Red knot occurrence, prey density, island morphology, and climate change in the Virginia Barrier Islands (2009-2023)

Global climate change is reshaping dynamic coastal ecosystems, with uncertain consequences for migratory shorebirds such as the federally threatened red knot (Calidris canutus rufa) that rely on coastal staging sites during migration. Understanding how sea-level rise and changing climate drivers affect red knot foraging ecology is critical for informing conservation and management at coastal staging sites. We integrated long-term biological, geomorphological, and climatological data to examine the direct and indirect pathways influencing red knots and their prey at intertidal foraging sites on the Virginia Barrier Islands during spring migration (May 21 - 28, 2009-2023). Using piecewise structural equation modeling, we tested hypothesized two causal networks linking 1) red knot occurrence and 2) densities of their main invertebrate prey to habitat characteristics, island morphology, geomorphic change, and climate drivers of ecosystem change. Red knots were indirectly affected by geomorphic change and climate drivers through bottom-up effects on invertebrate communities mediated by island morphology. Accelerated shoreline change narrowed islands, reducing invertebrate density and richness and indirectly decreasing red knot occurrence. Storms interacted with global climate oscillations to drive erosion or accretion of beaches, with variable effects on invertebrate density and red knot occurrence. Invertebrate responses were taxon-specific: shoreline change directly increased blue mussel density but indirectly reduced coquina clam and crustacean densities by narrowing island width, while storms impacts on crustacean density were mediated by beach width. Our findings suggest that accelerated ecosystem change under future climate scenarios may alter foraging conditions for red knots and other migratory shorebirds in the Virginia Barrier Islands, with broader implications for long-term population resilience.