Heavily male-biased adult sex ratio is associated with lower female survival in the federally endangered Anas wyvilliana (Koloa Maoli or Hawaiian Duck) on north Kaua‘i, Hawai‘i, USA

Abstract Anas wyvilliana (Koloa Maoli or Hawaiian Duck) is a federally endangered, non-migratory dabbling duck that experienced substantial population declines and range contraction during the 20th century. A critical obstacle in assessing threats and developing effective conservation actions involves the lack of information on population abundance, structure, and key demographic rates. We used a 5-yr capture–mark–recapture (CMR) dataset and robust design CMR models to estimate abundance, sex ratio, and survival of adult and first-year A. wyvilliana at Hanalei National Wildlife Refuge (NWR), a core wetland site, on north Kaua‘i, Hawai‘i, USA. During 2010–2015, 792 A. wyvilliana (nfemale = 150, nmale = 642) were captured 1,236 times. Model-averaged, sex-specific abundance estimates revealed a heavily distorted, male-biased sex ratio (3:1) that was relatively consistent across seasons and years. Annual apparent survival was lower for females (0.43) compared to males (0.69). We found strong evidence for temporary emigration, though the particular form (i.e., random vs. Markovian) was not clearly distinguishable. The top-ranked model indicated temporary emigration followed a random process but varied seasonally, with individuals having a higher probability of being unavailable for capture during summer (0.76) compared to winter (0.64). Seasonal abundance of adult and first-year A. wyvilliana available for capture at Hanalei NWR averaged 407 individuals (range: 252–625). Assuming random temporary emigration, the total number of A. wyvilliana available and temporarily unavailable for capture on the refuge averaged 1,283 individuals (range: 929–1,822). Our study provides robust estimates for several demographic parameters that establish a quantitative foundation for future population modeling and targeted, evidence-based conservation decisions for the recovery of A. wyvilliana.