Movements of organisms between habitat remnants can affect metapopulation structure, community assembly dynamics, gene flow and conservation strategy. In the tropical landscapes that support the majority of global biodiversity and where forest fragmentation is accelerating, there is particular urgency to understand how dispersal across habitats mediates the demography, distribution and differentiation of organisms. By employing unique dispersal challenge experiments coupled with exhaustive inventories of birds in a Panamanian lacustrine archipelago, we show that the ability to fly even short distances (< 100 m) between habitat fragments varies dramatically and consistently among species of forest birds, and that this variation correlates strongly with species' extinction histories and current distributions across the archipelago. This extreme variation in flight capability indicates that species' persistence in isolated forest remnants will be differentially mediated by their respective dispersal abilities, and that corridors connecting such fragments will be essential for the maintenance of avian diversity in fragmented tropical landscapes.

Greater nest predation in tropical than temperate birds has been hypothesized to be a primary selective force generating latitudinal differences in avian life history traits. Few extensive data sets, however, have been available from tropical forests to compare with data from temperate forests. To increase the amount of empirical information available for addressing issues related to the evolution of life history traits of tropical birds, we measured the nesting success of understory birds in lowland forest of central Panama. We found and monitored the fates of 696 nests of 71 species over two breeding seasons. Daily nest predation rates for the ten species for which we obtained the largest samples ranged from 1.6 to 8.3%, equivalent to a loss of 43 to 92% of nests. These values overlapped extensively the range of daily predation rates experienced by ecologically similar species in North America. Proportion of nests fledging young, estimated with the Mayfield method, was significantly lower in tropical (range: 8 to 57%) than temperate (27 to 60%) species. Nesting success in Panama varied among years, however, being greater in 1996 than 1997. In 1996, nesting success was similar to that of species breeding in forest fragments of midwestern North America. When compared with success of nests in large, contiguous forest tracts of North America, however, tropical avian nesting success was consistently lower by approximately 23%. We conclude that nesting success in central Panama may be poor in most breeding seasons, but also may be punctuated by occasional years of relatively exceptional success, a possibility heretofore unappreciated because of a general paucity of data from the tropics. Furthermore, our results indicate substantial variation in levels of nesting success among species, and almost no variation in clutch size. Such large interspecific variation, as well as potentially large annual variation, in nesting success does not support the hypothesis that uniformly low levels of nesting success select for small tropical clutch sizes.

Summary 1.It has been suggested that immune defences are shaped by life history and ecology, but few general patterns have been described across species. We hypothesized that 'fast' life-history traits (e.g. short development times, large clutch sizes) would be associated with developmentally inexpensive immune defences, minimizing the resource demands of young animals' immune systems during periods of rapid growth. Conversely, 'slow' life histories should be associated with well developed antibody-mediated defences, which are developmentally costly. 2. We therefore predicted that 'fast-living' species would exhibit higher levels of complement proteins, a component of non-specific innate defence, but lower levels of constitutive ('natural') antibodies. Additionally, we predicted that constitutive immune defences in general would be higher in species with ecological characteristics that might increase exposure to pathogens, such as open nests, omnivorous diets, gregariousness, and closed forested habitat. 3. Across 70 Neotropical bird species, we found a strongly positive relationship between incubation period and natural antibody levels in adult birds, suggesting that longer developmental times might allow the production of a more diverse and/or more reactive adaptive immune system. Complement activity was positively, although weakly, correlated with clutch size, providing some support for the hypothesis that faster-living species rely more on innate defences, such as complement. Unexpectedly, solitary species had higher natural antibody titres than species that frequently join flocks. 4. Our results suggest that, despite probably widespread differences in the intensity and diversity of pathogen exposure, species-level variation in constitutive immune defences is understandable within the context of life-history theory.

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