Effect of Nest-Site Supplementation on Polygynous Behavior in the House Wren (Troglodytes aedon)

Abstract

Several recent studies have documented repeated incidents of polygyny in populations of secondary-cavity nesting birds provided with an abundance of nest boxes (Dhondt 1987, Marks et al. 1989, Korpimiki 1989, Pinxten et al. 1989, Alatalo and Lundberg 1990, Petit 1991, and references in each). During a study of House Wrens (Troglodytes aedon) in Wyoming, we found that over half of males with at least two nest boxes on their territories became polygynous. Price (1986) and Quinn (1989) have also recorded high rates of polygyny among box-nesting House Wrens in Illinois and Alberta, respectively. In the Wyoming population, polygyny appeared to be costly to second-mated females because they received less parental assistance from mates and fledged fewer young than monogamous or primary females (in prep.). Reduced reproductive success for second-mated females has been documented in other boxnesting species including Pied Flycatchers (Ficedula hypoleuca; Alatalo et al. 1981, Stenmark et al. 1988), Blue Tits (Parus caeruleus; Dhondt 1987), Boreal (Tengmalm's) Owls (Aegoliusfunereus; Solheim 1983, Carlsson et al. 1987, Korpimiki 1991), European Starlings (Sturnus vulgaris; Pinxten and Eens 1990), Prothonotary Warblers (Protonotaria citrea; Petit 1991), and House Sparrows (Passer domesticus; Veiga 1990). To explain the occurrence of polygyny in these species, most researchers have invoked one of several formal models that outline conditions under which polygyny, although costly to fitness, can still be an adaptive choice made by females (Searcy and Yasukawa 1989). However, these models assume that polygyny evolved in situations where females were routinely faced with a choice of paired and unpaired mates. Alatalo and Lundberg (1984) cautioned that if in the the abundance and dispersion of suitable cavities is such that males rarely control more than one nest site, there may be little selection on females to identify and consider the pairing status of a potential mate. Thus, the seemingly maladaptive choice of polygyny made by females in box-nesting populations could be an artifact of providing males with a surplus of high quality nest sites. Alatalo and Lundberg (1984) rejected this hypothesis for Pied Flycatchers by showing that similar proportions of males in areas with and without nest boxes attempted to attract second mates. In contrast, Petit (1991) reported that in the natural situation male Prothonotary Warblers very rarely have access to more than one natural nest cavity and polygyny is extremely rare. She too suggested that polygyny may occur much more frequently in this species when males have a surplus of nest boxes because females are evolutionarily naive to the costs of polygyny. This hypothesis was not considered in the other studies cited above with the exception of Marks et al. (1989; see also Korpimiiki 1991:44). The purpose of this study was to determine whether the occurrence of polygyny among box-nesting House Wrens was an artifact of allowing males access to more than one nest box per territory. We asked if and how frequently in the natural situation female House Wrens pair polygynously, and how frequently paired males attempt to attract second mates. We compare these data to similar data gathered when we provided each male with several boxes per territory. Because the frequency of polygyny has not been documented for any naturally-nesting population of House Wrens, we felt that this study was critical prior to testing formal models for the occurrence of polygyny in this species.