Genetic mark–recapture analysis reveals large annual variation in pre‐breeding sex ratio of greater sage‐grouse

Abstract

Sex ratio, and the extent to which it varies over time, is an important factor in the demography, management, and conservation of wildlife populations. Greater sage‐grouse Centrocercus urophasianus populations in western North America are monitored using counts of males at leks in spring. Population estimates derived from lek‐count data typically assume a constant, female‐biased sex ratio, yet few rigorous, empirically derived estimates of sex ratio are available to test that assumption. We estimated pre‐breeding sex ratio of greater sage‐grouse in a peripheral, geographically isolated population in northwestern Colorado during two consecutive winters using closed‐population, robust‐design, multi‐state, genetic mark–recapture models in program MARK. Sex ratio varied markedly between years, with estimates of 3.29 (95% CI: 2.36–4.59) females per male in winter 2012–2013 and 1.54 (95% CI: 1.22–1.95) females per male in winter 2013–2014. Rather than assuming a constant sex ratio, biologists should consider the potential for large annual variation in sex ratio of greater sage‐grouse populations when estimating population size or trend from male lek‐count data.