Unequal sex ratios and their consequences in Herring Gulls (Larus argentatus)

Summary We observed the sex ratios and age composition of courting, nesting, and nonbreeding birds (in clubs) in four breeding colonies of Herring Gulls in Maine and New York. Breeding pairs, with eggs or chicks, often contained an immature male, but rarely contained an immature female. Similarly, courting pairs contained immature males, but rarely immature females. A higher proportion of courting pairs contained immatures compared with pairs with eggs or chicks. In all four colonies there was an excess of females in the clubs. A simulated colony which assumed unequal survival for males and females had an age ratio for unpaired birds that was similar to the observed values. We conclude that in Herring Gulls, mate competition exists and stems in part from an unequal tertiary sex ratio.