THE EFFECTS OF HABITAT FRAGMENTATION ON MIDWESTERN GRASSLAND BIRD COMMUNITIES

The influence of area and vegetation structure on breeding bird commun ities associated with 24 Illinois grassland fragments (0.5-600 ha) was studied between 1987 and 1989 to document the effects of habitat frag mentation in a severely fragmented midwestern landscape. Fragment area strongly influenced bird communities within grasslands and accounted for a high percentage of the variation in mean breeding bird species r ichness among fragments (R2 = 0.84). Breeding bird species richness pa tterns within 4.5-ha subsections of these grasslands also significantl y increased with fragment size. Eight of the 1 5 (53%) most common bir d species had distributions among fragments that were significantly in fluenced by habitat area, whereas six species (40%) had distributions within fragments that were significantly influenced by vegetation stru cture only. The Dickcissel (Spiza americana) was the only species with a distribution within fragments that was not significantly associated with either habitat area or vegetation structure. Four groups of bird s were identified by an analysis of habitat area and vegetation struct ure preferences of individual species: area-sensitive species (5 speci es), edge species (3), vegetation-restricted species (6), and the Dick cissel. Estimates of minimal area requirements for the five area-sensi tive species ranged from 5 to 55 ha. Discriminant analyses of habitat suitability within fragments suggests that the absence of area-sensiti ve grassland bird species from some small fragments may result, in par t, from limited habitat availability. All five area-sensitive species, however, also regularly avoided structurally suitable habitat on smal l grassland fragments. As a result of the considerable extent to which native and, more recently, agricultural grasslands have declined in t he Midwest, habitat fragmentation is likely to have caused midwestern grassland bird declines, especially for area-sensitive species.