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
Citation
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