Scale-dependent habitat use in three species of prairie wetland birds

We evaluated the influence of scale on habitat use for three wetland-obliga te bird species with divergent life history characteristics and possible sc ale-dependent criteria for nesting and foraging in South Dakota, USA. A str atified, two-stage cluster sample was used to randomly select survey wetlan ds within strata defined by region, wetland density, and wetland surface ar ea. We used 18-m (0.1 ha) fixed radius circular-plots to survey birds in 41 2 semipermanent wetlands during the summers of 1995 and 1996. Variation in habitat use by pied-billed grebes (Podilymbus podiceps) and yellow-headed b lackbirds (Xanthocephalus xanthocephalus), two sedentary species that rarel y exploit resources outside the vicinity of nest wetlands, was explained so lely by within-patch variation. Yellow-headed blackbirds were a cosmopolita n species that commonly nested in small wetlands, whereas pied-billed grebe s were an area-sensitive species that used larger wetlands regardless of la ndscape pattern. Area requirements for black terns (Chlidonias niger), a va gile species that typically forages up to 4 km away from the nest wetland, fluctuated in response to landscape structure. Black tern area requirements were small (6.5 ha) in heterogeneous landscapes compared to those in homog eneous landscapes (15.4-32.6 ha). Low wetland density landscapes composed o f small wetlands, where few nesting wetlands occurred and potential food so urces were spread over large distances, were not widely used by black terns . Landscape-level measurements related to black tern occurrence extended pa st relationships between wetlands into the surrounding matrix. Black terns were more likely to occur in landscapes where grasslands had not been tille d for agricultural production. Our findings represent empirical evidence th at characteristics of entire landscapes, rather than individual patches, mu st be quantified to assess habitat suitability for wide-ranging species tha t use resources over large areas.