Determinants of mallard and gadwall nesting on constructed islands in North Dakota
Constructed islands with adequate nesting cover provide secure nesting sites for ducks because islands restrict access by mammalian predators. These islands are costly to construct and should be placed in areas that ensure the greatest use by nesting ducks. We studied mallard (Anas platyrhynchos) and gadwall (A. strepera) nesting on constructed islands in North Dakota in 1996 (n = 20) and 1997 (n = 22) to evaluate factors—particularly amount of perennial grass cover in the surrounding landscape and density of breeding pairs—that possibly influence numbers of initiated nests. We also examined effects of island characteristics, such as island vegetation, on numbers of nests. Numbers of mallard and gadwall nests on islands were negatively related to amounts of perennial grass cover in the surrounding uplands. Numbers of mallard nests were positively related to percentages of tall dense cover on islands. We found no effects of breeding-pair density on numbers of nests initiated by either species, possibly because breeding pairs were abundant on all study sites. Percent shrub cover on islands was a better predictor of island use than was percent tall dense cover. Island use by these species increased with island age and distance from mainland shore. Amounts of perennial cover in landscapes should be primary considerations in determining where to build islands. Our data suggest that use of islands by nesting mallards and gadwalls is greatest in landscapes with little perennial grass cover (i.e., high amounts of cropland). Other researchers documented a positive relation between nest success in upland covers and amount of perennial grass cover in the landscape. Therefore, islands constructed in landscapes with little perennial cover should provide greater gains in duck recruitment rates than islands constructed in landscapes with greater amounts of perennial grass cover.