Ardea
Official journal of the Netherlands Ornithologists' Union

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Rendell W.B. & Robertson R.J. (1994) Defense of extra nest-sites by a cavity nesting bird, the Tree Swallow Tachycineta bicolor. ARDEA 82 (2): 273-285
We use data from field experiments and observations of Tree Swallows Tachycineta bicolor defending single and extra nest-sites at nestbox and cavity populations to determine the function of extra nest-site defense behaviour. About 50% of pairs travelled up to 56 m to defend neighbouring nest-sites for a month or longer. Both sexes defended extra nestsites. Males spent more time than females at extra nest-sites, especially during later stages of the breeding cycle. In Experiment 1, 60% of extra nest-sites erected beside occupied nestboxes were defended by a neighbouring settled pair. In Experiment 2, males, and perhaps females, preferred to settle on territories with two nest-sites versus those with one. There were no age differences for either sex when comparing birds which defended extra nest-sites with those that did not. Physical characteristics of males were not different between defense types, but females that defended extra nest-sites had longer wings than those that defended one nest-site. Although they may be beneficial in cases where a nest-site is destroyed by a predator, extra nest-sites were usually not essential for renesting. Residents were evicted from territories with extra nest-sites as often as those with only one nest-site. Polygyny was not observed in the natural populations, but one quarter of males with extra nest-sites in the nestbox populations attracted a second female. Our data suggest that male Tree Swallows exhibit resource defense polygyny, and females probably defend extra nest-sites to prevent or delay settlement by other females.


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