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In its spread west across North America in 2002, West Nile virus (WNV) reached a population of marked American Crows (Corvus brachyrhynchos) in Stillwater, Oklahoma, in late summer. Within two months, 46 of 120 individuals were missing or known to be dead, 39 of which (33% of the population) are estimated to have died for WNV-related reasons. In 2003, 56 of 78 marked crows disappeared or were found dead between June and November. Five of the 28 juvenile losses were possibly unrelated to WNV, thus we estimate that 65% of our population died because of this pathogen in 2003. The total loss of 72% of population members, including 82% of juveniles, in a single year of WNV exposure raises concern for precipitous declines in American Crow populations in coming years. El Virus del Nilo Occidental Devasta una Población de Corvus brachyrhynchos Resumen. En su diseminación hacia el oeste de América del Norte durante 2002, el Virus del Nilo Occidental alcanzó a fines del verano una población marcada de Corvus brachyrhynchos en Stillwater, Oklahoma. En menos de dos meses, 46 de los 120 individuos registrados desaparecieron o murieron, 39 de los cuales (33% de la población) estimamos que murieron por causas relacionadas con el virus. En 2003, 56 de los 78 cuervos marcados desaparecieron o fueron encontrados muertos entre junio y noviembre. Cinco de las 28 pérdidas de juveniles posiblemente no estuvieron relacionadas con el virus, por lo que estimamos que el 65% de nuestra población murió a causa de este patógeno en 2003. La pérdida total del 72% de los miembros de la población, incluyendo el 82% de los juveniles, en un solo año de exposición al virus plantea preocupaciones en cuanto a la posibilidad de una disminución precipitada de las poblaciones de C. brachyrhynchos en los próximos años.

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I measured annual reproductive success for a resident population of cooperatively breeding Western American Crows (Corvus brachyrhynchos hesperis) over 6 years, and examined the relationship between it and several ecological and social variables. Most nests (57%) failed to fledge any young, due largely to predation. Success in fledging young was associated with three interrelated variables: (1) the presence of helpers, (2) early nesting, and (3) consistent differences between pairs. Assisted pairs began incubation earlier, some pairs consistently nested early, and the proportion of years that pairs were successful was related to the proportion of years in which they had help. Although both help and incubation date were related to nesting success independent of each other, the effect of help became nonsignificant after controlling for differences between pairs. The slight increase in fledging success possibly attributable to helpers may have been the result of consistently successful pairs succeeding, with help, in otherwise poor years. Postfledging survival was related in part to the size of individuals; larger nestlings tended to have a greater chance of fledging, and once fledged, to have a greater chance of surviving the following 2-week period. Larger nestlings survived to one year of age significantly more often than smaller nestlings.

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