Trophic collapse in oversummering areas: a hidden driver of Red Knot (Calidris canutus rufa) declines?

Abstract

Red Knots (Calidris canutus rufa) have undergone a dramatic population decline in recent decades, with the Tierra del Fuego population falling by over 75% since the 1980s. While the loss of horseshoe crab (Limulus polyphemus) eggs at key staging areas has been recognized as a major cause, other potential drivers remain understudied. Here, we investigated the diet and foraging ecology of oversummering Red Knots in the Pampas region of Argentina. Based on fecal and video observations, we found that their diet consisted almost exclusively of two bivalves: wedge clams (Donax hanleyanus) and yellow clams (Amarilladesma mactroides, formerly Mesodesma mactroides), with consistent selection of intermediate prey sizes. Foraging performance varied with tidal timing, conspecific density, and time since arrival but not with age proxies. Historical records indicate that the yellow clams, once the dominant intertidal bivalve across the region, collapsed in the 20th century due to overharvesting, habitat degradation, and unexplained mass mortality events. This previously overlooked trophic collapse may have reduced food quality and availability during a vulnerable life stage, contributing to cumulative population stress. Our findings underscore the importance of integrating oversummering areas, long neglected in research and conservation planning, into regional and flyway-scale strategies. Failing to protect these critical habitats may jeopardize the recovery of rufa Red Knots and other long-distance migrants.