Factors Affecting Clutch Size in Arctic Passerines

Clutch size and related aspects of breeding biology were investigated in the Lapland Longspur (Calcarius lapponicus) and Snow Bunting (Plectrophenax nivalis) in the North American Arctic, and in allied temperate and subarctic species. Evidence was sought to test Lack's hypothesis that the increase in clutch size with latitude is related to the length of daylight available for the adults to collect food for their young, and that clutch size is ultimately determined by the average maximum number of young for which the parents can find food. The arctic species were studied mainly at 76° N on Devon Island; some additional data were collected elsewhere. Increase in clutch size with latitude was correlated with a decrease in the adults' night rest period from 7 hr at 50° N to 3—5 hr in the Arctic. In the Lapland Longspur in Canada clutches were larger at high latitudes and at localities with early breeding seasons. Clutch size and latitude were not significantly related for those localities where activity of adults attending nestlings is not restricted by daylength. Differences in clutch size were not attributable to geographic or interspecific variation in hatching asynchrony, hatching success, or growth of the young. Hatching of Lapland Longspur and Snow Bunting clutches on Devon Island corresponded closely with the emergence of the adult insects which predominate in the nestlings' diet, but other factors may also influence the timing of the breeding season. The decline in clutch size in these single—brooded species as the season progressed apparently was not related to changes in the environmental food supply. In the multiple—brooded Chestnut—collared Longspur (C. ornatus) in Saskatchewan clutch sizes changes little during the season. Annual differences in clutch size were demonstrated in the Snow Bunting, but their significance is unknown. Although Snow Bunting clutches of seven are rare on Devon Island, more young were fledged from experimental broods of seven than from smaller broods, but the young averaged lighter in weight. Adults feeding large broods made more visits to the nest each day and were lighter in weight than those with small broods, but the visiting rate per young was less in large broods. Much of the variation in clutch size, including the increase with latitude, is related to environmental factors influencing the food—gathering potential of the adults. But this does not mean that clutch size has evolved to a limit set by the environmental food supply, because food—gathering behavior and related morphology have also evolved by natural selection. Clutch size must be determined by the effects that changes in it and other aspects of reproductive strategy have on the probability of survival of both the adults and their offspring.