By J. Michael Checkett
North American waterfowl nest across an immense geographical area ranging from the High Arctic to the marshes along the Gulf Coast. Reproductive strategies have evolved in keeping with the selective pressures (habitat variability, predation, etc.) associated with these varied environments. Renesting is one of several reproductive strategies employed by hens to exploit local conditions and improve the odds of successfully hatching a nest.
Nesting waterfowl live in an ever-changing environment where favorable wetland conditions may occur only four years out of 10. Predators loom, and tilling, haying and burning of peripheral cover during the nesting season further reduce available nesting habitat and destroy nests. Each year a large portion of the breeding duck population loses initial nest attempts, and individual hens must decide if they will attempt to nest again. Most will shift to new sites, and some may travel to new regions to continue the nesting effort.
Renesting, laying a replacement clutch following the loss of a previous nest, is an important reproductive strategy ducks of many species employ. It allows individual hens to have additional opportunities to hatch a nest after their earlier nesting attempts failed, compensating for high rates of initial nest loss common to ground-nesting birds. The old adage "persistence pays" has never held truer than for many waterfowl species. When nesting conditions are favorable, many duck species will go to great lengths to pull off a successful clutch. Whether a duck renests depends on many factors that vary annually and geographically, resulting in wide differences in renesting propensity and productivity among populations and species.
Renesting propensity generally varies with length of breeding season and depends on stage of incubation and date at the time of nest loss. Further, older birds may be more likely to renest, possibly because they tend to begin nesting earlier and may be in better physical condition. Productive wetlands are also vital for nesting because they provide hens with the nutrients required for egg production. Favorable habitat conditions benefit ducks in many ways, but good wetland conditions, allowing for adequate body reserves for egg laying and providing food and cover for ducklings, are believed to be the major factor affecting frequency of renesting. Conversely, in years of average conditions or areas with poor habitat, birds will be less likely to try to renest, and in drought years, some may forego nesting altogether.
Renesting behavior is different for each duck and goose species. Mallards, for example, are more likely to renest than are lesser scaup and gadwalls, because mallards are early nesters (three to four weeks earlier). If a mallard hen's first nest fails, she usually has time left to renest and have the brood gain flight before the end of the brood-rearing season. Mallards have been documented to make up to six nesting attempts, but many late-nesting prairie species (scaup, gadwalls and blue-winged teal) may have time or resources to renest only one to three times. Some species like northern pintails are not frequent renesters.
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.