SURVIVAL RATES OF A NEOTROPICAL PARROT: IMPLICATIONS FOR LATITUDINAL COMPARISONS OF AVIAN DEMOGRAPHY

Abstract

Latitudinal variation in avian demography played an important early role in the development of life history theory, especially in the idea of a cost of reproduction. Recent attempts to determine the survivorship of tropical birds with mark-recapture sta- tistics have proved controversial. Here, we use a small neotropical bird, the Green-rumped Parrotlet (Forpus passerinus), as a model system for investigating sources of heterogeneity that might bias interspecific comparisons. Mark-resighting data were collected on 1334 adult parrotlets over a decade. We expected adult survival to be low because this parrot lays a large clutch (mean 5 7 eggs), is a cavity nester, and breeds in a highly seasonal environment. A two-age-class term in local survival was nonsignificant, indicating that an age or transience effect was unimportant. Local survival of males did not vary annually, but 19.3% of the yearly variation in female survival was explained by rates of nest loss during stages when females were incubating or brooding young. The overall local survival rate of parrotlets ( f5 0.565) was identical to temperate hole-nesting species of the same body size but was lower than that of tropical birds that lay smaller clutches. However, we also detected considerable heterogeneity in parrotlet survival. Females and males that were sighted but did not breed comprised a mean 23.5% and 52.9% of our population, respec- tively. Using multistate models, we found that breeders had significantly higher probabilities of local survival ( f5 0.678 vs. 0.486), of retaining their status as breeders ( c5 0.719 vs. 0.279), and of detection ( p 5 0.997 vs. 0.375) than did nonbreeders. Overall, males and females had comparable local survival rates (breeders f5 0.698 vs. 0.658, nonbreeders f5 0.536 vs. 0.436). Our estimates of local survival could be affected by breeding dispersal, but site fidelity of parrotlets was strong: 95% of adults moved ,500 m in consecutive years. A literature review for tropical birds showed that mark-resighting studies usually report return rates based on resightings of breeding or territorial adults, whereas mist net studies rely on recaptures and pool birds of different age and social status in their calcu- lations of local survival. Future studies should attempt to compare subsets of avian pop- ulations that are similar in demography. Because rates of site fidelity and social system may differ among species, these factors must also be considered in interspecific comparisons of avian life histories.