Buitron, Deborah Patricia. 1982. Behavior of Black-Billed Magpies During the Breeding Season. PhD Dissertation. University of Minnesota, Minneapolis, MN. 135 p.
During four breeding seasons (1978-1981) I studied the behavior of a small population of individually marked black-billed magpies (Pica pica) in Wind Cave National Park, South Dakota. This thesis is divided into three parts: (1) the roles of males and females in raising offspring; (2) the defense of offspring from potential predators; and (3) the occurrance and characteristics of extra-pair courtship.
In the first part, I describe the contributions made by each sex to reproduction in an area where magpies nested at lower densities than has been found in other studies. Both males and females participated extensively at each stage of the reproductive cycle. Although females laid and incubated eggs, and brooded young nestlings, males provided their mates with almost all the food required during 30 days of laying and incubation. Both sexes spent about equal amounts of time working on nests, but males brought more material to the nest. Males also fed nestlings and fledglings at higher rates than females, and were more active in driving away potential predators, especially when the females were inside their nests caring for eggs and nestlings.
The second section examines one aspect of parental care in detail, how magpies respond to different classes of predators. The level of response by magpies was found to depend on (1) the type of predator and the threat it posed to eggs, nestlings, fledglings, and adults; (2) the behavior of the predator; and (3) the reproductive stage of the magpies. Reactions to egg and nestling predators was most frequent and intense during laying and incubation. Predators that are primarily dangerous to young fledglings, such as raptors in flight and ground predators, were responded to most vigorously during the second half of the nestling period and the first two weeks of flegding. Raptors perched near a nest or a family were always mobbed vigorously. Vigorous mobbing often appeared successful in driving away potential predators; also alarm calling was judged to make it difficult for a predator to continue hunting effectively in that area.
In the final section I describe the behavior of males and females taking part in extra-pair courtship. Intruding males were usually paired birds whose own females were already incubating eggs. Females received higher rates of extra-pair visits during their fertile periods than either before or after, and typically they responded by following and begging from the intruders. The timing of visits and the behavior of intruding males and of visited females suggest that extra-pair copulations may have been the main objectives of both males and females, although none were seen. Guarding of fertile mates by males suggests an adaption for protecting their reproductive success.