Patterns and causes of understory bird declines in human-disturbed tropical forest landscapes: A case study from Central America

Tropical forest understory birds are declining globally for unknown reasons, indicating an urgent need to understand the causes. We review and synthesize studies investigating causes of these declines focusing on the Sarapiquí region of the Caribbean slope of Costa Rica. We discuss evidence for five potential causes of population declines motivated by current understanding of the effects of fragmentation, disturbance of remnant forests, climate change, and their possible interactions: (1) reduced forest area increases the probability of stochastic extirpation; (2) reduced connectivity among forest patches decreases population rescue opportunities; (3) degradation of preferred microhabitats due to, for example, abundant large mammals, jeopardizes specialized birds' foraging opportunities; (4) high nest predation rates reduce productivity below replacement levels; and (5) changes in macro- and microclimate increase energetic demands and reduce survival. Our review documents how tropical forest loss and degradation likely impact understory birds through a variety of direct, indirect, and interrelated causes spanning multiple temporal and spatial scales and levels of biological organization. We propose that the processes affecting understory birds in the Sarapiquí region may be broadly representative of threats experienced by rainforest understory birds pantropically. Effective conservation will require consideration of such diverse and interacting factors.