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In territorial species, the reproductive success of a male is dependent on the quality of his territory. One important component of territory quality is spatial location. High-quality territories not only should be located in areas of high food abundance and low predation, but also should be located in areas that offer optimal amounts of social interaction. Such optima might be different for individuals according to their sex, dominance, or genotype. We studied territory quality (size, vegetation structure, and placement) in a socially monogamous, polymorphic passerine, the White-throated Sparrow (Zonotrichia albicollis), in order to determine how spatial attributes contribute to selection intensity on two genotypes. In this species, plumage (white and tan), behavior, and life-history characteristics have a genetic basis and are correlated with the presence or absence of a chromosomal inversion. Using remote sensing and Geographic Information Systems (GIS), we found that the territories of white and tan males do not differ in size or vegetation structure, suggesting that these factors are not of primary importance to males when deciding where to establish a territory. Instead, we suggest that the placement of white and tan territories depends on the number of neighbors (particularly, white male neighbors). Tan males settle in low-density, neighbor-restrictive habitats where intruder pressure from white males seeking extra-pair copulations is reduced. In contrast, white males tend to settle in high-density areas where the probability of encountering neighboring fertile females is greatest. This segregation has led to intraspecific niche partitioning in the two disassortative pair types so that each male morph can best exploit its respective reproductive strategies. These factors may, in turn, contribute to the maintenance of this unusual mating system and, ultimately, the stability of the polymorphism in this species. Similar forces may be operating in other species without distinct morphological markers; we suggest that researchers keep social factors in mind when examining habitat selection.


This article was originally published in Ecology, 85(4): 1125-1136. © 2004 Ecological Society of America