The understory insectivore guild is disproportionately affected by deforestation, and knowing the underlying mechanisms is critical to effective conservation. I investigated demographic, ecological, and behavioral responses of Chestnut-backed Antbird (Myrmeciza exsul) populations (a persistent understory insectivore) to a fragmented Costa Rican rainforest landscape where many ecologically similar species have declined. I estimated demographic rates to parameterize population models in three habitats differentially affected by forest fragmentation: contiguous, peninsular, and fragment. Models indicated that M. exsul are declining in the peninsula (λ=0.83), but increasing in fragments (λ=1.41). Sensitivity analyses suggested that population growth was most sensitive to adult survival and nesting success, suggesting these two variables as potentially important explanatory demographic parameters in this landscape. I studied nest predation using digital video and quantified breeding success and population density in each site. Nest predation rate was so high in the peninsula that few nests fledged any young, much lower in the fragments, and intermediate in the contiguous forest, inversely tracking M. exsul population density and corroborating population growth rate findings. Using 22,000 hours of active nest video recordings, one primary predator emerged, the bird-eating snake (Pseustes poecilonotus), responsible for 80% of nest attacks. Pseustes’ prevalence in the peninsula where predation rates were highest implies possible predation-limitation. Populations both declining and growing locally provided unique insights into the mechanisms of change in a deforestation-impacted landscape, but represent only some fragmentation consequences. Therefore, I reviewed regional studies to assess other potential contributions to understory insectivorous bird decline in the Sarapiquí. Empirical studies supported effects of habitat area loss, dispersal limitation, reduced microhabitat availability, and low physiological tolerances to changing climates. / acase@tulane.edu