Sexual Size Dimorphism and Parental Care Patterns in a Monomorphic and a Dimorphic Larid

I examined parental care patterns of the Black Skimmer (Rynchops niger) and the Caspian Tern (Sterna caspia) to determine correlates of sexual size dimorphism and parental care. In both species, the sexes had quantitatively different parental roles, particularly during the chick-feeding period. Females were generally more involved in parental activities at the nest site than males. Caspian Tern males provided greater numbers and total mass of prey than their mates. Black Skimmer males fed chicks less frequently than did their mates (especially when the broods were very young), but males delivered substantially larger prey and thus contributed greater prey-mass, particularly as broods aged. The sexually dimorphic skimmer pattern of parental roles may have increased the feeding efficiency and reduced predation risks to the young. No overt behavioral manifestations of sexual conflict were observed between paired Caspian Terns or Black Skimmers.