Adaptive Significance of the Variable Dispersion Pattern of Breeding Northern Orioles

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Abstract
Spacing patterns in the northern oriole [Icterus galbula bullockii] were studied. Any riparian site can be represented by a point on the plane whose axes are nest site density and nest habitat food density. The plane is contoured with respect to the expected degree of territoriality for populations in each section. For all cases an alternative foraging habitat surrounds the riparian one. For any given nest site density, as food increases, populations can be arranged along a continuum from Type B (nesting) territoriality, through semi- (nesting and part of feeding) territoriality, to Type A (all-purpose) territoriality. This is due to the increase in defendability of their food supply. For any given level of food availability, as nest sites increase so will nesting bird density, so that the Type B territories of the bottom section become colonies. As nest site density increases, the threshold for any increase in defense of territory lies at a greater level of food density because of increased pressure from birds trying to use those nest sites. For any nest site density, there comes a point where Type A territoriality is the predicted behavior because of high food density and perhaps large extent of the riparian habitat. This pattern should be found in other typically riparian birds of a body size large enough to permit utilization of both habitats. The abundance of nest sites and of food act in opposition to one another in their effects on the spacing pattern of northern orioles in riparian woodland. The surrounding treeless habitat offers a food supply and serves as a buffer allowing territoriality to be reduced in accordance with the demand for nest sites and with the worth of the nest habitat. In its extreme, such a system becomes a colonial one, although not for the usually cited reasons of highly clumped or unpredictable food supply.

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