Time-of-Day Effects on the Numbers and Behavior of Non-Breeding Raptors Seen on Roadside Surveys in Eastern Pennsylvania

-Results are presented of 30 pairs of morning and afternoon roadside surveys of New World vultures and diurnal birds of prey in farmland habitat in eastern Pennsylvania during the non-breeding season. Eight-hundred-twenty-seven raptors, including members of two species of vultures and 10 species of diurnal birds of prey, were counted on 60 surveys between October 1992 and December 1994. Three species of raptors, Turkey Vulture (Cathartes aura), Red-tailed Hawk (Buteo jamaicensis) and American Kestrel (Falco sparverius), comprised 90% of all birds sighted. Although almost equal numbers of birds were seen on morning and afternoon surveys, species composition, the numbers of birds, and the flight behavior of individual species, differed significantly on morning and afternoon surveys. Time of day of survey affected different species differently. Numbers of Turkey Vultures increased significantly, those of Red-tailed Hawks decreased significantly, and those of American Kestrels remained the same between morning and afternoon surveys. In general, sampling variance was higher on morning than on afternoon surveys. These results, which clearly demonstrate the need to take time of day into account when conducting roadside surveys of raptors, suggest that the ability to compare data from different sites is limited by the extent to which those data were collected at the same time of day. EFECTOS DE LA HORA-DELDIA EN EL NOMERO Y COMPORTAMIENTO DE RAPTORES NO-APAREADOS VISTOS EN MUESTREOS POR CARRETERAS EN LA PENNSYLVANIA ORIENTAL Sinopsis.-Se presentan los resultados de 30 pares de muestreos matutinos y vespertinos de aves carrofieras y rapaces diurnos llevados a cabo en las carreteras en habitat de granjas del en el este de Pennsylvania durante la epoca de no-apareamiento. Ochocientos veintisiete raptores, incluyendo dos especies de buitres y 10 especies de raptores diurnos, se contaron en 60 muestreos entre octubre del 1992 y diciembre del 1994. Tres especies de raptores, Cathartes aura, Buteo jamaicensis y Falco sparverius, sumaron 90% de todas las aves vistas. Aunque se observaron casi la misma cantidad de aves en los sondeos matutinos y vespertinos, la composici6n de especies, los ntlmeros de aves, y el comportamiento de vuelo de especies individuales fueron significativamente diferentes en los sondeos matutinos y vespertinos. La hora del dia afect6 diferentemente a diferentes especies. Los ndmeros de C aura aumentaron significativamente, los de B. jamaicensis decrecieron significativamente y los de F sparverius se mantuvieron iguales en sondeos de mafiana y de tarde. En general, la varianza del muestreo fue mayor en los sondeos matutinos que en los vespertinos. Estos resultados, que demuestran claramente la necesidad de considerar la hora del dia al conducir sondeos de raptores por las carreteras, sugieren que la habilidad para comparar datos de lugares diferentes estA limitada en la medida a que esos datos se hallan obtenido en horas similares. As populations of open-habitat raptors are widespread and occur at relatively low densities, they are often counted and observed with the use 'Current address: 184 Islington Road, Newton, Massachusetts 02166 USA. 2 Current address: Department of Entomology and Applied Ecology, University of Delaware, Newark, Delaware 19717 USA. 3 Author to whom reprint requests should be sent.