A model to predict breeding-season productivity for multibrooded songbirds

A3STRACT.-Breeding-season productivity (the per capita number of offspring surviving to the end of the breeding season) is seldom estimated for multibrooded songbirds because of cost and logistical constraints. However, this parameter is critical for predictions of population growth rates and comparisons of seasonal productivity across geographic or temporal scales. We constructed a dynamic, stochastic, individual-based model of breeding-season productivity using demographic data from Wood Thrushes (Hylocichla mustelina) in central Georgia from 1993 to 1996. The model predicts breeding-season productivity as a function of adult survival, juvenile survival, nesting success, season length, renesting interval, and juvenile-care intervals. The model predicted 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, nesting 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 variation in breeding-season productivity. This modeling exercise emphasizes the need for demographic data for songbird species, and we encourage biologists to use similar models to evaluate productivity in songbird populations.

Received 1 July 1999, accepted 8 February 1999.

BREEDING-SEASON PRODUCTIVITY, which we define as the per capita number of offspring surviving to the end of the breeding season, is an important life-history parameter that often is required for models of population growth (Pulliam 1996). However, most analyses of population viability rely on estimates of nesting success or seasonal fecundity as surrogate parameters for breeding-season productivity (e.g. Ricklefs and Bloom 1977, Holmes et al. 1992, Roth and Johnson 1993, Pease and Grzybowski 1995). Population modeling exercises have emphasized the need for demographic data for songbird species that might be used to predict breeding-season productivity (Thompson 1993, Donovan 1995a, Powell et al. 2000).

For songbirds that nest multiple times during a season, direct estimates of breeding-season productivity require estimates of season length, renesting-interval length, nesting success, female survival, and juvenile survival, which are parameters that require constant monitoring of females and young during the breeding season. Pease and Grzybowski (1995) developed a model that used nesting success and lengths of the nesting season and renesting interval to estimate seasonal fecundity for a multibrooded species. However, their model probably overestimates seasonal fecundity because it does not incorporate female mortality during the breeding season. Pease and Grzybowski's (1995) model also does not incorporate juvenile mortality, which is necessary to predict breeding-season productivity.

For animal species with short bursts of reproductive effort, breeding-season productivity could be calculated as the product of (1) nesting success (probability of nest surviving until nestlings fledge), (2) mean number of offspring per successful nest, and (3) mean number of nesting attempts per female per year. However, multibrooded species produce juveniles over a period of several months. Juvenile survival during the breeding season is not 100% (Anders et al. 1997, Powell et al. 2000). Also, juveniles produced early in the breeding season may have a lower chance of surviving to the end of the breeding season than juveniles produced late in the breeding season (Krementz et al. 1989). Some viability analyses for multibrooded songbirds (Donovan et al. 1995b, Anders et al. 1997, Trine 1998) impose a "general" juvenile mortality rate on the numbers of juveniles produced during the year in an attempt to estimate productivity. This approach seems unwise at worst and imprecise at best, in light of the previous discussion.

Our objective was to construct a simulation model that would predict breeding-season productivity for a multibrooded songbird species. Beyond predicting a critical parameter value that we could not measure in the field, the model allows manipulations of demographic parameters, season length, and renesting-interval length that are not possible in empirical studies.

METHODS

We studied multibrooded Wood Thrushes (Hylocichla mustelina) at the Piedmont National Wildlife Refuge (PNWR) in central Georgia from 1993 to 1996. Radio-marked females began nesting in late April, and the last radio-marked fledglings left the nest in early August. Therefore, the 45-day battery life of our transmitters was shorter than the breeding season.

We estimated daily survival of females and juveniles from radio-marked birds using program SURVIV (White 1983), which uses likelihood-ratio tests (LRT) and Akaike Information Criteria (AIC) values to test between time-specific and constant-survival models. Survival estimates were obtained from 63 female and 38 juvenile Wood Thrushes (Powell et al. 1998). We used aerial telemetry to relocate birds that dispersed beyond the range of our hand-held telemetry equipment.