273 DEFENSE OF EXTRA NEST-SITES BY A CAVITY NESTING BIRD, THE TREE SWALLOW Taehyeineta hieolor WALLY B. RENDELL & RALEIGH J. ROBERTSON ABSTRACT We use data from field experiments and observations of Tree Swallows Tachycineta hicolor defending single and extra nest-sites at nest- box and cavity populations to determine the function of extra nest-site de- fense behaviour. About 50% of pairs travelled up to 56 m to defend neigh- bouring nest-sites for a month or longer. Both sexes defended extra nest- sites. 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 neigh- bouring settled pair. In Experiment 2, males, and perhaps females, pre- ferred 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. Al- though 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 on- ly one nest-site. Polygyny was not observed in the natural populations, but one quarter of males with extra nest-sites in the nestbox populations attract- ed a second female. Our data suggest that male Tree Swallows exhibit re- source defense polygyny, and females probably defend extra nest-sites to prevent or delay settlement by other females. Department of Biology, Queen's University, Kingston, Ontario, Canada K7L3N6. INTRODUCTION An important function of territoriality is to defend resources essential for breeding (Brown 1964). For secondary cavity-nesting birds suitable nest- sites are often a limiting resource (Von Haartman 1957,1969; Holroyd 1975), so territorial behaviour functions to acquire and defend cavities. In Tree Swallows Taehyeineta hieoior limited availability of cavities results in considerable competition. Males and females arrive at breeding sites in very early spring and begin defending one or more cavities (Robertson et ai. 1992). The area immedi- ately surrounding the cavity or cavities is defend- ed against intruders as a territory. Territorial de- fense involves one or both members of a pair spending extensive periods of time perched on or near the nest site, and intercepting and chasing in- Accepted 22 June 1994 truders that encroach and persist for more than a few seconds in the vicinity of the nest-site. Fe- males are often more vigilant and aggressive than males, especially when repelling female intruders (Stutchbury & Robertson 1987d). When birds are off territory they are generally foraging at dis- tances from 50 to several hundred metres from the territory. While within sight of the territory, they respond to intruders; otherwise intruders are evicted upon the return of the residents to the ter- ritory. Those birds that do not get nest-sites be- come floaters (Stutchbury & Robertson 1985), in- trude on residents' territories (Lombardo 1986b) and will displace or quickly replace residents (Stutchbury & Robertson 1987a). Competition for nest-sites can escalate to fights causing injury and death (Leffelaar & Robertson 1985, Lombardo 1986a, Robertson et ai. 1986). Despite this severe ARDEA 82: 273-285