The structure of western warbler assemblages: Analysis of foraging behavior and habitat selection in Oregon

Abstract

-This study examines the foraging behavior and habitat selection of a MacGillivray's (Oporornis tolmiei)-Orange-crowned (Vermivora celata)-Wilson's (Wilsonia pusilla) warbler assemblage that occurred on early-growth clearcuts in western Oregon during breeding. Sites were divided into two groups based on the presence or absence of deciduous trees. Density estimates for each species were nearly identical between site classes except for Wilson's, whose density declined on nondeciduous tree sites. Analysis of vegetation parameters within the territories of the species identified deciduous tree cover as the variable of primary importance in the separation of warblers on each site, so that the assemblage could be arranged on a continuum of increasing deciduous tree cover. MacGillivray's and Wilson's extensively used shrub cover and deciduous tree cover, respectively; Orange-crowns were associated with both vegetation types. When the deciduous tree cover was reduced, Orange-crowns concentrated foraging activities in shrub cover and maintained nondisturbance densities. Indices of foraging-height diversity showed a marked decrease after the removal of deciduous trees. All species except MacGillivray's foraged lower in the vegatative substrate on the nondeciduous tree sites; MacGillivray's concentrated foraging activities in the low shrub cover on both sites. Indices of foraging overlap revealed a general pattern of decreased segregation by habitat after removal of deciduous trees. I suggest that the basic patterns of foraging behavior and habitat selection evidenced today in western North America were initially developed by ancestral warblers before their invasion of the west. Species successfully colonizing western habitats were probably preadapted to the conditions they encountered, with new habitats occupied without obvious evolutionary modifications. Received 18 August 1980, accepted 20 February 1981. STUDIES of similar coexisting species usually enumerate ecological differences that presumably reduce the demand upon common resources and allow coexistence (Williams and Batzli 1979). MacArthur's (1958) classic study of niche segregation by sympatric Dendroica, followed by a series of similar studies by Morse (1967, 1968, 1971, 1973, 1976) and Ficken et al. (1968), which dealt with several genera, led to the conclusion that competition for limited resources played the primary role in structuring warbler species assemblages in the eastern United States. The identification of ecological differences alone, however, does not demonstrate the role of competition in structuring avian communities (Schoener 1974). Experimental manipulations are useful means of examining the role of foraging plasticity in species coexistence (Williams and Batzli 1979, Landres and MacMahon 1980). This study was designed to (1) document the foraging behavior, habitat selection, and population densities of a MacGillivray's (Oporornis tolmiei)-Orange-crowned (Vermnivora celata)-Wilson's (Wilsonia pusilla) warbler assemblage that occurred on clearcuts in the Oregon Coast Range, and (2) assess the response of this assemblage to removal of the deciduous tree component on the clearcuts. Results are related to an hypothesis about the formation and structure of western warbler assemblages. The species observed in this study breed commonly in many communities in western North America. In addition, races of the Wilson's Warbler range eastward 5 78 The Auk 98: 578-588. July 1981 This content downloaded from 157.55.39.215 on Wed, 31 Aug 2016 05:01:02 UTC All use subject to http://about.jstor.org/terms July 1981] Western Warbler Assemblages 579 TABLE 1. Description of study sites. Years Site postUSFS size Elevation plantSite name number (ha) (m) Aspect inge Location Deciduous tree (DT) group Bennerb.c 3403-51 21 300-450 NE 5 Lincoln County, Oregon (T14S, R9W; sec. 14, 23) Dill Pickleb 3502-16 31 110-280 NE 7 Lane County, Oregon (T15S, R9W; sec. 17) Non-deciduous tree (ND) group Green Divideb,c 3503-38 36 150-400 N 5 Lane County, Oregon (T15S, R9W; sec. 31) Bear Creekb 3404-51 24 150-370 NE/SE 5 Lincoln County, Oregon (T15S, RlOW; sec. 4, 5) Age of conifer at beginning of study. b Studied in 1979. c Studied in 1980. across Canada and the northern United States to Newfoundland, Ontario, Vermont, and Maine (A.O.U. 1957). These warblers are usually described as common inhabitants of brushlands and second-growth forests, although the Wilson's and, to a lesser extent, the Orange-crown, are often associated with riparian vegetation (Gabrielson and Jewett 1940, Bent 1953). In western Oregon, the species are primarily summer residents, arriving in early May and departing in August or September for southern wintering grounds (Gabrielson and Jewett 1940, A.O.U. 1957). STUDY AREAS AND METHODS