Habitat Selection by Birds of Riparian Communities: Evaluating Effects of Habitat Alterations

Abstract

Avifauna of riparian communities were studied in Iowa during late spring and early summer. Birds were censused on 28 study plots representing a habitat gradient from hayfields to closed-canopy woodlands. An index of nesting niche breadth was determined for 18 open-nesting bird species on the basis of vegetation life form(s) used for nesting, and for 10 cavity-nesting species on the basis of type(s) of nest-cavity support. Cavity-nesters preferred soft snags as nest sites. Floodplain woodlands supported higher densities of breeding birds than upland woodland or herbaceous habitats. Bird species richness increased (P < 0.01) with the width of wooded riparian habitats. Wooded habitats supported a maximum of 32 species; herbaceous habitats, 8. Observation frequencies of 41 bird species in 6 general habitat types were used to calculate indices of tolerance to habitat alteration. Microhabitat characteristics selected by each species were determined by comparing bird observation frequencies with 36 vegetation variables, using stepwise multiple regression. The potential effects of 6 alterations to wooded riparian habitats on the 41 species are predicted. J. WILDL. MANAGE. 44(1):1-15 Riparian habitats are critical to wildlife, especially in regions with intensive agriculture where streamside habitat is being depleted rapidly. Nongame birds constitute a major component of riparian wildlife communities, but most research concerning nongame bird management has been local in nature or concerned with endangered species (Robbins and Erskine 1975). Recently, contributors to several symposia have emphasized the need for research on management of all nongame birds (Smith 1975; DeGraaf 1978a,b). Such research is essential to assess the impacts of habitat alterations on the associated wildlife (Lennartz and Bjugstad 1975). Our study is an attempt to satisfy part of that need. Our objectives were to quantify factors critical in nest-site and habitat selection by breeding birds of riparian communities over a range of habitat types and to use the results to evaluate effects of habitat perturbations on avian communities. The results are applicable to both riparian and upland habitats of the Central Plains and, with caution, could be used in other areas as well. We are grateful to the landowners of Guthrie County, Iowa, for their cooperation in all phases of the field work. T. Rosburg and R. Deitchler assisted in collecting field data. D. F. Cox, L. Wolins, M. Hand, and B. J. Meador provided help with the statistics and computer analysis. J. J. Dinsmore, R. B. Dahlgren, and R. Q. Landers reviewed an earlier draft of the manuscript. The manuscript also benefited from the suggestions of 2 anonymous referees.