Unlike most ducks that breed in northern latitudes, some southern-breeding species like wood ducks begin nesting in late January, which allows them to employ a strategy called “double brooding,” or hatching two nests in one season. By nesting in tree cavities, wood ducks improve their initial nest success, and up to 11.5 percent of females may produce two broods in a single nesting season.
On the other end of the spectrum, renesting is virtually absent in swans and Arctic-nesting geese. These birds rarely renest because they are subject to greater time and energy constraints imposed by a particularly short breeding season and because they rely on nutrient reserves acquired during spring migration and carried within their bodies to the breeding grounds.
Interestingly, most duck species tend to have a high reproductive potential (allowing them to exploit favorable conditions) but correspondingly low nest success and adult survival rates because predation has always taken a large toll on duck nests and ducks. Predators and their prey have been involved in evolutionary warfare for eons, each adapting to improve efficiency at either capturing prey or escaping predators. Although populations may wax and wane with changes in weather and local habitat conditions, given time and a fairly natural environment, an uneasy balance is generally maintained that allows populations of both to persist. For waterfowl, ultimately only a small percentage of nests need to hatch to maintain most populations. Recent estimates by the Institute for Wetland and Waterfowl Research (IWWR), DU’s research arm in Canada, indicate that midcontinent mallard population stability may be achieved with an average nest success rate of 11.5 percent, rather than 15 percent as was believed in the past. Other duck species are believed to need 15 to 20 percent nest success.
Research on nest success looking back to the 1930s, even before so many wetlands were drained and grass was plowed under, indicates nest success for prairie dabbling ducks was variable and never very high. Current research indicates that nest success continues to fluctuate over time and generally averages between 10 and 40 percent. Though predators have always taken the majority of nests, the good news is that each year some percentage of duck nests will always hatch. Renesting offsets the effects of nest predation as the more nests that are attempted, the more nests will hatch. However, there is a vital trade-off. The more times a hen nests, the more she exposes herself to becoming prey. Thus, hens will most commonly renest when good habitat provides reliable food resources and cover for hens and ducklings. In dynamic environments such as the Prairie Pothole Region, annual recruitment rates of ducks are sensitive to the amount of renesting that occurs. Recent research conducted by IWWR and partners highlighted the importance of events that occur on the breeding grounds to population growth.
The research revealed that nest success (which includes renesting) is mainly related to landscape habitat and annual conditions. These findings have big implications for duck hunters. Because of ongoing habitat loss, waterfowl today have fewer wetlands and grasslands to nest in, and the fragmentation of this habitat has changed the predator-prey relationship. As would be expected, fragmentation and loss of habitat concentrate ducks and predators on available habitat. Low nest success and low renesting probability are simply symptoms of poor habitat.
While there is little we can do about annual precipitation, we can improve future reproductive success by protecting existing wetlands and restoring enough grassland in the right places to allow enough nests to hit their mark in favorable cover. Habitat keeps the predator-prey relationship in check and improves the likelihood a hen will hatch a nest, whether it is her first attempt or a renest. Therefore, when conditions are good, for many hens, persistence will pay off.
Mike Checkett is a regional biologist at DU’s national headquarters in Memphis.