Importance of the post-fledging period in population dynamics of tropical countryside birds

We need demographic data from all stages of the avian life cycle to understand and predict population dynamics of tropical bird species, many of which are little known and are threatened with extinction. The post-fledging period, between leaving the nest and juvenile dispersal, is little studied, but is likely to be critical in shaping bird populations in tropical ecosystems. Between 1999-2006, we conducted mark-recapture, radio tracking, and nest monitoring to study the population demographics of three tropical forest bird species representing a range of forest requirements. We compared their population trends, breeding success, adult and juvenile survivorship in various types of forest remnants versus deforested habitats around Las Cruces Biological Station, Costa Rica. Mark-recapture estimates of adult survivorship did not explain the differences in long-term population trends. For two species, nest productivity did not differ, but for the most forest-dependent species, Turdus assimilis, productivity in forest remnants was a third of that in deforested countryside, suggesting an ecological trap. However, in deforested habitats, daily post-fledging mortality was nine times higher for T. assimilis and for all species combined. Overall mortality rate of 156 radio tracked adults was 3.2% and 0.08%/day/bird, but for 18 tracked fledglings, this rate was 50% and 5.5%/day/bird. While 86% of fledglings in coffee plantations were predated, this was the case for 27% of fledglings in forest fragments. These results emphasize the conservation importance of combining various methods to study tropical birds´ post-fledging period, which is a critical but understudied stage of avian life history.