Juvenile Mortality Increases with Clutch Size in a Neotropical Bird

The change in avian clutch size with latitude is a celebrated example of geographic variation in a vertebrate life-history trait. Alternative hypotheses for this pattern invoke nest predation, limited food for nestlings, or post-fledging juvenile mortality as selection pressures leading to small clutch size of tropical birds. We manipulated the clutch size of Spotted Antbirds (Hylophylax naevioides) in central Panama to test these hypotheses. We observed that rates of nest predation were not influenced by parental activity at nests and parents could successfully feed nestlings in enlarged broods. Although larger broods produced the most fledged juveniles, these individuals were less likely to survive to dispersal than were juveniles that fledged from smaller broods. Consequently, nest productivity did not vary with clutch size. Post-fledging mortality was not related to nestling mass two to three days prior to fledging. Rather, differences in the allocation of parental investment per juvenile among differently sized broods appeared to influence juvenile survival probability. These results identify parental care during the post-fledging period as a potential key factor influencing the evolution of small clutch size in tropical birds.