The influence of habitat features on grassland birds nesting in the Prairie Pothole Region of North Dakota
Results of habitat fragmentation studies on grassland and wetland birds are not consistent. Some studies have found positive relationships among abundance, nest success, field size, and distance to edges, whereas others have found no relationship. One reason for differing results may be the landscape composition in which the study took place. I examined how landscape composition influenced relations among: (1) occurrence and abundance of grassland songbirds and field size, and (2) nest success of ducks, field size, and edges. I also investigated the effects of mowing on grassland songbirds, and how landscape features, such as amount of perennial grassland, and predator community composition influenced the nest success of ducks in fields. The study took place in the Prairie Pothole Region of central North Dakota during the 1996--1997 breeding seasons. Two types of 6.4 x 6.4 km study areas were selected based on the amount of perennial grassland they contained: 15--20%, and 51--55%. The remaining portion of the study areas was primarily cropland and wetland. within the four study areas used, I conducted 281 point counts and found 1,810 duck nests in;46 grassland fields, enrolled in the Conservation Reserve Program, ranging in size from 2--606 ha. Landscape composition influenced relations between field size and relative abundance of 3 grassland songbird species. As the amount of perennial grassland in the landscape increased, the species, abundances in smaller fields increased. Nest success of ducks in fields increased as field size increased, and was greater in study areas with 51--55% grassland compared to 15--20% grassland. I detected an interaction between nest success of ducks and distance to nearest field edge. In study areas with 51--55% grassland, I found a positive relationship between probability of an individual nest hatching and distance to nearest field edge, while in study areas with 15--20% grassland, I found no relationship and probability of hatching was low. Future studies examining the influence of habitat loss and fragmentation on grassland and wetland birds should take into account the landscape context in which the study takes place.