Seasonal Variation in the Foraging Behavior of Some Migratory Western Wood Warblers

-I observed the foraging behavior of four warbler species (Dendroica petechia, Oporornis tolmiei, Geothlypis trichas, and Wilsonia pusilla) in the summer in Wyoming and in the winter in Nayarit, Mexico. Of six variables (absolute foraging height, relative foraging height, vegetation density, horizontal foraging position, feeding method, and foraging substrate) believed to be potentially important in distinguishing the warbler species ecologically, the two foragingheight variables provided the greatest separation of the four species in both summer and winter. An analysis of the behavioral similarity of each species from summer to winter revealed that feeding method was the least changed behavior and that absolute foraging height involved the greatest behavioral flexibility. The behaviors that are most flexible are possibly the least well tied to the birds' morphology and are also the ones that have been shown by other workers to reveal the effects of competitors through "niche shifts." Therefore, ecological relationships among coexisting species (in terms of overlaps or positions in niche space) may never be fully derivable from morphological information alone. Received 6 March 1980, accepted 24 March 1981. ECOLOGICAL studies of Nearctic migrants on their breeding grounds are relatively common, but such studies on their wintering grounds are less common (but see Eaton 1953, Schwartz 1964, Willis 1966, Lack and Lack 1972, Leck 1972, Tramer 1974, Rappole 1975, Chipley 1976, Karr 1976, Mills 1976, Post 1978, Wilz and Giampa 1978, or some of the more recent studies included in Keast and Morton 1980). Comparative studies that deal with the ecology of migratory species on both their breeding and wintering grounds are rarer still (but see Root 1967, Baker and Baker 1973, Lack 1976, Bennett 1980, Rabenold 1980). Only through such comparative studies will we begin to understand the extent to which the morphology of a species represents a compromise among behaviors that vary seasonally (Fretwell 1972). In this paper I quantify the foraging behavior of four species of migratory wood warblers (Yellow Warbler, Dendroica petechia; MacGillivray's Warbler, Oporornis tolmiei; Common Yellowthroat, Geothlypis trichas; and Wilson's Warbler, Wilsonia pusilla) that can be found syntopically during both the breeding and nonbreeding seasons in western North America to determine (1) whether several potential mechanisms of ecological isolation change seasonally, and (2) which aspect of foraging behavior shows the grestest seasonal flexibility. STUDY SITES AND METHODS I collected foraging data on the four warbler species from 20 May-20 June 1975 within 4 ha of willow (Salix) habitat adjacent to the Jackson Hole Biological Research Station, Grand Teton National Park Wyoming (43?52'N, 1 10'34'W). Willows were the only large plants growing in the study area, and the open areas between them were filled with grasses and sedges. A foliage-height profile, which depicts the porportion of vegetation at different heights, is given in Fig. 1. Winter foraging data were collected from 15 January to 15 February 1975 and 1976 in second growth Present address: Department of Zoology, University of Montana, Missoula, Montana 59812. 765 The Auk 98: 765-777. October 1981 This content downloaded from 150.131.112.212 on Wed, 16 Oct 2013 20:33:09 PM All use subject to JSTOR Terms and Conditions 766 RICHARD L. HUTTO [Auk, Vol. 98