Annual Moisture Levels Drive Population Dynamics Across the Breeding Range of a Declining Waterbird

In order to understand causes of avian population declines and establish conservation strategies, we require knowledge of how changes to the environment are linked to changes in population abundance. Such linkages can be examined with large-scale, long-term databases, such as the North American Breeding Bird Survey (BBS). The BBS is generally under-utilized for waterbirds, in part due to the notion that it is unreliable; however, for some readily detectable species such as Black Terns (Chlidonias niger), this may be a valuable data source. We applied Bayesian hierarchical generalized additive models to BBS counts to examine population trends and drivers of abundance across the Black Tern range. We uncovered long-term declines and strong spatial variation in trends, with declines particularly prominent in peripheral areas at the edge of the species' range. There was a strong positive effect of spring moisture (measured via the Standardized Precipitation Evapotranspiration Index) on abundance, whereas moisture in the core of the species' range in the prairies had a negative effect on abundance in peripheral areas, suggesting birds are attracted to the core of the range in wet years, and vice versa. There was a weak and spatially varying effect of the North Atlantic Oscillation Index on annual abundance. Ongoing wetland loss coupled with climate extremes will become increasingly important for predicting population fluctuations in this and other waterbirds.