BIRD ABUNDANCE AND SPECIES RICHNESS IN GRASSED WATERWAYS IN IOWA ROWCROP FIELDS

Grassed waterways have been used for decades to prevent soil erosion in agricultural cropland, but their benefits to wildlife had not been evaluated prior to our study. We documented bird species composition and relative abundance during the breeding season in 44 grassed waterways in central Iowa. The waterways were planted predominately to smooth brome (Bromus inermis) and were in cornfields and soybean fields. Forty-eight bird species were observed in waterways, compared with only 14 in the surrounding crop fields. Red-winged blackbirds (Agelaius phoeniceus), dickcissels (Spiza americana), barn swallows (Hirundo rustica), western meadowlarks (Sturnella neglecta), brown-headed cowbirds (Molothrus ater), grasshopper sparrows (Ammodramus savannarum) and song sparrows (Melospiza melodia) were the most abundant bird species in the grassed waterways. Total bird abundance in the grassed waterways averaged 2198 birds observed/census/100 ha compared with 682 in crop fields. Temporal patterns in bird abundance were attributed primarily to aspects of the waterways and surrounding cropland that changed over time, such as vegetation height. Because most (53%) of the bird species were at peak abundance in the waterways during 4-22 july, grassed waterways should not be mowed until the end of August or early September.