By J. Brian Davis, Ph.D.
The 1930s marked the rise of the wildlife management profession, as well as experimentation with the use of manmade nest boxes to help revive wood duck populations depleted by years of habitat loss and overharvest. Dr. Frank Bellrose, regarded as the father of wood duck management, and his colleague Arthur Hawkins began using nest boxes and studying nesting female wood ducks at Chautauqua National Wildlife Refuge in Illinois. Since their pioneering efforts, nest box programs have become popular among government agencies, conservation organizations, and private citizens for studying and propagating wood ducks.
The basic breeding habitat requirements of wood ducks are nesting cavities and adjacent wetland habitat containing trees, shrubs, or dense herbaceous vegetation that provide cover and an abundance of aquatic insects for ducklings. The use of nest boxes by wood ducks varies among regions and is influenced by the availability of suitable natural tree cavities, the proximity of attractive wetlands, and other factors. Researchers have determined that the use of nest boxes by wood ducks may average 44 percent in northern states in the Atlantic Flyway and only 16 percent in northern Mississippi Flyway states. However, in the southern United States it is not uncommon for 75 to 100 percent of nest boxes to be used by wood ducks on specific wetlands. While data about the availability of natural nest cavities in southern states has not been updated in nearly 30 years, the high occupancy rates of nest boxes by wood ducks in this region might indicate an inadequate supply of suitable nest sites in some areas.
Habitat conditions for wood ducks are much different in the Central and Pacific Flyways, where hardwood forests are often sparse or nonexistent. In the Central Valley of California, for example, natural nesting cavities are scarce, but managed wetlands, riparian corridors, and perhaps flooded rice fields provide an abundance of aquatic insects for breeding wood ducks and their broods. Thus, nest boxes are especially beneficial to wood duck populations in the Central Valley and other regions where natural nesting cavities are in short supply.
Regular maintenance of wood duck boxes is essential to maximize their potential productivity. Boxes should first be inspected in late January or February. Old nesting materials should be cleaned out and replaced with three to four inches of fresh wood shavings. In natural cavities, female wood ducks deposit their clutch of eggs among their own down feathers, rotted wood, and other natural materials. However, wood ducks cannot bring nesting materials into manmade structures. Consequently, it's important for managers to place fresh wood shavings in boxes to create a favorable nesting environment. Managers should also check to ensure that wood duck boxes and predator guards are secure and in the right positions.
Because wood duck boxes may be used multiple times during a single breeding season, they should be checked about once a month from March through late June. The remains of abandoned wood duck nests as well as nesting material placed in boxes by starlings, squirrels, and other unwanted species should be removed during each visit. Federal law prohibits tampering with the eggs of migratory birds, so be certain that nests are no longer viable before conducting maintenance activities. If you do find a female wood duck or an active nest in a box, quietly move away and return at a later date to determine the fate of the nest.
Waterfowl populations are strongly influenced by the survival of breeding female ducks, the success of nesting efforts, and the survival of ducklings. These factors greatly influence recruitment, which is the number of ducklings that survive to join the adult population. In graduate research conducted at Mississippi State University during the late 1990s and early 2000s, my colleagues and I examined these factors in relation to wood ducks using nest boxes on study sites in Mississippi and Alabama. During our research, we found that second and third nesting attempts accounted for 38 to 65 percent of all the ducklings produced during the breeding season. These results underscored the importance of regular wood duck box maintenance. Had we not removed the remains of previous wood duck nesting attempts and other debris from the boxes during our monthly maintenance checks, far fewer ducklings would have been produced on our study sites.
To study duckling survival and other aspects of their ecology after the birds had left our nest boxes, we equipped 135 female wood ducks and more than 400 ducklings with radio transmitters and monitored the birds from March through July.
We determined that duckling survival was low-about 20 percent overall. However, we also discovered that when hens and broods moved, sometimes over long distances, to densely vegetated scrub-shrub habitats that did not contain nest boxes, duckling survival was as high as 60 percent.
Nest boxes are often placed in high densities over water in visible locations. As attractive as this may be to nesting wood ducks and people who enjoy watching them, our research suggests that this practice may also benefit predators of ducklings. As a result, we recommend that nest boxes be widely spaced in flooded scrub-shrub habitat and not placed in open environments, which may attract predators.
While continuing to assess other parts of our nesting and duckling survival data over the past two years, we determined that recruitment of young wood ducks on our two study areas largely occurred from production and survival of ducklings in late spring and summer. In fact, on one study site 68 percent of total duckling recruits were hatched and reared from June through August, while on another study site 91 percent were reared during this same period. We attributed these findings to more vegetation being present in wetlands in late spring and summer than earlier in the breeding season, which likely resulted in more cover and aquatic insects for ducklings. These results again highlight he importance of monitoring and maintaining wood duck nest boxes throughout the breeding season to ensure that they are able to produce ducklings in late spring and summer, when their chances of survival are greatest.
Dr. Brian Davis is a professor of wildlife management in Mississippi State University's Department of Wildlife, Fisheries, and Aquaculture.