Variation and Migration Overlap in Flight Feather Molt of the Rose-Breasted Grosbeak

Rose-breasted Grosbeaks (Pheucticus ludovicianus) are extremely variable in several aspects of their flight feather molt. They have a high frequency of arrested molt of the remiges. They commonly show arrested and "omissive" molt in the prealternate rectrix molt of the first-year males, of which as many as half may replace only part of the tail. The coloring of adult and juvenal feathers shows great variability. Finally, most Rose-breasted Grosbeaks that strike TV towers during their nocturnal migration in the autumn are still molting flight feathers. No other species studied thus far shows such a prevalence of active molt during migration. The reasons for variability in molt and plumage are unknown, as are the reasons for an apparent relative lack of variability in the molt and plumage of other species. We hope that this paper will prompt further investigation of molt variation, and of overlap between molt and migration, in Rose-breasted Grosbeaks and other species.