Renesting Determines Seasonal Fecundity in Songbirds: What do we Know? What Should we Assume?

Because of the difficulty of following female songbirds through an entire breeding season, field ornithologists are seldom able to directly measure seasonal fecundity (defined as number of offspring produced per female during an entire breeding season). Instead, it is more commonly inferred from some measure of nest-productivity data (e.g. average number of offspring fledged per nesting attempt) using algorithms that make assumptions about the propensity of females to renest after a nest failure or after successfully fledging a brood. Recent analyses have often assumed set maximum numbers of nesting attempts and successful broods, and that all females breed up to those maxima. However, whereas data from songbirds intensively followed for an entire breeding season show that they are capable of up to 4–8 nesting attempts, many authors, in estimating seasonal fecundity, assume a maximum of only 1–4 nesting attempts. We applied a model to a Prairie Warbler (Dendroica discolor) data set (Nolan 1978) that allowed direct comparisons of (1) seasonal-fecundity estimates obtained assuming fixed maximum numbers of renestings and broods with (2) estimates obtained assuming that numbers of renesting attempts and successful nests are constrained only indirectly by length of breeding season. Although results under the latter assumption are concordant with Nolan's (1978) direct empirical measure of Prairie Warbler seasonal fecundity, estimates under assumptions of fixed maxima of renestings or broods are in serious error for many parameter choices. As such, our analyses disclose that essentially all estimates of seasonal fecundity in the literature derived by assuming a limited maximum number of nesting attempts or of successful broods are biased. Most commonly, when nest mortality is high, seasonal fecundity is underestimated; in some cases where nest mortality is low, seasonal fecundity is overestimated. We recommend that researchers estimating seasonal fecundity from nest-productivity data use a model that explicitly sets breeding-season length and thereby only indirectly constrains the possible number of nesting attempts and successful broods. La Nidificación Repetida Determina la Fecundidad Estacional en las Aves Canoras: ¿Qué Sabemos? ¿Qué Deberíamos Suponer?

Winter Site Fidelity of Prairie Warblers in the Dominican Republic

Wintering Prairie Warblers (Dendroica discolor) were studied in pine forest, desert thorn scrub, and desert wash habitats of the Sierra de Bahoruco, Dominican Republic from October–April 1996–1998 and January–February 1999. We used mist netting, color banding, and extensive resighting of color-banded birds to quantify habitat-specific demographies, site fidelity, and fitness indices. Males predominated early in the season in pine habitat while females predominated in the desert. Sex ratios were more equal through the drier months of late winter although females continued to be more frequent in the desert. Adult birds predominated throughout all habitats and time periods. Nonterritorial wanderers were far more common in desert habitats than in pine forest, and females predominated among these wanderers. Site persistence was moderately high, but variable between habitats and time periods, and was significantly lower in the drier months in thorn scrub than it was in the desert wash or pine forest. Annual return rates were generally high and did not vary significantly among years. Birds showed significant declines in pectoral-muscle-mass scores in both of the desert habitats but not in the pine forest. Data presented here emphasize the importance of dry-season events and habitat heterogeneity in the winter ecology of migratory birds. Fidelidad al Territorio Invernal de Dendroica discolor en la República Dominicana Resumen. En los meses de octubre a abril de 1996 a 1998 y enero y febrero de 1999, investigamos a Dendroica discolor en el bosque de pino, en los matorrales del desierto y en el arroyo seco de desierto en la Sierra de Bahoruco, República Dominicana. Para llevar a cabo esto, se utilizaron redes ornitológicas y anillos colorados. Se realizó una revisión extensiva de las aves anilladas para cuantificar la demografía en los tres hábitats, la fidelidad al territorio invernal, y los índices de aptitud (“fitness”). Al comenzar el invierno, los machos predominaron en el bosque de pino mientras que las hembras lo hicieron en el desierto. La proporción entre los machos y las hembras mantuvo una mayor igualidad entre los sexos en los meses más secos del invierno, aunque hay que considerar que las hembras predominaban en el desierto. Se observó que las aves adultas predominaban en los tres hábitats durante todo el período investigado. Las aves sin territorio eran más comunes en el desierto y eran mayormente hembras. La fidelidad al lugar fue moderadamente alta pero variable entre los hábitats y los períodos y fue significativamente más baja en los meses secos en los matorrales del desierto que en los arroyos secos y los bosques de pino. La tasa anual de retorno de esta especie fue generalmente alta y no varió significativamente entre años. La masa de los músculos pectorales bajó significativamente en las aves que se encontraban en los matorrales del desierto y en el arroyo seco pero no en los bosques de pino. Los datos mencionados aquí acentúan la importancia de eventos que ocurren en la época seca del período invernal y de la heterogenidad del hábitat en la ecología invernal de las aves migratorias.