Geographic Variation in Habitat Structure of the Black-Throated Green Warbler (Dendroica virens)

--Habitat structure of the Black-throated Green Warbler (Dendroica virens) was examined at five study sites: (1) Mount Desert Island, Maine; (2) Mount Blue State Park, Maine; (3) White Mountain National Forest, New Hampshire; (4) southern Adirondacks, New York; and (5) Itasca State Park, Minnesota. Principal component analysis of 13 habitatstructure variables measured at each site produced habitat gradients from tall to shorter canopies, large to smaller trees, and coniferous to deciduous forests. A second ordination indicated that the habitat sampled included five plant-community types: pine forests, sprucearbor vitae, balsam fir, mixed spruce-fir-deciduous, and beech-maple-birch. Consistent structural features within the total range of habitats sampled were difficult to identify. I suggest that widely occurring species such as the Black-throated Green Warbler have a wide range of habitats with a suitable structure and that regional analyses, even within a single plant-community type, may be of limited value with regard to habitat management when considering the entire range of many species. Received 10 September 1981, resubmitted 24 May 1982, accepted 20 October 1982. GEOGRAPHIC variation has been analyzed for several aspects of avian biology--song dialects (Nottebohm 1975), morphology (James 1970), foraging behavior (Roth 1979), and plumage (Pratt 1980)--yet most analyses of avian habitat have been confined to single-site comparisions (Whitmore 1975, Smith 1977, Collins et al. 1982). Community-level comparisons between sites are common (e.g. Karr 1971, Rabenold 1978, Rotenberry and Wiens 1980), but these studies were based upon a defined habitat structure so that bird-species diversity, niche parameters, and behavior could be compared within a similar habitat type. Some detailed analyses of geographic habitat differences for a single species do exist (Noon and Able 1978, Cody 1978, Noon et al. 1980, Noon 1981). In particular, Noon et al. (1980) determined that habitat structure was consistent within the range of most eastern United States breeding birds. Their analysis was cast in a community-oriented context, however, so some important differences in habitat structure between sites may not have been detectable with their techniques. In this study I have used the 0.04-ha circle technique, a more direct approach, to elucidate local habitat structure at five sites within the range of the Black-throated Green Warbler (Dendroica virens). In this paper, "habitat" refers to the specific life form or structure of the vegetation in which a species 382 occurs (Grinnell 1917, James 1971). The Blackthroated Green Warbler was selected because of its wide geographic range and because this species is reported from a number of plantcommunity types (Brooks 1940, Bent 1953), including pine forests (Collins et al. 1982), deciduous vegetation (Maurer and Whitmore 1981), and spruce-fir (Picea-Abies) forests in the northeast (MacArthur 1958). Although differences in vegetation structure between plantcommunity types would appear obvious intuitively some characteristics of this species' habitat may not vary between sites. Therefore, data were gathered in Maine, New Hampshire, New York, and Minnesota to determine: (1) is the habitat structure of the Black-throated Green Warbler different at several points in its range, and (2) if differences exist, which structural variables change and which are similar between sites? Knowledge of the range of habitat structure may facilitate development of regional models for predicting the future extent of a species' habitat (Shugart 1981). MATERIALS AND METHODS To analyze patterns of habitat structure of the Blackthroated Green Warbler, five study sites were selected: (1) Mount Desert Island (MDI), a large island about 300 m off the coast of Maine, the vegetation of which consists of large areas of spruce and arbor vitae (Thuja occidentalis) forests (Davis 1966) and of The Auk 100: 382-389. April 1983 April 1983] Geographic Variation in Dendroica Habitat 383 TABLE 1. Mean and standard error for the habitat-structure variables per 0.04-ha sample. Differences were tested by Kruskal-Wallis one-way analysis of variance (H-statistic). Site abbreviations are given in the text; variable abbreviations are presented in Table 2. Site GC SC CC CH CO SPT T1 T2 T3 T4 T5 T6 T7 MDI J 52.5 21.0 84.0 46.2 79.0 5.5 44.2 43.2 15.7 7.0 2.9 0.0 0.0 (n = 10) SE 6.8 4.5 2.9 1.9 3.5 0.7 6.7 3.4 1.4 0.8 1.0 0.0 0.0 MBP J 36.8 32.7 92.7 46.7 34.5 8.5 34.4 21.4 9.9 3.8 1.5 0.6 0.2 (n = 11) SE 3.9 5.8 1.9 2.3 5.4 0.6 4.0 3.1 1.4 0.6 0.5 0.4 0.1 WMF ;r 45.5 43.5 94.5 49.9 23.5 6.9 29.5 19.0 10.6 4.9 3.9 1.4 0.3 (n = 10) SE 5.9 7.7 1.8 1.6 5.2 0.5 2.6 2.2 1.1 0.7 0.6 0.2 0.2 NYA J 28.8 78.8 92.5 66.8 36.2 7.0 25.5 6.5 2.2 4.2 4.8 3.0 2.2 (n = 4) SE 6.2 2.7 2.2 4.1 5.7 0.9 2.8 1.3 0.4 1.0 0.9 0.6 0.7 ISP J 52.7 39.0 84.3 66.1 50.3 5.7 15.1 10.5 5.7 4.4 3.3 1.9 0.9 (n = 15) SE 4.3 5.7 3.4 3.8 7.7 0.3 2.2 1.5 0.7 0.9 0.7 0.7 3.9