Regional differences in population trends and environmental drivers of population dynamics in an at-risk waterbird

Understanding the drivers of population dynamics in migratory birds is essential for their conservation and management. At the same time, population trajectories and the importance of certain environmental factors can vary spatially and temporally. Surveys on the nonbreeding grounds can be valuable for analyzing broad-scale drivers of population dynamics. This is especially valuable for species that have a large portion of their breeding range in inaccessible and under-surveyed areas such as the boreal forest, as is the case for Podiceps auritus cornutus (the North American subspecies of the Horned Grebe). Here we used data from the Christmas Bird Count, which encompasses the entirety of the P. auritus cornutus nonbreeding range, to examine the relative influence of breeding ground (pond abundance, maximum temperature, and precipitation) and nonbreeding ground (minimum temperature, and precipitation) influences on regional population trends. We uncovered significant regional variation in population trends, with nonbreeding population sizes increasing in the interior of the continent, but stable or decreasing in coastal areas. The abundance of the subspecies on their nonbreeding grounds was negatively influenced by the amount of precipitation in the Prairie Pothole Region in 32% of the strata and positively influenced by the amount of precipitation in the boreal forest during the previous breeding season in 95% of the strata. Warmer minimum temperatures on the nonbreeding grounds were also associated with higher abundance in 60% of the strata. Our findings highlight the importance of understanding regional variation in drivers of trends and underscore the need for more focused research on migratory connectivity in P. auritus and on the portion of the population that breeds in the boreal forest.