A System-Level Analysis of Migratory Shorebird Habitat Use, Diet, and the Influence of Climate-Driven Ecosystem Change in the Virginia Barrier Islands

Abstract

Migratory shorebirds rely on high-quality coastal staging habitats to refuel during migration, but these sites are threatened by climate change. The Virginia barrier island system supports over 100,000 shorebirds annually, including federally threatened red knots (Calidris canutus rufa) and other state and regional species of conservation concern such as dunlin (Calidris alpina) and semipalmated sandpipers (Calidris pusilla). The goals of my thesis were: 1) to advance understanding of the foraging ecology of these imperiled shorebird species in the understudied mudflat habitats of the coastal lagoon behind the barrier islands, and 2) to examine how a changing climate may impact red knots by altering barrier island habitats and the invertebrate prey that sustain them during migration. I compared habitat use and prey availability across sand, peat, and mudflat substrates during spring migration (May 14 – June 2, 2023 – 2024) and used fecal DNA metabarcoding to examine diet and prey selection on mudflats. Mudflats supported comparable shorebird numbers and invertebrate densities to barrier island sand substrate. The composition of shorebird and invertebrate communities differed across substrates. Dunlin and semipalmated sandpipers primarily used mudflats and consumed mostly crustaceans while red knots primarily foraged on barrier islands, feeding mainly on bivalves. Red knots foraging on mudflats preferred bivalves. Dunlin similarly showed a preference for bivalves but to a lesser extent than red knots. Semipalmated sandpipers exhibited a generalist diet strategy. All species consumed diatoms, providing the first evidence of biofilm grazing in this system. These findings highlight the ecological importance of mudflats in supporting diverse shorebird foraging strategies. To assess how climate-driven ecosystem change influences red knots and their prey, I used piecewise structural equation modeling integrating long-term data from peak spring migration when red knots are most abundant in Virginia (May 21 – 28, 2009 – 2023). I found that red knots were indirectly influenced by climate change via bottom-up effects on invertebrate prey mediated by barrier island morphology. Prey responses were taxon-specific, suggesting future change may alter invertebrate community structure. I then examined how barrier island morphology has changed since long-term red knot monitoring began in 2007. Half of the barrier islands in our study area significantly narrowed in island width or beach width between 2007 and 2023, though a lack of system-level trends in either indicated relative stability for the barrier island system as a whole. However, accelerated geomorphic change and increasing climate variability may alter foraging conditions, with broader implications for red knot population resilience. My research highlights the importance of preserving habitat heterogeneity at coastal staging sites and incorporating climate-driven landscape-scale processes into conservation planning for migratory shorebirds along the U.S. Atlantic flyway.