NATAL AND BREEDING DISPERSAL IN THE MOUNTAIN WHITE-CROWNED SPARROW ZONOTRICHIA-LEUCOPHRYS-ORIANTHA

Dispersal data were collected on Mountain White-crowned Sparrow Zonotr ichia leucophrys oriantha, an open-nesting migratory songbird breeding in high elevation meadows of the Sierra Nevada of California. Young b irds began to move soon after fledging. Although some remained on thei r meadow of birth until migration, most moved away. During this post-f ledging period juveniles could have been locating future breeding area s. Especially in males that remained near the natal site, the rate of return as adults was very high (28.5%). Juveniles may have also been s earching out favourable areas for foraging during this time. For most young birds, especially females, the final phase of natal dispersal an d selection of a breeding site probably did not occur until the pre-br eeding period, the time between arrival from wintering areas as one-ye ar-olds to settling on territory. Between-year breeding dispersal dist ance for the population did not vary inter-annually (n = 15), although gender differences were delectable. Males tended to be more site-tena cious than females. For example, dispersal distance decreased signific antly with age in females and increased when they changed rather than retained mates between years. The same trends occurred in males but th e differences were not significant. When within-year breeding dispersa l occurred following loss of a nest or fledging of young, the pair rem ained together and renested on the same territory. Dispersal distance did not vary according to reason for nest failure or with nesting succ ess. It is speculated that dispersal behaviours in Z. l. oriantha may be influenced by their use of small, scattered meadows - the ecologica l equivalent of islands - for breeding habitat.