A comparison of historical and contemporary reproductive traits in a declining population of Glaucous-winged Gulls (Larus glaucescens)

Understanding how organisms manage life history trade-offs under variable environmental conditions is an aim that is central to ecology. Comparing modern reproductive data with those from historical studies can increase understanding of the range of historical conditions that have acted on a given species over time. We use recent (2008-2010) and historical (1960s-1980s) reproductive data from a single study colony, Mandarte Island, Canada, to help understand the recent population declines experienced by the Glaucous-winged Gull (Larus glaucescens) in North America's Salish Sea, a highly urbanized area. Because this long-lived species has also undergone regional long-term declines in reproductive investment via decreasing egg and clutch size, we assessed whether modern reproductive outcomes were similarly affected. Although our statistical power was hampered by small sample sizes, it appears that hatching success declined over time, from 0.83 (1960s) and 0.76 (1970s-1980s) to 0.60 (2008-2010). An apparent decline in nesting success, from 0.63 (1962) to 0.52 (this study), was not statistically significant. Recent clutch sizes showed intra-seasonal declines, a pattern unchanged from historical trends. In contrast, egg mass historically was constant within a given nesting season, but recent egg-mass data show intra-seasonal declines. We conclude that most gulls currently breeding on Mandarte Island cannot attain historical levels of reproductive success — perhaps because of declining environmental quality in the form of reduced availability of high-quality fish prey — although some high-quality pairs in this population are still able to maximize reproductive output. Our study highlights the importance of long-term study systems for identifying consequences of large-scale ecosystem changes; however, methodological clarity is essential to ensure data comparability through time. Ultimately, further study is needed to identify the drivers of reproductive changes in this population, something that could be used to inform future management decisions