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The Foraging of Warblers Isolated on Small Island

1971; Wiley; Volume: 52; Issue: 2 Linguagem: Inglês

10.2307/1934580

1939-9170

Douglass H. Morse,

Rangeland and Wildlife Management

In this paper I attempt to answer the question, What (if any) differences occur in the foraging of isolated wood warblers (Parulidae) and ones in large multispecies populations? Parula (Parula americana), Myrtle (Dendroica coronata), and Black‐throated Green (D. virens) Warblers were studied on seven small spruce‐clad islands off the coast of Maine, each island supporting one pair of one to three of these species. Data obtained were compared with earlier studies of these species in large adjacent populations. Though seasonally isolated, all individuals belonged to the large gene pools of the appropriate species on the adjoining mainland. While the insular forests resemble those of the mainland, they are in general shorter and more open. When only one species was present, it was always the Parula Warbler; when two species were present, they were always the Parula and Myrtle. Black‐throated Green Warblers only occurred in the presence of the latter two species. diversity increased as the size of forest and proportion of foliage in tree crowns grew. Parula and Myrtle Warblers experienced greater nesting success than Black‐throated Green Warblers. Black‐throated Green Warblers foraged much as they did in large populations; however, Myrtle and Parula Warblers demonstrated considerably more plasticity in foraging. When Black‐throated Green Warblers were absent, the two other species tended to modify their foraging. On the one island studied in detail where only Parula Warblers occurred, they appeared to expand their foraging range farther than where Myrtle Warblers were present. Overlap in foraging usually decreased as species compositions became more depauperate. Though variability occurred among individuals of each species, variability of an individual in two different years appeared as great. Black‐throated Green Warblers dominate the other two species socially, and the implications of this situation upon distributions in larger populations and on small islands are discussed. I hypothesize that the stereotypy of the Black‐throated Green Warbler represents an adaptation to the extremely high density it experiences in large populations, which make up the major part of the gene pool. The higher degree of plasticity seen in Parula and Myrtle Warblers may be appropriate to their subordinate roles.