John T. Lokemoen David E. Sharp Harold F. Duebbert 1983 <p>Island-nesting mallards (<i>Anas platyrhynchos</i>) and gadwalls (<i>A. strepera</i>) were studied on a 4.5-ha island in 385-ha Miller Lake in northwestern North Dakota during 1976-80. During the 5-year study, 2,561 duck nests of 9 species were found on Island A located 180 m offshore; 59% were mallard and 34% were gadwall. In patches of shrub cover, which contained the greatest concentrations of nests, densities ranged from 241 to 389 mallard nests/ha and from 139 to 237 gadwall nests/ha. Over 97% of the nests were placed in 4 patches of shrubs totaling about 1 ha of western snowberry (<i>Symphoricarpos occidentalis</i>)--Woods rose (<i>Rosa woodsii</i>) cover, which composed about 30% of the island's vegetation. Average hatching success was 85% for clutches of all species. Abandonment averaged 14% (348 of 2,426 nests) and was the major cause of egg failure. Only 15 nests (&lt;1%) were destroyed, primarily by ring-billed (<i>Larus delawarensis</i>) or California gulls (<i>L. californicus</i>). A minimum of 15,960 ducklings including 9,300 mallards and 5,150 gadwalls hatched on 4.5-ha Island A. Hatching rates of eggs in successful nests averaged 83% for mallards and 87% for gadwalls. Despite the close spacing of nests, most individual hens maintained normal nesting regimes. Eighty-one percent of the mallard clutches contained 7-13 eggs and 81% of the gadwall clutches contained 8-14 eggs. Island A in Miller Lake provides an outstanding example of the potential for high reproduction levels of mallards and gadwalls nesting in small areas of predator-free habitats.</p> application/pdf 10.2307/3808608 en Wiley Concentrated nesting of mallards and gadwalls on Miller Lake Island, North Dakota article