| California Gulls (Larus californicus) nesting at Mono Lake, California, lay abnormally small clutches, and I studied the breeding biology of the gulls nesting there and at Great Salt Lake, Utah, in an effort to explain this clutch size reduction. Increases in chick mortality and decreases in the numbers of nesting adults at Mono Lake are tied to declines in the level of the lake. Consistent differences in chick productivity between the two sites are not due to differences in the intensity of predation, parasitism, or weather, but are most attributable to differences in the food supply.;A review of the theory of clutch size determination reveals six hypotheses most able to explain clutch size variation in the Laridae: (1) optimal working capacity, (2) gametic repackaging, (3) bet-hedging, (4) most productive brood size, (5) egg formation ability, and (6) age structure.;Eggs at Mono Lake are smaller than at Great Salt Lake, thus rejecting the gametic repackaging hypothesis. Birds from Mono Lake average slightly lower ages at death than those from Great Salt Lake, thus rejecting the bet-hedging hypothesis. The ages of the Mono Lake birds are sufficiently close to those from Great Salt Lake to reject the age structure hypothesis. Broods of three would be more productive than broods of two at Mono Lake if the Mono Lake females laid three egg clutches, thus rejecting the most productive brood size hypothesis. Costs of parental care do not appear to climb with increasing brood size at Mono Lake, suggesting that the optimal working capacity hypothesis does not play a large role in explaining the clutch size reduction at Mono Lake. All evidence is consistent with the interpretation that clutch size is lower at Mono Lake because the early spring food supply there is too poor to allow the laying of additional eggs.;Reserve levels determine the optimal working capacity, and more attention needs to be given to the variation of reserve level dynamics among birds of varying phylogenies and ecologies. Ecologists must also consider the possibility of both constraint and adaptation in the determination of clutch size. |
