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

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.