Predator responses to similarity and dispersion of artificial nest sites: Implications for the structure of Boreal Forest Songbird communities

Auk, The, Jan 2001 by Rangen, Sheila A, Clark, Robert G, Hobson, Keith A

ABSTRACT.-Relatively little is known about the role of predation in shaping patterns of coexistence and nest dispersion of songbird species. It has been hypothesized that predators diversify songbird communities by preying more heavily on individuals and species with greatest similarity in nest-site use. To investigate the importance of predation, we tested how predators responded to assemblages of artificial songbird nests that varied in nest-site placement, vegetation features, and nest dispersion patterns in boreal forest of west-central Alberta, Canada. Variability among nest sites was achieved by deploying wicker nests throughout a gradient of vegetation cover and by deploying nests to simulate two- and three-species assemblages. Two-species assemblages, comprising 20 simulated White-throated Sparrow (Zonotrichia leucophrys) and 3 simulated Hermit Thrush (Catharus guttatus) nests, and threespecies assemblages, comprising 10 simulated White-throated Sparrow, 9 simulated Hermit Thrush, and 4 simulated Chipping Sparrow (Spizella passerina) nests, were deployed in eight replicate plots. We hypothesized that predators would be more adept at locating and depredating (1) nests characterized by similar vegetation features in nest patches; (2) nests of similar appearance or nest guild; and (3) clumped versus randomly distributed nests. Contrary to predictions, predation did not increase as variance in vegetation of nest sites decreased across 16 nest-predation plots, nor did variance in vegetation of successful nests increase as predation level increased across 15 nest-predation plots. The addition of one species' nest type to assemblages did not result in lower predation rates. Predators also did not depredate more clumped nests than randomly distributed nests. Overall, predation did not appear to influence patterns of songbird species coexistence or nest dispersion. Abilities of predators to discriminate among less-similar versus more-similar nest sites and nest-dispersion patterns are probably species-specific; that is probably related to the hunting behavior of predators (i.e. use of olfactory and visual cues) and territory size. Received 2 September 1999, accepted 8 August 2000.

A GENERAL GOAL of avian evolutionary ecology is to understand mechanisms driving patterns of community organization (aliens 1989). Hypotheses advanced to explain the structure of avian communities have focused primarily on interspecific competition and food limitation (Schoener 1974; Martin 1987b, 1991). However, recent research has shown that predation might also be an important factor influencing evolution of forest songbird community structure (Martin 1988a, b, c; Lima and Valone 1991, Forsman et al. 1998), particularly when predation is the primary cause of nesting mortality (Ricklefs 1969). If predators respond to accumulating densities of similar nesting songbird species as though they were one species, predation may provide a selective pressure for coexisting species to select different nest types and locations, presumably within constraints of stereotypic nest placement that arises from a species' evolutionary history (Martin 1988c, 1993). Such partitioning of nesting sites may yield a more diverse bird community that, in turn, forces predators to search more substrates and height levels, inhibiting the development of predator search images, and decreasing predator searching efficiency (Martin 1988b, 1993, 1996).

An inherent assumption of the predation hypothesis is that predators specialize on nest types. However, the extent to which different predator species hunt for nests of specific songbird species or in locations of specific vegetation types is unclear. Use of search images that lead to an intensified search for similar nests is generally ascribed to predators that rely on visual cues (Martin 1988b). Thus, if a considerable proportion of the predator community comprises nocturnal mammals, visual cues provided by nest construction, substrate, strata, and vegetation at nest sites may be of limited importance. Rather, olfactory search images may prevail with predators cueing on odors of nest contents, parent birds, and nests (Nams 1997, Pelech 1999). Ultimately, songbird community assemblage patterns may be subject to counteracting selection pressures from different species in the predator community.

Predation pressure may favor coexisting songbirds with different nest types and locations, but also species with nests that are well spaced from neighbors. For that to occur, predators must concentrate their search efforts after cueing on nests, which results in closely spaced nests incurring heavier predation (Tinbergen et al. 1967, Sonerud 1985). Some studies using artificial and natural nests have reported densitydependent predation, whereas others have found no relationship between predation rates and nest density (Zimmerman 1984, Reitsma 1992, Hogstad 1995, Lariviere and Messier 1998). Nonetheless, few studies have examined predator responses to nest-dispersion patterns of songbirds (Picman 1988, Major et al. 1994).