Competition is a major limiting factor of refueling in migratory passerines during stopover

For an avian migrant, refueling capacity attainable during stopovers governs the entire migration schedule and, ultimately, its fitness. Specifically, timely replenishing energy stores is critical when migration involves crossing ecological barriers, within which refueling may be limited. Here, we tested the hypothesis that fuel deposition rates (FDRs) of migratory passerines within barrier-edge stopover sites are constrained by the density of potential competitors, irrespective of migration season, phenology, and local environmental conditions. We also evaluated diverse intra- and inter-specific competition scenarios and explored a potential mediation of density-dependence by environmental factors. The analyzed data, collected by us over 13 consecutive years (2009–2022), contain information on seven species of long-distance migratory insectivorous passerines measured within eight desert-edge habitats throughout autumn and spring migrations. As predicted, our analyses revealed negative density-dependence regulation of FDRs, consistent across species and migration seasons. Notably, bird density exerted its effect above and beyond the other factors known to influence FDR, such as relative ambient temperature, phenology, temporal progress of stopover, and body mass next to landfall. As expected, FDR increased at higher relative ambient temperatures and with the stopover's progress. In spring, FDR also rose as the season advanced. These findings signify the substantial impact of competition on the refueling performance of migratory passerines during their stopover on an ecological barrier's edge, acting over and above the other environmental factors. The detected importance of competition and its interrelation with other predictors provides an insight into stopover's functioning; environmentally imposed and inescapable interspecific interference is thus a significant limiting factor of FDR, a deleterious relationship potentially remediable by informed habitat restoration and planning.