Dispersal by Juvenile American Crows (Corvus Brachyrhynchos) Influences Population Dynamics Across a Gradient of Urbanization

Abstract

Abstract American Crow (Corvus brachyrhynchos) populations are increasing across North America, often at high rates in urban areas. A monthly survey of American Crows in the Seattle Christmas Bird Count (CBC) circle suggested that winter counts reflected American Crow abundance at other times of the year, so we used CBC results for American Crows as a measure of population trend. In the Seattle area, local survival and fecundity appear unable to account for exponential population growth. We tested the hypothesis that juvenile dispersal from outlying suburban and exurban areas contributes to growth in the urban population by radiotagging 56 juveniles 5–46 km away from the central business district of Seattle and tracking their movements. Juvenile American Crows’ centers of activity were 0.2–22.2 km away from their natal territory during the first 3–12 months after fledging. An estimated 45% survived one year. Movements of dispersing American Crows varied in their consistency with simulated random-walk paths; the data suggested that, at the population level, American Crows were not drawn into urban areas, though some individuals may have been. Movements of dispersers produced a net influx into the city, because of greater reproductive success outside the city than in it. Simulations of urban population growth that included immigrants and emigrants accounted for most of the observed growth, which indicates the importance of distant suburban and exurban breeding pairs to urban population dynamics. La Dispersión de Juveniles de Corvus brachyrhynchos Influencia la Dinámica Poblacional a lo Largo de un Gradiente de Urbanización