Examining skewed sex ratio in the mountain plover (Charadrius montanus) population

Skewed sex ratios can have negative implications for population growth or persistence if not congruous for a species system. A skewed tertiary sex ratio (2.3 males per female) has been detected in the breeding population of a grassland shorebird experiencing population declines, the mountain plover (Charadrius montanus). To evaluate the ontogeny of the observed male skew this study examined the early life stages, from laying to fledging, of mountain plover young during their breeding season from 2010 – 2012 in eastern Colorado. The life stages between laying and fledging that allows for differentiation between production and survival of males and females. Early stages encompass the primary (eggs produced) ratio which allows for evaluation of applied sex allocation theory, the secondary sex ratio (successfully hatched chicks) which determines if a sex specific mortality is occurring pre-hatching, and the chick stage which determines if a sex specific mortality is occurring post-hatching. Mountain plovers are a sexually monomorphic species at all stages therefore DNA samples were used to determine the sex of individuals. The primary sex ratio was 1.01 (± 0.01) males per females. The secondary sex ratio consisted of 1.1 (± 0.02) males per female. Neither the primary nor secondary sex ratio was able to account for the magnitude of the skew observed later in this species adult population. Radio telemetry was used to evaluate the next stage of life, survival of male and female chicks from hatching until fledging. Using a multi-state mark recapture analysis, the top model for predicting chick survival rates estimates differed between males (0.55 ± 0.13) and females (0.47 ± 0.15). The estimated survival difference between the sexes during the chick stage can drive a population with equal survival rates at all other life stages to a ~2.1 :1 adult sex ratio. Results from this study suggest survival difference between males and females at the chick stage is possibly contributing to a male skewed population