Sex, Wingtip Shape, and Wing-Loading Predict Arrival Date at a Stopover Site in the Swainson's Thrush (Catharus Ustulatus)

Abstract

AbstractPointed wingtips and low wing-loading are believed to reduce the amount of energy used during migratory flight. If this is true, individuals with pointed wingtips or consistently low wing-loading should be able to migrate faster than conspecifics and may arrive on the breeding grounds and at stopover sites earlier. To test this hypothesis, I captured 97 Swainson's Thrushes (Catharus ustulatus; hereafter “thrushes”) during May in three consecutive field seasons, 2003-2005, at a stopover site in central Illinois. I used molecular techniques to sex 47 of these thrushes and created a discriminant function analysis using wing, tarsometatarsus, and tail lengths that predicted sex with 80.9% accuracy (P < 0.001). I applied this function to the remaining 50 thrushes so that I could control for any sexual dimorphism in morphology and arrival timing, and then examined seasonal trends in sex and morphology. Male thrushes arrived earlier at my stopover site than females (P < 0.001). After controlling for sex, thrushes with more pointed wingtips (P = 0.002) and lower wing-loading (P = 0.044) arrived earlier than thrushes with more rounded wingtips and higher wing-loading. The general linear model with sex, wingtip pointedness, and wing-loading explained nearly one-third of the variation in arrival date (P < 0.001, r2 = 0.281, n = 90). Pointed wingtips and lower wing-loading may help migratory birds cover distance more quickly and arrive at the breeding grounds sooner; alternatively, thrushes with more pointed wingtips and lower wing-loading may have departed from the wintering grounds earlier. If wing shape and size affect rate of migration, these variables may be important determinants of individual quality in migratory passerines.El Sexo, la Forma de las Puntas de las Alas y la Carga Alar Predicen la Fecha de Arribo de Catharus ustulatus a un Sitio de Escala Migratoria