The concept described here is called the “operational sex ratio” (OSR). Whereas the sex ratio is simply a head count of the number of males and females in a population (and, in all but a very few exceptional species, this ratio hovers near 1:1), the operational sex ratio accounts for the fact that not all individuals are actually available for reproducing at any point in time. It is defined as the ratio of reproductively available males to reproductively available females. The OSR can skew in the direction of females, as it does in jacanas, but it is typically skewed toward an excess of available males. The extent of skew is a good metric for the intensity of sexual selection likely to be acting in the population.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 688