Asynchronous Polygyny in the House Wren (Troglodytes aedon)

Abstract

Polygyny occurs regularly (5% or more males polygynous) in only 14 (5%) of the 291 species of passerines analyzed by Verner and Willson (1969). Orians (1961) suggested that polygyny in passerines is most common in habitats where nest-site availability is limited. Suitable nest sites are limited for many secondary cavity nesting birds (von Haartman 1957, Holroyd 1975, Pinkowski 1979, Minot and Perrins 1986). Secondary cavity nesters, therefore, might exhibit polygynous mating systems if other factors (such as the need for male parental care) do not limit them to monogamy. Evidence from populations of Great Tits (Parus major; Kluyver 1951), Pied Flycatchers (Ficedula hypoleuca; Alatalo and Lundberg 1984), and House Wrens (Troglodytes aedon; Kendeigh 1941) supports this hypothesis. The question of why females choose to settle with already-mated males, when it is potentially costly to their fitness, is central to the study of avian mating systems. The polygyny-threshold model suggests that a female who chooses an already-mated male obtains a higher-quality breeding situation than if she were to choose an unmated male at the same time (Verner 1964, Verner and Willson 1966, Orians 1969). Although Vehrencamp and Bradbury (1984) suggested that an adequate test has never been made, the polygyny-threshold model is widely cited as the best explanation for the evolution of territorial polygyny in many bird species (Searcy and Yasukawa 1989). A key prediction of the polygyny-threshold model is that secondary females should be at least as successful as monogamous females that settle at the same time (Garson et al. 1981). Tests of this prediction however, have produced little support (Leonard 1990). We report results that support the prediction for a singlebrooded population of House Wrens and discuss how timing of polygyny may have been a factor. We studied House Wrens in 1986 at the Beaverhill Bird Observatory, 72 km east of Edmonton, Alberta (53?24'N; 112?31'W). The study comprised an area with