Existing information on bird species composition, abundance and nesting status during the breeding season (May through July) was compiled for habitats characteristic of the agricultural landscapes of Iowa. Data were derived from 60 sources for 144 bird species in 20 habitats. Total numbers of breeding bird species were highest in floodplain forest (107 species) and upland forest (85), and lowest in small grains (31) and herbaceous fencerows (27). Species abundances were standardized and categorized on a scale from 0 (absent) through 5 (very abundant with >250 individual birds/census count/100 ha). Bird species abundances are lowest in some agricultural habitats (e.g., tilled row crops and small grains) and highest in narrow, strip-cover habitats (e.g., railroad rights-of-way, wooded fencerows and farmstead shelterbelts). Species abundance patterns in natural habitats (forest, marsh and prairie) are intermediate between those in agricultural and strip-cover habitats. Twenty-five species occurred only in forest habitats and 14 only in marshes. Other species selectively (through not exclusively) use tilled row crop, grassland or wooded habitats. Principal components analysis was used to assess the relative similarities in use of the 20 habitats by the assemblage of breeding birds in Iowa. Predicted numbers of nesting species increased from 18 to 93 over four landscape scenarios representing a progression from an intensively farmed row-crop monoculture to a diverse mosaic of crop and noncrop habitats. Although laborious, the approach developed in our study has been useful for standardizing and synthesizing a diverse literature in efforts to conduct ecological risk assessments for farmland birds. It can also provide valuable baseline data for landscape-level research.
