Reproductive Performance of Female-female Pairs and Polygynous Trios of Ring-Billed Gulls

Abstract

Abstract We studied female-female pairs of Ring-billed Gulls (Larus delawarensis) on Granite Island, northern Lake Superior, during the breeding seasons of 1979 and 1980. In 1979 the colony consisted of approximately 2,400 nesting pairs, with a total of 99 nests containing 5-7 eggs (superclutches). In 1980 the colony had increased in size to 2,600 nests and contained a total of 71 superclutches. We discuss the difficulty of distinguishing nests in which superclutches have been laid by female-female pairs from single-cup nests used by polygynous groups or from nests receiving dump eggs. Nests containing superclutches were larger than those containing normal-sized clutches. They were not differentially located by substrate, nest density, or location within the colony. Nearest-neighbour distance was also similar for the two clutch types. Eggs laid in superclutches were slightly smaller than those from normal-sized clutches (1-4 eggs) but did not differ in shape. Significantly more eggs from superclutches rolled from the nest or were destroyed or abandoned than from normal-sized clutches. The proportion of nests that hatched at least one chick did not differ significantly between the two clutch types. Hatching success for superclutches was 34% in 1979 and 30% in 1980, whereas for normal-sized clutches it was 77% in 1979 and 61% in 1980. Chicks from superclutches had a higher rate of mortality during the week following hatching than did chicks from normal-sized clutches. Chicks from the former hatched at significantly lighter weights than did those from the latter during both years of study, but their weights did not differ after the first week posthatch. Tarsal and culmen measurements followed a similar pattern to that of weight. Chicks from normal-sized clutches had a significantly higher fledging success than did those from superclutches. The reproductive success of four polygynous groups is also reported.