Hayes, F. E. 1991. Raptor populations along the Paraguay River: seasonal, geographical and time of day
variation. Journal of Raptor Research 25:101-108.
Abstract.--Nineteen species of diurnal raptors were recorded during four censuses from ships along 859 km of the Paraguay River in June, August and October 1988 and January 1989. Seasonal, geographical and time of day variation in linear densities was documented for several of the common species. Most species of non-migratory hunting raptors were most common in June; they may have been exploiting prey concentrated in emergent vegetation when the river was flooded. The Lesser Yellow-headed Vulture (Cathartes burrovianus) appeared to be the most abundant when water levels were low. Both the Lesser Yellow-headed Vulture and Snail Kite (Rostrhamus sociabilis) were more common farther north where marshes were more extensive; the Black Vulture (Coragyps atratus) was more common farther south where human habitations were more prevalent. Both the Snail Kite and Crested Caracara (Polyborus plancus) were most active in the early morning whereas the Lesser Yellow-headed and Black Vultures were more active at midday.
Resumen.--Se registraron 19 especies de rapaces diurnas, durante cuatro censos desde barcos, a lo largo de 859 km del r�o Paraguay durante los meses de junio, agosto y octubre de 1988, y enero de 1989. Se document� la variaci�n de densidades lineales para algunas especies de rapaces comunes durante diferentes �pocas, regiones geogr�ficas y per�odos del d�a. La mayor�a de las especies de rapaces cazadoras no-migratorias fue m�s com�n en junio; pudiera ser que las rapaces estuvieran explotando presas concentradas en la vegetaci�n emergente cuando el r�o estaba desbordado. El Cathartes burrovianus parec�a ser m�s com�n cuando los niveles de agua estaban bajos. El C. burrovianus y el Rostrhamus sociabilis (caracolero) eran m�s comunes hacia el norte donde los esteros son m�s extensos; el Coragyps atratus result� ser m�s com�n hacia el sur donde los asentamientos humanos prevalecen. El R. sociabilis y el Polyborus plancus estuvieron m�s activos temprano por la ma�ana, mientras que el C. burrovianus y el C. atratus desplegaron mayor actividad al medio d�a. |
