Twenty years of coastal waterbird trends suggest regional patterns of environmental pressure in British Columbia, Canada

Waterbirds are often used as indicators of ecosystem function across broad spatial and temporal scales. Resolving which species are declining and the ecological characteristics they have in common can offer insights into ecosystem changes and their underlying mechanisms. Using 20 years of citizen science data collected by the British Columbia Coastal Waterbird Survey, we examine species-specific trends in abundance of 50 species in the Salish Sea and 37 species along the outer Pacific Ocean coast that we considered to form the core wintering coastal bird community of British Columbia, Canada. Further, we explore whether ecological commonalities increase the likelihood of a species undergoing declines by testing the hypotheses that waterbird abundance trends are influenced by dietary specialization and migration distance to breeding grounds. Results suggest that most populations are stable (i.e., temporal trends are not significant) in both the Salish Sea (36 of 50 spp.) and Pacific coast (32 of 37 spp.) regions. Twelve species displayed significant decline trends in the Salish Sea, whereas two had significant increases. Along the Pacific coast, only three species displayed significant decline trends, and two significant increases. This result is corroborated by guild-specific trends indicating that waterbirds occupying the Salish Sea are faring significantly worse than those residing along the outer coastal regions, almost irrespective of dietary specialization or migration distance. Our results provide evidence that differential environmental pressures between the inner and outer coastal regions may be causing overall loss of wintering waterbirds within, or abundance shifts away from, the Salish Sea. Potential mechanisms responsible for these observed patterns are discussed, including environmental (e.g., climate) and human-induced (e.g., nutrient and chemical pollution) pressures. Collaborative, inter-disciplinary research priorities to help understand these mechanisms are suggested.