Invited Commentary: Fifty Years of Raptor ResearchTranscript of a Plenary Address Presented at the Raptor Research Foundation 50-year Anniversary Conference, Cape May, New Jersey, 17 October 2016
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. Este análisis examina los principales progresos que han ocurrido durante los últimos 50 años en nuestro conocimiento acerca de tres aspectos de la biología de las aves rapaces: (1) los factores naturales que limitan las densidades de cría; (2) la influencia de químicos tóxicos; y (3) los movimientos y las migraciones. Las primeras pruebas indicaban que las densidades de aves rapaces reproductoras estaban limitadas de forma natural por la disponibilidad de presas o de lugares de nidificación, cualquiera que se encontrase en menor medida en el área de estudio. Evidencias más recientes han demostrado que la depredación puede tener una influencia adicional, siendo las aves rapaces de mayor tamaño y los búhos los que limiten el número de rapaces más pequeñas a niveles menores de lo que el alimento o los lugares de nidificación permitirían. Además, se ha demostrado que algunas especies de rapaces migratorias, como en otras aves migratorias, pueden estar limitadas en sus áreas de migración e invernada a niveles menores de lo que permitirían las condiciones en las áreas de cría. A medida que las poblaciones de rapaces se fueron recuperando de los impactos de pesticidas organoclorados, la atención ha recaído sobre otros agentes limitantes, incluyendo el plomo (proveniente de municiones), que está impidiendo actualmente el establecimiento de poblaciones sostenibles por sí mismas de Gymnogyps californianus en estado silvestre, y los medicamentos antiinflamatorios veterinarios, los cuales han causado graves declives en las poblaciones de buitres asiáticos. El desarrollo del seguimiento por radio-tracking ha permitido el estudio de los movimientos locales a nivel individual en rapaces, aportando nueva información sobre los territorios y el uso del espacio, mientras que el seguimiento vía satélite ha mostrado las rutas migratorias, las áreas de invernada y el comportamiento de cientos de aves a nivel individual. [Traducción del equipo editorial]Abstract
Resumen
Spacing of sparrowhawk nests (shown by mean nearest neighbor distance) in relation to prey density (as indexed by point counts of all prey species together, expressed as numbers and biomass). From Newton et al. 1986.
Numbers of Short-eared Owls (Asio flammeus) breeding in an area of Finland each year in relation to the abundance of Microtus voles. Adapted from Korpimäki and Norrdahl 1991.
Modes of action of DDE (from the insecticide DDT) and HEOD (from aldrin and dieldrin) on raptor populations. From Newton 1986.
Shell-thickness index of Eurasian Sparrowhawks (Accipiter nisus) in Britain, 1870–1997; the research program came to an end in 1998. Shell-thinning became apparent very quickly from 1947, following the widespread introduction of DDT in agriculture. The problem improved from the 1970s, following progressive restrictions in the use of the chemical, which was banned altogether from 1986. Each dot represents the mean shell index of a clutch (or part-clutch), and more than 2000 clutches are represented from all regions of Britain. Shell index was measured as shell weight (mg)/shell length × breadth (mm). Adapted from Newton 1986.
Shell-thinning and population trend in Peregrine Falcons (Falco peregrinus) in different parts of the world. All populations showing more than 17% shell-thinning (associated with a mean level of 15–20 ppm DDE in fresh egg content) declined, some to the point of extinction. In the one exception, extra eggs and young were added by biologists to maintain numbers. From Peakall and Kiff 1988.
Migration of a satellite tracked radio-tagged Short-toed Eagle (Circaetus gallicus) from France to Niger, showing the daily distances and the nightly stopping places. Adapted from Meyburg et al. 1998, with permission from Alauda journal.
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