Age-Assortative Mating Related to Reproductive Success in Black-Crowned Night Herons

Abstract

-Age-assortative mating occurs frequently in birds, and may be produced by active selection for older and more successful mates, but also simply by age-related differences in breeding time, or by mate fidelity, without age-related selection. We describe the occurrence of age-assortative breeding in the Black-crowned Night Heron (Nycticorax nycticorax) near Wuxi, China. Throughout the whole breeding season, 59% of the breeders were adults i.e., birds older than two years, and 41% were younger birds injuvenile plumage i.e., birds in their second year. Most of 206 pairs were strictly assorted by age, and only 2.4% had one adult and one juvenile. A part of this assortment was due to age-related differences in the breeding period, but even when this seasonality is accounted for, the observed frequency of mixed pairs is much lower than expected. Mate fidelity is not likely to account for the observed assortment. We conclude that the strict age assortment we observed in Black-crowned Night Herons is to be ascribed to active mate selection by age. Reproductive success was higher for pairs of adults than for those in juvenile plumage, while no difference was found for egg size, chick condition or asymmetry. Pairing between adults may therefore be adaptive because it enhances reproductive success. The high frequency of Black-crowned Night Herons breeding in juvenile plumage in China (41%) contrasts sharply with the situation in other parts of the species range, where birds injuvenile plumage rarely breed, are even seldom seen near colonies, and are believed to remain in their wintering areas. Received 2 February 2001, accepted 25 March 2001.