TEMPORAL PATTERNS OF FITNESS AND SURVIVAL IN THE WOOD THRUSH

Abstract

Data from a 22-yr study of individually marked Wood Thrushes (Hylocichla mustelina) in a Delaware forest fragment were analyzed to determine temporal and demographic patterns of nesting success, recruitment of local young into the population (fitness), and adult return rates (survival). Clutch size, number of eggs incubated, and number of eggs hatched were greater in early nests than in late nests. Although no difference existed in the average number of fledglings produced from early and late nests, fledglings from early nests were more likely to be recruited as breeders in the local population. Pairs of yearling (SY) birds produced equal numbers of recruits per nest from early and late nests, whereas pairs of older (≥2-yr-old, ASY) birds produced more recruits from early nests. The probability of fledging at least one young tended to be greater for late nests, and the percentage of eggs that eventually fledged was also greater for late nests. All nests were most likely to fail in the laying stage than during any other nest stage. Early nests failed in the laying stage at a greater rate than late nests. The number of Wood Thrushes that hatched was the most important demographic, reproductive, or date variable measured that positively influenced seasonal fledging success. Year of breeding also positively influenced success. ASY birds returned to breed at higher rates than did SY birds. SY males returned at a higher rate than did SY females. There was no difference in return rates between ASY males and ASY females. Producing at least one Wood Thrush fledgling positively influenced the likelihood of return for SY females. Neither annual success nor any other demographic, reproductive, or date variable that we measured influenced the return probabilities of other age or sex classes of adult birds. Based on these temporal patterns of fitness and survival, we propose that life history strategies among Wood Thrushes vary as a function of late-season reproductive effort and winter-related factors that affect survival.