined meta-analysis of historical reproductive traits, statistical modeling of population trend, and stable isotope analysis (δ¹³C, δ¹⁵N) of historical and modern gull feathers and forage fish, with modeled population trend showing a continued decline in gull numbers from the 1970s to the present. Meta-analytical results pointed to decreasing egg and clutch size and a delayed lay date over the twentieth century to the present, while stable isotope analysis showed declining feather δ¹³C and δ¹⁵N since 1860, all of which was consistent with a growing reliance by gulls on non-fish foods. Demographic modeling showed that declining clutch size and productivity were largely sufficient to account for the gull population decline, and pointed to recovery from cessation of nineteenth-century egging as being an important contributor to the increase phase. These modeling results implied that declining consumption of forage fish affected gull productivity. Additional results from stable isotope analysis also supported a hypothesis of dietary change; namely, declining forage fish C:N ratios over time indicated a decrease in fish lipid content, and thus a decline in prey quality. Overall, my results highlight the value of compiling multiple retrospective studies to better understand the complex factors affecting long-term trends in animal populations. === Forestry, Faculty of === Graduate
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