Guild structure of a riparian avifauna relative to seasonal cattle grazing
The avifauna within the willow ( Salix spp.) community on the Arapaho National Wildlife Refuge (NWR) was dominated (96% of all observations each year) by 11 species of passerine birds during the summers of 1980-81. Using 28 vegetation variables measured or calculated for randomly selected points and points where birds were sighted, we assigned the species to 3 distinct response guilds relative to historical patterns of seasonal grazing. A eurytopic response guild (habitat generalists) included yellow warblers ( Dendroica petechia ) (YEWA), savannah sparrows ( Passerculus sandwichensis ) (SASP), and song sparrows ( Melospiza melodia ) (SOSP). A stenotopic response guild (habitat specialists) included willow flycatchers ( Empidonax traillii ) (WIFL). Lincoln's sparrows ( Melospiza lincolnii ) (LISP), and white-crowned sparrows ( Zonotrichia leucophrys ) (WCSP). The intermediate, mesotopic response guild included American robins ( Turdus migratorius ) (AMRO), red-winged blackbirds ( Agelaius phoeniceus ) (RWBL), and brown-headed cowbirds ( Molothrus ater ) (BHCO). Population densities of the eurytopic response guild differed little between healthy (historically winter-grazed) and decadent (historically summer-grazed) willow communities within a year. Densities of species in the mesotopic response guild differed more dramatically, and stenotopic response-guild species were absent or accidental in decadent willows. Information on habitat use patterns of the individual species between years supported the definition of response guilds; vegetation structure was most variable in habitats of eurytopic species and least variable in habitats of stenotopic species. Comparisons between used and available vegetation features indicated that species in the stenotopic response guild used locations that differed from random on the basis of bush spacing. We hypothesize that the response-guild structure primarily reflects the impact of cattle upon the horizontal patterning of the vegetative community.