LONG-DISTANCE DISPERSAL AND POPULATION-GROWTH OF THE YELLOW-HEADED BLACKBIRD XANTHOCEPHALUS-XANTHOCEPHALUS

In this paper, I sought to account for inter-annual variability in the frequency with which Yellow-headed Blackbirds disperse eastwards from the core area of their breeding range to the east coast of North Amer ica. It is hypothesized that eastwards dispersal is a consequence of e nhanced reproduction, that is, the number of Yellow-headed Blackbirds recorded during a given autumn migration at the east coast is related to the number of offspring produced within the breeding range. The maj or premise of this study was that the tendency for any individual to d isperse is constant, so that larger numbers of individuals moving east during any autumn reflects larger numbers of young having been produc ed the previous summer (most individuals found at the east coast have been juveniles). Records of vagrant Yellow-headed Blackbirds from Mass achusetts and elsewere within eastern North America were used to estim ate the yearly magnitude of dispersal, and data from the US Fish and W ildlife Service's Breeding Bird Survey (BBS) to estimate annual reprod uction. Eastwards dispersal of Yellow-headed Blackbirds was strongly c oherent throughout the Eastern United States, which is consistent with the notion that vagrancy is a consequence of a broad-scale, populatio n-level process such as reproduction. Dispersal was also significantly related to reproductive success in the north central North America. T hus, the number of Yellow-headed Blackbirds recorded in Massachusetts during a given autumn was significantly related to estimated reproduct ive success at the northern and eastern periphery of the breeding rang e, the same area where growth and spread of the population has been mo st evident.