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

Abstract

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 -February  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 . individuals per km  ; 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 . individuals per km  and a population of  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.

Terrestrial camera traps: essential tool for the detection and future monitoring of the Critically Endangered Sira curassow Pauxi koepckeae

“…Reliable identification of individuals could ultimately result in the determination of density estimates. The use of HD cameras could also facilitate the identification of an individual's sex from the size and shape of the blue casque (as for the helmeted curassow; NavedaRodríguez & Strahl 2006), which would result in more accurate population size estimates (see Alves et al 2015). Should identification of unique individuals prove impossible, researchers looking to perform robust population monitoring should adopt an occupancy modelling approach (such as that employed here), where the observation of marked individuals is not required (O'Brien & Kinnaird 2008).…”

“…Camera traps have been used previously to detail the population size, population trends, behaviour and ecology of a variety of different bird species (O'Brien & Kinnaird 2008) and have already been successfully utilised to study other endangered members of the cracid family (e.g. Alves et al 2015). As such, camera-trapping methodologies have the potential to elucidate novel and otherwise hard-to-obtain information in the case of the Critically Endangered Sira curassow.…”

Tamanho populacional, razão sexual e uso do habitat por uma população de mutum-do-sudeste (<i>Crax blumenbachii</i> Spix, 1825) reintroduzida em Ipaba, MG

“…A maior população remanescente do mutum-do-sudeste ocorre nas matas de tabuleiro da Reserva Natural da Vale, ES, e foi recentemente estimada em 325 indivíduos e densidade 1,6 indivíduos/km² (ALVES et al, 2015). Evidências indicam que Crax blumenbachii é monogâmico na natureza, de forma de que seria esperada uma razão sexual 1:1.…”

“…Trata-se de um valor muito superior ao encontrado para populações da mesma espécie (ALVES et al, 2015) e da maioria das espécies do gênero (SETINA, 2009). A priori, o crescimento denso-dependente pode ser esperado para qualquer população real que persista por tempo suficiente para atingir a capacidade de suporte do ambiente (SINCLAIR, 1996).…”

“…. Em busca de fatores que justifiquem essa discrepância, a primeira diferença óbvia que surge entre as populações da FM e da Vale é que a segunda é a maior população remanescente de mutum-do-sudeste conhecida, estimada em 325 indivíduos(ALVES et al, 2015). Portanto, é de se considerar que o desvio encontrado na população da FM esteja associado, de alguma forma, ao seu tamanho, seja por mera deriva genética ou por processos do efeito Allee que afetem diferencialmente machos e fêmeas.…”