SEX-BIASED PARENTAL-CARE ALLOCATION IN 3 TERN SPECIES (LARIDAE, AVES)

We studied parental-care allocation by males and females in three tern species. Female Common Terns (Sterna hirundo) and Little Terns (S. al bifrons) performed more incubation and brooding than males, whereas in the Sandwich Tern (S. sandvicensis) the sexes shared these duties equ ally. In all three species, agonistic behaviors were performed equally by females and males. Prey types brought by males and females of each species were similar, but males tended to bring larger prey and had h igher delivery rates than females. Information on parental-care alloca tion by female and male. seabirds of various species, 5 gulls, 6 terns , and 1 skimmer, indicates that females perform most of the incubation and brooding in both gulls and terns, whereas males perform most terr itory attendance and agonistic behavior (gulls) and more prey provisio ning (terns). These patterns are qualitatively consistent with the exp lanation that the differences between gulls and terns in sex-biased pa rental care are related to the fact that gulls exhibit sexual size dim orphism but terns do not. Contrary to theoretical predictions that in monogamous birds, females contribute more reproductive effort than mal es, in all the seabird species studied so far the total parental expen diture by males seems to equal or outweigh that by females.