By Tanner Gue, Kaylan Carrlson, and Stuart Slattery, Ph.D.
We have all heard the expression "timing is everything." As humans, we use a variety of information to help us plan for the future. This is true whether you're a farmer watching next week's weather forecast in anticipation of planting a crop or a stockbroker monitoring market data to make buying and trading decisions. Or maybe you'll use a moon phase calendar to help schedule some vacation time for deer camp or a long-awaited fishing trip. Good timing can maximize reward while reducing risk, effort, or both. This also couldn't be more true for North America's waterfowl.
Unlike tropical nesting birds, which have long nesting seasons, most of this continent's waterfowl migrate north to nest in temperate or tundra latitudes, where the window of opportunity to reproduce is relatively short. Given the importance of the nesting period in their annual life cycle, waterfowl need to have good timing. In general, ducks and geese nest when food resources and habitat conditions are optimal for egg formation and brood rearing.
Like people, waterfowl rely on certain cues to prepare them for important activities. For example, as day length increases in late winter, waterfowl sense that it's time to start "packing" for their long journey north to the breeding grounds. They prepare for the trip by consuming large quantities of high-energy foods such as moist-soil plant seeds, invertebrates, and agricultural waste grains. The objective is to put on fat, which waterfowl use for energy during their long journey north and during the breeding season. Once the birds have acquired enough fat, and day length reaches a certain duration, they catch the next favorable wind toward their breeding grounds. Along the way, they visit staging areas that have an abundance of food, where they stop to feed and rest. Waterfowl rely on these stopover areas to refuel and acquire additional energy and protein reserves that will be used for nesting once they reach the breeding grounds.
Not all waterfowl species begin the migration north at the same time. Most geese and some ducks, such as mallards and northern pintails, seemingly race north in early spring, right on the heels of the retreating snow line. The Canada goose is one of the earliest-nesting waterfowl species in the Prairie Pothole Region. In fact, it is not uncommon to see breeding pairs of Canada geese standing on frozen wetlands, claiming their territory and waiting for the thaw. In North Dakota, for example, Canada geese may initiate nests as early as April 1. These early-nesting species tend to be larger bodied and rely heavily on stored nutrients to help them through lean times.
There are several biological advantages for waterfowl that return early to the breeding grounds. First, by arriving shortly after the spring thaw, dabbling ducks can feed in expanses of sheet water formed by melting snow. While deeper potholes and marshes are still frozen, these temporary and seasonal wetlands warm quickly, supporting multitudes of tiny invertebrates that provide breeding hens with the nutrients required to successfully nest and produce eggs. Nesting early also gives female waterfowl the opportunity to renest if their first attempts fail and to raise their broods before temporary and seasonal wetlands go dry in summer. Of course, a significant disadvantage of this strategy is that early-nesting waterfowl are more likely to encounter cold temperatures and spring storms, which can result in nest losses. In addition, early nesters must rely on residual vegetation from the previous growing season for nesting cover, which can leave hens vulnerable to predators, especially on intensively cultivated landscapes like the prairies.
Mallards and pintails, which begin nesting in late April and early May, are followed by canvasbacks and northern shovelers, whose peak nest initiation dates range from mid- to late May. Next are blue-winged teal, the majority of which begin nesting in late May. Gadwalls and lesser scaup are among the last to get started, as many females of these species don't begin nesting until early to mid-June.
Late-nesting waterfowl species tend to have smaller body size than early-nesting species and rely more on nutrients acquired on the breeding grounds than on those stored during spring migration. These birds also have less time for renesting and brood rearing than do early-nesting species. On the other hand, late-nesting species are less likely to suffer nest loss due to inclement weather. Moreover, as spring progresses, upland nesting cover is typically lusher and there is a greater abundance of alternative food sources for predators such as skunks and raccoons. Several studies of upland-nesting ducks on the prairies have found that these factors are positively related to nest survival.
When waterfowl begin to nest can also vary considerably among members of the same species. For example, female wood ducks in southern portions of the Mississippi Alluvial Valley begin laying eggs in late February to early March, while those that migrate north to Minnesota may not initiate nests until early May. These differences are largely due to the availability of food resources needed for egg formation, which are simply available earlier in the South than in the Upper Midwest.
Age is another factor that can determine when waterfowl nest. Older females often arrive on the breeding grounds in better physical condition and nest earlier in the spring than first-year birds. Of course, weather and habitat conditions on the breeding grounds can also have a significant impact on waterfowl nesting efforts. Late spring thaws and dry conditions can make wetland invertebrates inaccessible or hard to find for breeding ducks. Without an abundance of these high-protein foods, the birds may delay or forgo nesting altogether.
Clearly, a variety of factors influence waterfowl nesting chronology, including species-specific traits like body size, diet, and other ecological requirements. Environmental conditions, however, such as the timing of the spring thaw and the availability of habitat on the landscape, can also have a profound impact on waterfowl nesting activities and the success of those efforts. That's why conserving wetlands and other crucial habitats on North America's most important waterfowl breeding areas is among Ducks Unlimited's highest priorities. It's the only way to ensure that waterfowl will have the resources they need to successfully nest, raise their broods, and sustain their populations.
Tanner Gue is a biologist and Kaylan Carrlson is director of conservation planning in DU's Great Plains Region. Dr. Stuart Slattery is manager of Boreal conservation science and planning with DU Canada.
The Importance of Spring
Migration Habitat On their energetically demanding journey north to the breeding grounds, waterfowl stop to rest and refuel at key spring staging or stopover areas. As long as these wetland complexes provide sufficient food resources to meet the nutritional needs of waterfowl, ducks and geese will arrive on the breeding grounds in good physical condition. This is important, because among some species of waterfowl, females that arrive on the breeding grounds with greater fat reserves tend to nest earlier and lay larger clutches than birds with less body fat, and are more likely to renest if earlier nesting attempts fail. Consequently, Ducks Unlimited works hard to conserve crucial waterfowl habitats not only on the breeding grounds but also on important staging areas across the United States.