Experimental Evidence That Female-Female Pairs in Gulls Result from a Shortage of Breeding Males

We tested the hypothesis that female-female pairings in Ring-billed (Larus delawarensis) and California gulls (L. californicus) result from a shortage of males in the breeding population. This hypothesis was tested by removing males from one Ring-billed and four California gull colonies early in the breeding season. The frequency of 4-6 egg clutches, which we used as an index of female-female pairings, was significantly higher in these colonies than in nearby control colonies, thus supporting our hypothesis. Our results indicate that female-female pairings allow females the chance to breed when they are unable to obtain a male partner.