Mate switching in multibrooded House Wrens

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-We studied within-season mate switching in two populations of House Wrens (Troglodytes aedon) in central Illinois over nine breeding seasons. On the East Bay site, 35.2% of the monogamous pairs switched mates. On this less-preferred habitat, there were fewer potential mates. In contrast, on the Mackinaw site, 58.8% of the monogamous pairs switched mates, and there were potential mates available throughout the breeding season. There was no consistent immediate reproductive cost or benefit to mate switching. We suggest that mate switching in these House Wren populations is a consequence of varying mate availability and differing gender-related costs of facultative polygyny and territoriality. Because females incubate the eggs and brood the chicks, males have the first opportunity to desert their mate and offspring, which often results in polygynous matings. After the chicks leave the nest, males that feed fledglings may lose their territory and the opportunity to breed again. Females do not incur such a cost, and they easily move to another territory and another mate. Frequencies and relative costs and benefits of mate switching also varied considerably among three other House Wren populations. This reveals considerable flexibility in response to different demographic and environmental situations. Knowledge of the basis for this variability is important to understand mate choice in birds. Received 14 February 1990, accepted 14 July 1990. THE DEGREE to which mated birds maintain the pair bond between breeding attempts within a single breeding season varies among species and among populations within a species. Multibrooded passerines often display almost complete mate fidelity, with the male caring for the offspring while his mate begins the next nest (see reviews in Nice 1930, Burns 1983). In these species, the advantages of mate fidelity (e.g. familiarity of mate and territory) outweigh the advantages of mate switching (finding a better mate or territory). Some species that regularly switch mates within seasons include European Starlings (Sturnus vulgaris; Feare and Burnham 1978), Prairie Warblers (Dendroica discolor; Nolan 1978), Indigo Buntings (Passerina cyanea; Carey and Nolan 1979), Winter Wrens (Troglodytes troglodytes; Garson 1980), Barn Swallows (Hirundo rustica; Shields 1984), Song Sparrows (Melodia melospiza; Weatherhead and Boak 1986), Fan-tailed Cisticola (Cisticola juncidis; Ueda 1986), and Pinon Jays (Gymnorhinus cyanocephalus; Marzluff and Balda 1988). The decision of a pair to separate will depend upon the relative costs and benefits of switching to a new partner, and upon mate availability (Maynard Smith 1977). If no potential mates are available, a bird must choose between breeding with its previous mate or not breeding. If new potential mates are available, birds that suffer from low reproductive success at their first nest may have higher subsequent reproductive success if they switch mates. Thus, if there is a direct relationship between immediate reproductive success and mate switching, birds with failed nests should be more likely to switch mates than those with successful nests, and birds that switch mates after nest failure should be more successful subsequently than those not switching. Birds switch mates for various reasons. They may be attempting to increase their reproductive success by finding a more experienced, older mate. They may be attempting to minimize the time between nesting attempts, or to obtain a higher-quality territory (Shields 1984). Switching to an older, more experienced mate may result in higher reproductive success because, in some species, more experienced or older birds have higher reproductive success than less experienced, younger individuals (Rowley 1983). If so, the frequency of mate switching should be lowest in old/experienced pairs, intermediate in mixed old/experienced-young/ inexperienced pairs, and highest in young/inexperienced pairs. Switching could also increase reproductive success by minimizing the time between nesting attempts, if breeding seasons are short or reproductive success decreases as the breeding season progresses (Burns 1983). 60 The Auk 108: 60-70. January 1991 This content downloaded from 157.55.39.51 on Wed, 10 Aug 2016 05:35:54 UTC All use subject to http://about.jstor.org/terms January 1991] Mate Switching in House Wrens 61 In this case, if one parent can raise at least some of the brood alone, the other parent should abandon the first brood and switch to another