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Owners and Satellites: The Economics of Territory Defence in the Pied Wagtail, Motacilla alba

N. B. Davies and A. I. Houston
Journal of Animal Ecology
Vol. 50, No. 1 (Feb., 1981), pp. 157-180
DOI: 10.2307/4038
Stable URL: http://www.jstor.org/stable/4038
Page Count: 24
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Owners and Satellites: The Economics of Territory Defence in the Pied Wagtail, Motacilla alba
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Abstract

(1) Pied wagtails defended winter feeding territories along a river. They fed on insects that were washed up onto the river banks. These formed a renewing food supply: after a stretch had been depleted, time was needed for prey abundance to return to a profitable level. The wagtail's feeding rate therefore depended on the time that had elapsed since the stretch was last depleted (the return time). (2) Territory owners increased their return time, and hence feeding rate, by systematic search around the territory and by evicting intruding conspecifics and other species that depressed their food supply. (3) Sometimes intruders landed on a territory undetected. This often decreased the owner's feeding rate because it visited stretches soon after they had been depleted by the undetected intruder. Juvenile intruders were more likely to land on a territory undetected, probably because they had duller plumage than the adults. (4) A territory was more profitable to an owner than to an intruder because the owner achieved greater return times. Intruders did worse because they did not know where the recently depleted stretches of the territory were. Even if intruders landed undetected, they often fed over areas that had recently been depleted by the owner. (5) Owners defended a territory of a fixed length, but their defence behaviour varied. Often the owner defended the territory alone but sometimes it associated with a subordinate (a satellite). Owners showed rapid changes in behaviour towards satellites; sometimes they chased them off and sometimes they allowed them to reside on the territory unmolested. (6) Satellites imposed a cost on the owner because they depleted the food supply on the territory. However, they also brought a benefit in the form of help with territory defence against intruders. (7) We develop a model which quantifies these costs and benefits, and show that owners vary their tolerance of satellites so as to maximize their own daily feeding rate. On days of high food abundance, when intruder pressure is greatest, owners tolerate satellites. We show that owners increase their feeding rate by this association, because the benefits gained through help with defence outweigh the costs incurred through sharing the food supply with another bird. On days of low food abundance, when an owner would have a higher feeding rate by being alone, it evicts the satellite from the territory.

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