Trends in colony sizes for five colonial waterbird species in the Atlantic Flyway

Robust estimates of colonial waterbird (CWB) breeding population trends are deficient owing to a lack of range wide, standardized survey efforts. Evaluating conservation priorities and effectiveness of management requires reliable trend estimates across multiple spatial scales. One potential data source for CWB trend estimation is the Colonial Waterbird Database, created in 2003 by U.S. Geological Survey and the U.S. Fish and Wildlife Service and intermittently updated since then. The database combines state or provincial survey data, particularly from the United States Atlantic Flyway, with historical colony counts obtained from publications. We combined recently collected survey data from Atlantic Flyway states and provinces with data archived in the database to generate population size trend estimates for five species: Double-crested Cormorant (Phalacrocorax auritus), Laughing Gull (Leucophaeus atricilla), Least Tern (Sternula antillarum), Common Tern (Sterna hirundo), and Black Skimmer (Rynchops niger). These species represent two actively managed conflict species and three species of conservation concern, respectively. We used mixed effects models to fit an exponential growth model to determine yearly trends in populations at Atlantic Flyway- and state-scales with survey data collected between 1964 and 2019. Direction of within-state trend estimates varied. Trends for some species (Common Tern, Laughing Gull) were increasing in northern states and decreasing further south. At the Flyway scale, Double-crested Cormorant increased (2.08 ± 0.28 % year-1) and Least Tern (-1.40 ± 0.36 % year-1) and Black Skimmer (-1.13 ± 0.68 % year -1) decreased, while Flyway-scale trends in Common Tern and Laughing Gull were not significant. Our analysis provides cross-state trend estimates to inform CWB management actions along the Atlantic Flyway.