Sexual Dimorphism of Large Gulls (larus Spp.)

RECENT•.3⁄4 much attention has been paid to sexual dimorphism in birds in relation to niche utilization. Storer (1966) shows that in three species of accipiters the smaller males take smaller prey on the average than do the females, and he suggests intraspecific competition may be partly responsible for maintaining and increasing the dimorphism, although other basic adaptive functions may also be involved. Smith (1966) found for two species of gulls (Larus glaucoides and L. thayeri) that the difference between the sexes was greatest when the species were allopatric with closely similar species. He suggests (p. 85): "that in the absence of competitors, selection will favor divergence in adaptive features such as bill shape as a means of reducing intraspecific competition." Similarly Selander (1966) notes that in woodpeckers the greatest degree of sexual dimorphism is found in insular species that have no closely related competitors. He further demonstrates a relationship between degrees of sexual divergence in foraging behavior and in bill size in two species of the woodpecker genus Centurus. He believes that sexual divergence is adaptive in alleviating intersexual competition for food and points out that many groups of birds tend to be more sexually dimorphic in bill dimensions than in sizes of other body parts such as wing, tail, or tarsus. This is not always true, even in cases where differences between the sexes in feeding habits have been demonstrated. Thus the European Goldfinch (Carduelis carduelis) is no more dimorphic in bill size than in winglength, but Newton (1967) shows that the sexes take somewhat different food. Differences in foraging habits have even been demonstrated in the Red-cockaded Woodpecker (Dendrocopos borealis) which shows no sexual dimorphism in bill dimensions (Ligon, 1968). For further instances of intersexual differences in feeding habits see Selander (1966). Obviously more critical information on this subject is needed, and it therefore seems worthwhile to publish the following observations obtained during a study on the feeding habits of several species of large gulls in Iceland (Ingolfsson, 1967). The species studied were the Great Blackbacked Gull (Larus marinus), the Lesser Black-backed Gull (L. ]uscus), the Iceland Gull (L. glaucoides), the Glaucous Gull (L. hyperboreus), and hybrid populations of Herring Gulls (L. arg,ntatus) and Glaucous Gulls, referred to here as L. argentatus/hyperboreus (a paper describing this hybridization is in preparation). Altogether 1,405 gulls were collected at all times of the year, divided among 78 samples (a sample consists of a number of conspecific birds obtained in one locality within the span