Baeyens G. (1981) The role of the sexes in territory defence in the Magpie (Pica pica). ARDEA 69 (1): 69-82

Magpie breeding pairs in Haren defended their territories (ca. 6 ha) throughout the year. Non-breeders were non-territorial, mostly juvenile and unpaired birds, which roamed about into territories and in the no-man's-land between them. Territory boundaries were usually clearly demarcated. Breeding pairs trespassed them only occasionally, and not for large distances, even not when they were lured away by a feeding station in an adjoining territory. When non-breeders intruded into territories, they were usually attacked and chased away by the owners, but the intolerance of the latter varied in space and time. Interactions between territory owners and non-breeders at feeding sites within territories indicated that territorial aggression increased in the nest-building period (February, March and April). Caged Magpies, presented within territories, .were also more often attacked in the breeding season than in summer and winter. Intruders and conspecifics in cages were more readily attacked, the nearer they were to the nest. Males spent more time and energy in territory defence than females. They attacked male intruders, wild and caged ones, more readily than female intruders. Also they were more likely to assist their mates in chasing other males than in chasing other females. Females. were only slightly more aggressive to intruders of their own sex. I suggest that males might profit from .being tolerant to intruding females because they might gain additional fertilizations, an additional mate, and because it might facilitate mate replacement if that is needed. Extramarital courtship and bigamy have been observed. In a future study the effects of experimental removal of pair members on mate replacement will be reported. Females might also insure themselves against mate loss by tolerating potential replacing males. However, the aggression females display, is perhaps part of their strategy to test the male's future faithfulness since they need male assistance during incubation and chick-raising.