This study discusses the breeding biology of the Bufflehead (Bucephala albeola; Anatidae, Aves), and examines factors operating upon the species during the breeding season to evaluate their effectiveness in setting limits to its breeding distribution. A definition of the breeding habitat utilized by the species is offered, with special attention being paid to the form and spatial relationships of the nest-hole. Precise details on nesting are presented and compared with those of related species. Factors which may affect the survival of young under varying conditions are examined critically for correlations with observed mortality. On the basis of these data it is suggested that climate is of major importance in limiting the breeding distribution of Bufflehead. Climate limits the distribution of trees, and thus of nest-sites, while luxuriant growth of shrubs in regions of high rainfall may limit the fraction of young reaching the water. Adverse weather may cause mortality both by causing chilling and by preventing small young from feeding. It is felt that further studies on waterfowl which involve relationships between nest-sites and territories might profitably use this species as a vehicle. / Science, Faculty of / Zoology, Department of / Graduate

In this study, I investigate the adaptive significance and consequences of territorial behavior in buffleheads (Bucephala albeola) . The aims are: (1) to test the hypotheses that territorial males defend the food supply, the female or the nest site, (2) to test the hypothesis that brooding females defend the food supply, and (3) to examine whether territorial behavior or nest site availability limits breeding density. 1) Males defend a territory from the pre-laying stage until late incubation. The size of male territories was not related to food abundance, and food was poorly correlated with reproductive success. When the males of seven laying females were removed, four widowed females were evicted from their territory by neighboring males; widowed females also spent less time feeding and more time alert. Females tended to settle on a territory adjacent to their nest, and those that did not do so suffered a higher rate of nest parasitism. I suggest that protection of the female is a major function of male territories and that protection of the nest site is a secondary function. 2) Females become territorial after hatching the brood but they defend a different territory. The size of the brood territory was inversely correlated with food density and the relationship was hyperbolic. Growth rate and survival of ducklings was negatively correlated with brood density in one year and survival was positively correlated with food density in another year. I suggest that brood territories secure an adequate food supply for the young and that females adjust territory size according to both food and brood densities. 3) Natural nest sites were not in short supply and the addition of nest boxes did not increase breeding density. Breeding density was stable over the four years of this study. When I removed seven territorial males, four were replaced. I propose a model to explain the variability of territorial behavior in ducks. This model is based on the hypothesis that mate-guarding is a major function of territorial behavior, and predicts that the degree of territoriality is correlated with habitat variability. A review of the territorial status of 69 species of ducks is consistent with the model. I conclude that breeding buffleheads exhibit two kinds of territorial behavior: males are territorial during the nesting season to protect the female and provide her with an undisturbed feeding area (i.e. mate-guarding), and also to protect the nest site; females are territorial during the brood-rearing stage to secure food resources. Territorial behavior of nesting pairs further appears to limit breeding density. / Science, Faculty of / Zoology, Department of / Graduate