Bird abundance and nesting success in Iowa CRP fields: The importance of vegetation structure and composition

Bird use of Conservation Reserve Program (CRP) and row-crop fields was studied in central Iowa from May through July 1991-1993. Thirty-three bird species were recorded in CRP fields and 34 in row-crop fields. The most abundant species in both habitats was the red-winged blackbird (Agelaius phoeniceus), accounting for 35% of all birds in CRP and 24% in row-crop fields. The dickcissel (Spiza americana), grasshopper sparrow (Ammodramus savannarum), bobolink (Dolichonyx oryzivorus), common yellowthroat (Geothypis trichas), brown-headed cowbird (Molothrus ater), savannah sparrow (Passerculus sandwichensis) and ring-necked pheasant (Phasianus colchicus) were the next most abundant species in CRP plots. The horned lark (Eremophila alpestris), vesper sparrow (Pooecetes gramineus) and brown-headed cowbird were the next most abundant species in row-crop fields. Nests of 16 bird species were found in CRP fields, with red-winged blackbirds accounting for 48% of all nests found. The vesper sparrow and horned lark were the only species nesting in row-crop fields. The major cause of nest loss for all species was predation, accounting for 52% of all nest loss in CRP fields and 65% in row-crop fields. Mammals accounted for 89, 88 and 85% of the predation on grasshopper sparrow red-winged blackbird and dickcissel nests, respectively. The Conservation Reserve Program has likely contributed to an increase in the abundance of many bird species in central Iowa, inasmuch as the row-crop habitat that it replaced has lower bird abundance and supports fewer nesting species. The vegetation structure and composition of CRP fields in central Iowa are diverse, resulting in differences in the bird species communities using these fields. The effects of several land-management practices are discussed relative to bird species composition and nesting success.