Banding data reveal bias in age-class sampling of songbirds during spring migration

Population age structure and vital statistics are important for understanding songbird demography and for developing conservation strategies. Field-based estimates of survival rates based on mark–recapture methods are conservative because they are constrained by problems associated with detection probabilities and emigration. However, data collected at bird-banding stations during spring and fall migration can potentially provide useful demographic information. I used banding data collected over a 6-yr period (2005–2010) at Long Point Bird Observatory on the north shore of Lake Erie in Ontario, Canada, and Powdermill Avian Research Center in southeastern Pennsylvania, U.S.A., to determine if banding records could be used to estimate vital statistics for several species of songbirds. As reported in previous studies, I found the proportion of juveniles captured during fall migration to be unrealistically high to be representative of true proportions, especially at Long Point. The proportion of juvenile songbirds captured remained implausibly high during spring migration, with related estimates of longevity and generation time implausibly low and of fecundity implausibly high. Based on apparent adult survival estimates from the literature that suggest an average age structure for songbirds of >55% adults and 55% en adultos y < 45% en juveniles, encontré que la tasa de captura de juveniles durante la migración primaveral duplicaba al menos la de los adultos. Un paso más lento de los juveniles durante la migración primaveral, es responsable en gran medida de este sesgo. Dado el caso de que no se puede asumir que los datos representan un muestreo sin sesgo, con respecto a la estructura de edad de la población, mis resultados indican que los datos tomados de aves en estaciones de anillamiento, durante la migración, no son apropiados para ser utilizados en estudios demográficos.