Fragmented tropical forest landscapes are becoming more abundant, and loss of species following fragmentation are often predictable. Larger animals, tend to disappear first from fragments due to the bushmeat trade. However, another vulnerable group includes understory, insectivorous birds, and ant-following birds. Nest predation is one mechanism that may limit bird populations and has long been suspected as a factor threatening bird populations in temperate and tropical forest fragments. A potential influence on nest predation that remains understudied in the tropics is density dependence. Dense territories can increase predators’ ability to find the closely-spaced nests. Yet bird density and nest predation are not always positively correlated, and multiple life-history traits and contexts are relevant.
In a forthcoming paper in Biological Conservation Visco and Sherry (2015) compared nest predation rates, bird density, and predator identities in three habitats of lowland Caribbean Costa Rica: two fragments, a peninsular reserve (La Selva Biological Station), and unfragmented rainforest. Their results suggest an inversely density-dependent nest predation pattern: In fragments, chestnut-backed antbirds reached their highest density and—contrary to predictions—experienced their lowest nest predation rates; La Selva, on the other hand, experienced the lowest density and highest predation rate. Because nest predation decreased with fragmentation, it appears not to explain declines of understory insectivores from forest fragments generally.
Nest survival models indicated that habitat best-described nest predation likelihood. Video surveillance of nests documented the bird-eating snake (Pseustes poecilonotus) causing 80% of nest loss (37 of 46 nests) and a larger variety of predators in fragments; thus, landscape factors influenced an understory bird’s nest predation. Given the large effect on our focal species, Pseustes likely affects other understory nesters, a topic warranting further study. Tropical reserve conservation plans should consider potential impacts of specialized nest predators on vulnerable understory birds
Visco, D. M., & Sherry, T. W. (2015 in press). Increased abundance, but reduced nest predation in the chestnut-backed antbird in Costa Rican rainforest fragments: surprising impacts of a pervasive snake species. Biological Conservation.pseutes