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Some aspects of the economics of territoriality in North American hummingbirds

University of British Columbia

Territoriality of post-breeding North American hummingbirds is one of the clearest examples of energy-based feeding territoriality, and has played an important role in the formulation and testing of theories on territoriality. This thesis follows two lines of inquiry. First, what special foraging strategies are used by hummingbirds holding energy-based feeding territories? Second, are energy-based models applicable to territoriality of breeding as well as non-breeding hummingbirds? I use a simulation model to examine the possible benefits for a non-breeding territorial hummingbird of avoiding recently visited locations while foraging. The simulation results suggest that while birds might benefit by avoiding flowers visited recently during a foraging bout, they would not benefit significantly by avoiding patches of flowers visited on previous bouts. An important benefit of short-term avoidance of flowers for model birds is that they can empty all or nearly all flowers in each patch visited. They therefore create coarse-grained patterns of nectar availability that are easy to exploit later on. Results of a field study on breeding territoriality of male Calliope Hummingbirds indicate that a solely energy-based model of territoriality cannot account for the behavior of these males. Throughout the 2 month breeding season, flower sampling indicated that males could have obtained energy faster by foraging on nearby undefended areas than by foraging on their territories. During June, flowers blooming on the territories were sparse and/or contained very little nectar, and males could not have obtained enough energy from them even to compensate for the cost of hovering while foraging. Consequently, they did virtually all their foraging away from their territories at that time. During May, when nectar availability on the territories was at its peak, males did not leave, expand, or shift their territories in response to experimental exclosure of all flowers blooming on them. I develop a model of optimal territory size for a promiscuously breeding male that defends a breeding territory containing no energy sources, as did the Calliope males during June at my study site. This model is analogous in design to existing models of energy-based feeding territoriality, but is based on the premise that a male's optimal territory size is that which maximizes his immediate reproductive success. A prediction of this model is that a male's territory size will be limited by the rate at which he can obtain energy while foraging away from his territory. An experiment performed on a Calliope male's territory did not support this prediction, and suggested that the size of his territory may have been limited only by his ability to detect intruders.

Text

2010-06-14

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http://hdl.handle.net/2429/25743

Zoology

University of British Columbia

Graduate