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Entrance Holes
It is important to make the entrance hole large enough to accommodate the largest cavity-nesters found in your area. In most cases, an oval entrance hole 3 inches high by 4 inches wide will be suitable for wood ducks, hooded mergansers, and buffleheads. If common goldeneyes are found in your area, an oval entrance 3 ½ inches high by 4 ½ inches wide is needed (Figure 12).

Entrance Placement
In North Dakota, cavity-nesting ducks are found along streams, lakes and reservoirs which have wooded shorelines. Nest structures erected in ponds in open country far from trees are not likely to be used by cavity-nesting ducks. On the other hand, nest boxes placed on ponds near wooded streams may be used.

Wood ducks tolerate some human activity and will often nest in boxes placed in trees in towns and cities which are located along a river or stream. All the cavity-nesters discussed have been known to nest in trees as far as one-half mile from a stream or lake. In general, however, nest boxes placed closer to water are more likely to be used and are considered safer because they reduce the distance newly hatched ducklings must travel to water.

Cavity-nesting ducks usually do not object to others of their kind nesting nearby. For this reason two or more boxes placed together may be used successfully in the same season. In general, it is recommended that one to six nest structures be placed in an area during the first year. Houses should be placed in the best locations at least 50 to 100 feet apart. If the nest boxes are used successfully, and the population increases, more structures can be added in the area.

Because some cavity-nesting ducks begin to nest in North Dakota in late April or early May, it is advisable to have the houses in place by late March.

Nest boxes may be placed either over water or over land. Generally speaking, structures over water receive more use by cavity-nesters than those far from water. Overwater structures can be supported by trees or snags surrounded by water, or by poles or pipes driven into the pond or lake bottom.

When placed over land, nesting structures are attached to trees, wood poles, metal pipes or posts. If nest boxes are placed close to water's edge, extra precautions must be taken to ensure that raccoons can not reach the nests. Raccoons travel along the shoreline, and boxes placed there are more likely to be found and the nests destroyed. Those nest boxes placed a distance back from the shore are less likely to be encountered by raccoons.

In general, nest structures should never be located far from water or trees. Ideally, boxes on land should be 30 to 100 feet from the water's edge. While nest boxes can be placed up to half a mile from lakes, ponds, marshes and rivers, some precautions should be taken when putting them this far from water. Since the hen must lead her ducklings to water after they hatch, the terrain between the nest box location and the water's edge should be free of major obstacles like highways, fences with small mesh wire, and high street curbing. When hatching occurs away from the water, the female will immediately lead the young overland to water. At that time the ducklings are extremely vulnerable to predators. These factors should be carefully considered in the selection of nest box sites.

Nest boxes placed over land on trees or on poles or other supports should be at least 8 feet above ground. It is recommended that the box entrance face the water, and that there be no branches or other obstacles for at least 30 feet in front of the entrance. Place the house so that the entrance is clearly visible, with no limbs or branches obstructing the ducks' view or flying approach. Hens tend to avoid selecting houses that are shielded by saplings or overhanging branches. It appears that nest boxes placed in comparatively open stands of mature trees with large spreading limbs are more likely to be used than houses placed in dense stands of young trees. Cavity-nesting ducks prefer to fly to their nests through a relatively open canopy and to perch on large, horizontal limbs near their nests. In some situations, dead trees provide a satisfactory place to locate nesting boxes. Boxes should not be placed on or near trees where they are vulnerable to predators that may reach the box from above by means of branches or other trees.