This review examines the main developments that have occurred over the past 50 years in our understanding of three aspects of raptor biology: (1) natural factors that limit breeding densities; (2) influences of toxic chemicals; and (3) movements and migrations. Early evidence indicated that raptor breeding densities were limited naturally by the availability of either prey or nest sites, whichever was in shortest supply in the area concerned. More recent evidence has shown that predation can have additional influence, with larger raptors and owls limiting the numbers of smaller ones to below what food or nest sites would permit. In addition, it has become apparent that some migratory raptors, like other migratory birds, can be limited in their migration and wintering areas to levels below those that conditions in breeding areas would permit. As many raptor populations have recovered from the effects of organochlorine pesticides, attention has switched to other limiting agents, including lead (from ammunition), which is currently preventing California Condors (Gymnogyps californianus) from establishing self-sustaining populations in the wild, and anti-inflammatory veterinary drugs, which have caused massive declines in Asia vultures (Gyps spp.). The development of radio-tracking enabled studies of the local movements of individual raptors, providing new information on territories and ranging behavior, while satellite-based tracking has revealed the migration routes, wintering areas, and behavior of hundreds of individual birds.
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Vol. 51 • No. 2