The Ecology of Spacing Behavior in the Yellow-Billed Magpie (Pica nuttalli)

DOCTORATE DISSERTATION: The yellow-billed magpie is a highly social corvid that breeds in loosely spaced colonies. Nesting close to conspecifics, rather than nesting in dispersed locations, is known to increase competition (for food, nest sites and mates) and increase the spread of disease and parasites. For coloniality to be evolutionarily stable, some benefit, or combination of benefits, must counter these inherent costs. Among several alternative hypotheses for the evolution of avian coloniality, I examined a model which relates the variability of foraging areas to selection for nesting in colonies. In a simple model, Horn showed that when resources were evenly distributed, breeding pairs nesting in dispersed locations had shorter average travel distances to foraging areas than pairs nesting in a cluster. When resources were clumped in space and time, breeding pairs nesting colonially in a central location had shorter average travel distances to unpredictable foraging areas. Thus, the minimization of travel distance to foraging areas will select for coloniality when resources are clumped in space and time. The model makes several assumptions: 1. Food resources must be spatially and temporally variable while the colony is occupied. 2. Colony locations should be near the center of mass of the food distribution. 3. Every colony member should forage throughout the entire foraging area exploited by the colony. The model s prediction is that, given these assumptions, mean travel distances should be shorter for colony members than those calculated for members of a hypothetical population of equal size that is dispersed over the same food distribution. I evaluated the assumptions and prediction of Horn s model using field data from a long-term study of yellow-billed magpies. Yellow-billed magpie nests were clustered relative to available habitat and individuals were not forced to nest in colonies because of habitat limitation. The location of good foraging areas varied significantly in space and time. Yellow-billed magpie colonies were roughly centrally located with respect to the distribution of food. Foraging areas were not defended and individual home ranges, as determined by radio telemetry, exhibited significant overlap. Consistent with Horn s model, mean travel distances were significantly shorter for colony members than those calculated for members of hypothetical populations of equal size that were dispersed over the same food distribution.