Wing-Shape Variation and Differential Timing of Migration in Dark-Eyed Juncos

Wing shape among Dark-eyed Juncos, Junco hyemalis, (as described by principal components analysis of eight wing measurements) varied significantly with capture date during the fall migration in southwestern Pennsylvania. We assessed the relationship between this variation and the differential timing of migration among and within age/sex classes in a sample of 463 juncos. Differential timing of migration among morphologically distinct age/sex classes accounted for most of the temporal variation in wing shape. Adult juncos tended to migrate earlier than immatures and females tended to precede males. Significant variation remained, however, even after accounting for differential timing of age/sex classes. Specifically, wing shape varied with capture date among adult males and among immature males. Although some components of wing shape were correlated with body mass in juncos, body mass did not vary with capture date. Temporal variation in wing shape among Dark-eyed Juncos caught during fall migration seems to be the complex result of differential timing of migration both among and within age/sex classes. Variation in the distance migrated and/or timing of prebasic molt may contribute to these differences in migratory timing.