Habitat Selection by Breeding American Redstarts in Response to a Dominant Competitor, the Least Flycatcher

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Abstract
Through natural population changes and experimental field removals, we tested the hypothesis that Least Flycatchers (Empidonax minimus) restrict habitat use by socially subordinate American Redstarts (Setophaga ruticilla). On a 10-ha site 2-yr-and-older (ASY), but not yearling (SY), male redstarts avoided the sector occupied by flycatchers from 1975 to 1980, but preferred this sector from 1981 to 1985 when flycathcers were absent. Vegetation changed subtly on the site but could not account for the sudden shift in redstart settlement pattern. On 6 4-ha sites ASY male redstarts were most abundant in years of Least Flycatcher absence. On the 5 4-ha sites from which Least Flycatchers either disappeared independently or were removed experimentally between 1981 and 1984, redstart abundance increased on four and remained constant on the fifth; on three control areas redstart numbers declined during the same period. Least Flycatchers recolonized one removal site, and ASY redstart abundance subsequently declined. SY male redstart abundance varied inversely with that of ASY male redstarts. We conclude that flycatchers influenced the distribution of ASY male redstarts directly, and that of SY males indirectly, more than either vegetation structure or other habitat characteristics. At no spatial scale examined, however, did total redstart abundance (ASY + SY) vary inversely with that of Least Flycatchers; in fact, their total abundances correlated positively at a regional scale. These findings, combined with model for asymmetric competition for mutually preferred habitat (Pimm et al. 1985, Rosenzweig 1985), illustrate how a socially dominant competitor could lead to a broadening rather than a narrowing of the habitat breadth of a subordinate species. We show that competitor species abundances need not vary inversely and that age classes may be affected differentially. This species interaction illustrates subtleties and complexities of how competition can modify avian habitat selection.