Population Densities and Community Structure of Birds in Jackson Hole: A Reassessment After 25 Years

Abstract

There is a wide literature in bird community ecology that bears on questions of species composition and species densities, and the variations in these attributes within habitats between years and between different geographic regions, and between habitat types both locally and regionally (Cody 1975, 1985; Diamond & Case 1986). While there has always been considerable debate on the constancy of bird communities within habitats and among years, and the extent to which community attributes are predictable and deterministic, rather than variable, stochastic or even chaotic (e.g. Wiens 1985, 1988), recently other developments have accentuated the need for measuring and evaluating bird distributions and densities. There is a fast-developing literature that documents recent (ca. the last decade or two) declines in bird species' distributions and densities at both local and regional scales, and emphasizes in particular evidence for recent reductions in the ranges and densities of bird species that are neotropical migrants (e.g. Hutto 1980, 1986; Keast & Morton 1980; Terborgh 1989; Smithsonian 1991). Given especially the concern that bird species breeding in North American sites and overwintering at lower latitudes (where habitat destruction and fragmentation are particularly rapid), data on status changes in breeding bird communities need to be carefully monitored. The best, perhaps the only, way of doing this is to collect current data, to contrast with comparable data collected in a similar fashion in earlier periods. Thus the rationale for this study is apparent: during 1966-68 I obtained extensive data on the bird communities at two sites in Jackson Hole within Grand Teton National Park, located near the site of the old Research Station on the north side of the Snake River below the dam at Jackson Lake. During two field seasons 1991-92 I reassessed the bird communities at these two sites, with the major objective being a documentation of whether, in which ways, and to what extent, the bird communities of the two sites had changed over the 25-year period. The birds in Jackson Hole are particularly appropriate for this 25-year, then-and-now comparison, since none of the dominant species at the two study sites is resident. But while some species winter almost wholly within the United States (e.g. Fox sparrow (Passerella ilaca), other species winter in northern Mexico in desert habitats (e.g. Brewer's sparrow Spizella breweri, Chipping sparrow Spizella passerina) or west-coastal second growth habitats (e.g. Yellow warbler Dendroica petechia, Common Yellowthroat Geothlypis trichas), and yet others winter much further south into Central America (e.g. MacGillivray's warbler Oporornis tolmei, Wilson's warbler Wilsonia pusilla). The results of the comparison are presented in this report, where differences in species composition and density are revealed in both the short-term (adjacent years) and the longer term (between censuses over 25 years apart). But despite such variations, it will be noted that the overall community structure and composition of the sites has changed little over the quarter century.