Stopover ecology and population dynamics of migratory shorebirds

Abstract

Migratory animals are declining worldwide, and for many species there are multiple threats to population stability from throughout the annual cycle. Animals performing long-distance migrations use stopover sites en route to replenish fat stores, and the congregation of individuals at migratory stopover sites affords an opportunity to track population health, particularly for species that are highly mobile and difficult to study at other times of year. However, analysis of these data presents several challenges due to the flow-through nature of stopover and uncertainties about how stopover site conditions influence both demographics and migratory behaviors. Here, I explore the use of quantitative methods and hierarchical modeling to lend clarity to our understanding of the role of stopover in the annual cycle of migratory birds, considering both an ecological perspective on the direct and indirect effects of stopover on demographics and migratory behavior, and a statistical perspective on advancing new and existing modeling frameworks for analyzing complex mark-recapture datasets. Using counts, trapping, and mark-resight monitoring data collected for three species of Arctic-breeding shorebirds in Delaware Bay, USA, I evaluate the ways that stopover site conditions and phenology of resource availability influence the timing and rate of mass gain during stopover, variation in site use among years, and population vital rates. Disentangling these responses is important for understanding how populations respond to changes in stopover conditions, and how to interpret changes in stopover passage population size. Understanding the relative role of stopover in the context of the annual cycle is important for informing conservation strategies that will be most effective at halting declines and promoting population stability.