Population size assessment of the Endangered red-billed curassow Crax blumenbachii: accounting for variation in detectability and sex-biased estimates

The Endangered red-billed curassow Crax blumenbachii is endemic to the lowland Atlantic Forest of Brazil and is extinct across most of its range as a result of habitat loss and hunting pressure. Global population estimates are unreliable as the species has never been surveyed systematically. During March 2012–February 2013 we used line transects to estimate the density and size of one of the most important populations, in the Vale Nature Reserve. Using Distance we estimated a density of 1.3 individuals per km2; results stratified by sex indicated a male-skewed sex ratio. Data collected from motion-activated cameras confirmed that the population was male skewed but to a lesser degree than suggested by transect data. Sex-specific behavioural responses to the presence of an observer probably contributed to the difference between camera and transect data. Using the camera-derived sex ratios we corrected transect estimates to 1.6 individuals per km2 and a population of 325 individuals. Our systematic survey approach has revealed that Vale Nature Reserve has a larger population than previous estimates of the global population. We show that behavioural differences between the sexes in cryptic species can mislead population estimates, and we highlight the importance of motion-activated cameras as a tool for studying cryptic forest species.This study was funded by Conservation des Espèces et des Population Animales, the Mohamed bin Zayed Species Conservation Fund, Idea Wild and Vale Nature Reserve. FA and LFS were supported by São Paulo Research Foundation (FAPESP) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)