Influence of niche overlap and territoriality on hybridization between Hermit Warblers and Townsend's Warblers

Auk, The, Jan 2000 by Pearson, Scott F, Manuwal, David A

ABSTRACT.-Hybrid zones between Townsend's Warblers (Dendroica tou7nsendi) and Hermit Warblers (D. occidentalis) in the Pacific Northwest are narrow relative to estimated dispersal distances and appear to be moving, with Townsend's replacing Hermits. We examined whether the habitat-transition and parental-fitness asymmetry models can explain why these zones are narrow and moving by comparing habitat variables associated with warbler territories in the Washington Cascades hybrid zone. Habitat variables did not differ among phenotypes, suggesting that the habitat-transition model cannot explain the narrow and dynamic nature of this hybrid zone. Habitat characteristics of Hermit Warbler territories did not differ inside versus outside the hybrid zone, also suggesting that this zone is not associated with a region of habitat transition. The lack of difference in habitat use could be the result of comparing variables that are not important to pairing success. However, warblers tended to select territories on west-southwest aspects. South aspects in the southern Washington Cascades are dominated by Douglas fir (Pseudotsuga menziesii) and true fir, which is a habitat selected by female warblers when choosing among territories and males. The parental-fitness asymmetry model does not necessarily make predictions about habitat use within the hybrid zone but predicts the superiority of one parental species over the other. However, if significant overlap occurs in habitat use or niche (as in these warblers), then competition between parental species is likely to occur. To determine whether these species compete, we mapped 12 warbler territories and monitored an additional 94 territories throughout the breeding season and found that all males with neighbors compete for and hold exclusive territories. Thus, the pattern of habitat use and territoriality is consistent with the parental-fitness asymmetry model. Received 19 August 1998, accepted 16 June 1999.

HYBRID ZONES that are narrow relative to dispersal distances raise an interesting evolutionary question: what selective forces prevent them from widening? Results from previous studies indicate that narrow hybrid zones are usually maintained either by strong selection against hybrids, or by environmental adaptation (Barton and Hewitt 1985). Four general models can account for narrow hybrid zones (Rohwer and Wood 1998): (1) hybrid inferiority or tension zone, (2) habitat transition, (3) parental-fitness asymmetry, and (4) recent contact. In the hybrid-inferiority or tension-zone model, hybrids are inferior to both parental species, and selection against hybrids is balanced by immigration into the hybrid zone by individuals from the adjacent parental populations (Barton and Gale 1993; Barton and Hewitt 1985, 1989). The relative strengths of selection and dispersal determine zone width. In the habitat-transition model, hybrid zones exist between two different environments, each of which favors a different parental form (Moore 1977, Barton and Hewitt 1985, Grant and Grant 1992). If the area of ecological transition is narrow, then the associated hybrid zone will also be narrow. In the parental-fitness asymmetry model, one parental form is superior to the other, and hybrids are intermediate or lower in fitness, resulting in a moving zone (Hewitt 1989). The fitness of hybrids relative to the parental forms will affect zone width and movement. If hybrids are inferior to both parental species, then the zone will be narrow and the movement slow. If hybrids are intermediate in fitness, then the movement of the zone will be accelerated and the width will depend on the fitness differences between hybrids and parental species. Superior parental zones appear to be relatively rare because of their ephemeral nature (Gill 1980, 1997; Hewitt 1989; Harrison 1990). Finally, hybrid zones may be narrow because of a combination of recent contact and low dispersal rate (Endler 1977, Barrowclough 1980).

Only two of these models, habitat transition and parental-fitness asymmetry, can explain a hybrid zone that is both narrow and moving. In the habitat-transition model, zones move if the area of ecological transition moves (e.g. Northern Flicker [Colaptes auratus], Moore and Price 1993; Carrion Crow [Corvus corone coronel and Hooded Crow [C. corone cornix], Cook 1975). Many of these zones are associated with human-caused habitat changes and may move as the environment changes (e.g. Blue-winged Warbler [Vermivora pinus] and Golden-winged Warbler [V chrysoptera], Gill 1980; Common Grackle [Quiscalus quiscula], Yang and Selander 1968). In the parental-fitness asymmetry model, zones move as the competitively superior parental species replaces the other parental species (e.g. fire ants [Solenopsis invicta and S. richteri], Shoemaker et al. 1996; meadow katydids [Orchelimum nigripes and 0. pulchellum], Shapiro 1998).

Recently, Rohwer and Wood (1998) described hybrid zones between Townsend's Warblers (Dendroica townsendi) and Hermit Warblers (D. occidentalis) in the Olympic Mountains of Washington, the southern Cascades of Washington, and the Cascades of Oregon south of Mount Hood. The two Washington hybrid zones have been described in detail, and both appear to be narrow and moving. Transitions from one parental phenotype to the other occur over 100 to 125 km, a distance only three to four times greater than estimates of root mean square dispersal distance (Rohwer and Wood 1998). These rapid changes in phenotype suggest that selective forces or environmental adaptations are preventing the zones from widening (Barton and Hewitt 1985, 1989). In addition, all plumage characters change abruptly from the Townsend's phenotype to the midpoint phenotype (a distance of 25 km), but change more gradually from the midpoint phenotype to the Hermit phenotype (a distance of 100 km). This pattern suggests that Townsend's Warbler alleles are flowing uniformly into Hermit Warbler populations. These hybrid zones may be moving because of changing environmental conditions or because Townsend's Warblers have a selective advantage over Hermit Warblers.