The Autumn Passage of Yellow-bellied Flycatchers in South Texas'

The possession of a suitable spot on the wintering grounds may be important for an individual's survival in migrant species showing territoriality in the nonbreeding season (e.g., Rappole et al. 1989, Winker et al. 1990). If the earliest arrivals are favored in securing the best winter territories (e.g., Smallwood 1988), then selection might favor individuals who quickly return to wintering areas to secure suitable areas. The opposite phenomenon, the vernal "rush" to breeding grounds, is well documented, and is thought to be the result of selective pressure from competition for breeding territories (e.g., Oring and Lank 1982, Francis and Cooke 1986). Given that winter territories are defended on an individual basis (i.e., both sexes show aggressive responses to intruders; Rappole and Warner 1980, George 1987, Winker et al. 1990), a sexual bias in arrival time on the wintering grounds might not be predicted from wintering selective pressures alone (cf Jehl 1963). The hypothesis that some selective pressure exists for early arrival on the wintering grounds suggests that the first arrivals in winter quarters might be individual adults who failed to reproduce. Although we have found no means to test this hypothesis on the wintering grounds of migrant passerines, it may be possible to examine it from another perspective. If a larger part of the adult population failed to reproduce in a breeding season, our hypothesis predicts that (at the population level) adult passage in autumn migration in this season might be earlier than during a season