Influence of perennial upland cover on occupancy of nesting structures by mallards in northeastern North Dakota

Artificial nesting structures are considered to be most attractive to nesting female mallards (Anas platyrhynchos) in areas where natural nesting cover is scarce, leading to the management recommendation to place structures in agricultural landscapes with little perennial upland cover. In 1997-1998, we compared occupancy rates of 260 nesting structures installed on 13 10.4-km² sites in northeastern North Dakota where amount of wetland habitat was comparable but amount of perennial upland cover was either high (x̄=44.8%, grassland sites) or low (x̄=8.0%, cropland sites). Contrary to expectation, occupancy rates were >4 times greater on grassland sites than on cropland sites (17.8 vs. 3.9%, P=0.003). The difference was largely a function of greater average mallard densities on grassland versus cropland sites (15.2 vs. 9.2 pairs/km², P≤0.002). When pair density was controlled for statistically, occupancy rates of structures did not differ between grassland and cropland sites (14.5 vs. 7.9%, P=0.22). We conclude that placing structures in areas with high mallard density is the most effective way to maximize initial occupancy and that proportion of existing perennial upland cover in a landscape has little direct effect on structure occupancy.