Chick mortality leads to male-biased sex ratios in endangered Great Lakes Piping Plovers

Few investigators have studied the offspring sex ratios of monomorphic shorebirds because visually determining the sex of juveniles is not possible. We investigated the ontogeny of an observed male-biased adult sex ratio in the federally endangered Great Lakes population of Piping Plovers (Charadrius melodus). We determined sex ratios at hatching, banding (x- = 9.0 d old), and fledging (23 d old) to determine if the bias arises during the pre-fledging period and, if so, at what stage. For three consecutive years (2012-2014), we used a molecular technique to determine the sex of 307 chicks and followed individuals to a stage where survival to fledging could be inferred. Within fully-sexed broods at hatching, the average proportions of male chicks (2012-2014) were 0.47, 0.58, and 0.54, respectively. At banding, the sex ratio remained unbiased in 2012 (0.51), but was male-biased in 2013 (0.59) and 2014 (0.57). Overall, the sex ratio did not differ significantly from parity at fledging in 2012, but did differ during 2013 (P = 0.01) and 2014 (P = 0.03). Using logistic regression models fit using Bayesian inference, we found strong support for a sex effect on chick survival to fledging age, with higher male than female survival (μ_male = 0.83 [95% credible interval: 0.75-0.90]; μ_female = 0.71 [0.61-0.80]). These results suggest that the male-biased adult sex ratio in Piping Plovers arises, in part, due to differential survival during the pre-fledging period. This difference did not result from female chicks hatching later in the season or weighing less at banding than male chicks, factors that could potentially affect the likelihood of survival. Future investigations into possible behavioral- or weather-related influences on sex-specific survival are needed. Our results have important implications for (1) identifying management efforts needed to increase recruitment given female-biased chick mortality, and (2) conducting population viability analyses, which frequently assume an unbiased fledgling sex ratio.