Nest site vegetation structure influences nest predators and nesting success of understory birds in a dry evergreen forest in northeastern Thailand

Nest predation is typically the main cause of nest failure in forest understory birds; thus, identification of primary nest predators is key to understanding nest predation patterns. Furthermore, the responses of predators are likely affected by vegetation structure, but predator responses to micro-scale habitat characteristics are largely unknown, especially in tropical forests. We used a long-term study with one of the largest datasets of its kind to investigate the extent to which micro-habitat structure (5-m radius surrounding a nest) can predict the likelihood of predation and by which predator. In a secondary evergreen forest in northeastern Thailand, 2013-2021, we found 1,016 active nests of 13 species and 24-hour video-monitored 500 of them. We recorded 336 predation events from 16 nest predator species. From this and previous studies at our site, we identified the top 4 predator species/species groups accounting for similar to 83% of predation events: Macaca leonina (northern pig-tailed macaque, 36% of predation events), cat snakes (Boiga cyanea [green cat snake] and B. siamensis [gray cat snake]; 20%), Lycodon cf. davisonii (Blandford's bridle snake, 18%), and accipiters (Accipiter trivirgatus [Crested Goshawk] and A. badius [Shikra]; 9%). These 4 predator species differed in their responses to vegetation structure likely reflecting differences in foraging behaviors. Macaque and accipiters, both diurnal and visually oriented, tended to depredate more visible/open nests, but macaque depredated nests surrounded by more trees and short woody stems (<3 m tall) compared to raptors. For snakes, both nocturnal, cat snakes depredated nests with higher numbers of both short woody stems and woody climbers, whereas bridle snake depredated nests with more trees and fewer climbers. As noted previously, nest predator identity is critical to understanding habitat-predation patterns. Our data suggest that nest site vegetation characteristics influence the likelihood of a given species of predator locating a nest and that even small changes in vegetation structure could significantly alter predation patterns.