A model to predict breeding-season productivity for multibrooded songbirds

Breeding-season productivity (the per capita number of offspring surviving to the end of the breeding season) is seldom estimated for multibrooded son gbirds because of cost and logistical constraints. However, this parameter is critical far predictions of population growth rates and comparisons of s easonal productivity across geographic or temporal scales. We constructed a dynamic, stochastic, individual-based model of breeding-season productivit y using demographic data from Wood Thrushes (Hylocichla mustelina) in centr al Georgia from 1993 to 1996. The model predicts breeding-season productivi ty as a function of adult survival, juvenile survival, nesting success, sea son length, renesting interval, and juvenile-care intervals. The model pred icted that seasonal fecundity (number of fledglings produced) was 3.04, but only 2.04 juveniles per female survived to the end of the breeding season. Sensitivity analyses showed that differences in renesting interval, nestin g success, fledglings per successful nest, and adult and juvenile survival caused variation in breeding-season productivity. Contrary to commonly held notions, season length and fledgling-care interval length did not cause va riation in breeding-season productivity. This modeling exercise emphasizes the need for demographic data for songbird species, and we encourage biolog ists to use similar models to evaluate productivity in songbird populations .