More than just a roof under their feet: How spatial and structural features of a coastal city predict nest site selection in the urban-nesting Glaucous-winged Gull (Larus glaucescens)

How birds use and interact with an urban environment has been of interest for a variety of species across the globe. This has particularly been the case for gulls (Laridae) as species in this family nest and otherwise occupy habitat in urban areas on six continents, often coming into conflict with humans as they do so. In this study, we combine survey data of urban-nesting Glaucous-winged Gulls Larus glaucescens in Vancouver, Canada with detailed information on architectural and spatial features of the structures on which they nest to build an explanatory and predictive statistical model for nest-site selection in our study city. We find that building height is by far the most important explanatory variable in this selection process and that rooftops of higher structures are generally more attractive to nesters, but only up to a certain point. Flat rooftops in non-residential areas that are near water are also highly valued. However, complex relationships exist between these and other variables that offer detailed insights into urban nesting patterns. We use our modelling approach to predict that roughly 1,800 pairs of Glaucous-winged Gulls nested in the city in 2017 and over 2,000 pairs nested in the city in 2023. Combining this with high exact nest and rooftop recurrence rates derived from additional survey data taken between 2015 and 2019, and with historical population data, we argue that this urban population is likely to continue to expand in the near future at roughly 4% per annum. As this species has suffered declines in its natural breeding colonies across the region, this suggests its future in the region may be increasingly urban-centric.