Patterns of daily temporal variation in detectability of forest birds in Bolivia
Published quantitative descriptions of daily variation in detectability or vocal activity of tropical forest birds are few, but such information is critical to designing effective survey protocols. I examined daily temporal variation in detectability of a bird community during the dry season in a lowland Bolivian forestry concession. Whether analyzed by family, foraging guild or by sensitivity to habitat disturbance, the detectabibty of birds varied considerably across five one-hour census periods. The greatest number of birds (similar to 70%) was detected in surveys during the first two hours following sunrise. Pre-sunrise and pre- and post-dusk surveys were important for detecting several families, including Tinamidae, Cracidae, Odontophoridae, Falconidae, Strigidae, Caprimulgidae, Momotidae, Furnariidae, and Dendrocolaptidae. Among ecological guilds identified by diet and forest stratum, most diet groups contained at least one guild that was proportionately more detectable during times not typically surveyed in other studies. Birds considered highly sensitive to habitat fragmentation were detected proportionately more frequently in pre-sunrise surveys than birds with medium or low sensitivity. Nocturnal and pre-dusk surveys can contribute considerably to estimates of bird density and relative abundance, and should at least be explored in all tropical forest bird community studies.