by Johann Walker and Scott Stephens Ph.D.
After a long migration northward from the Gulf Coast, a hen pintail has finally arrived on the prairies of North Dakota in late April. She and her mate have established a territory on a shallow wetland in the middle of several square miles of native grassland. She has spent many days feeding and carefully inspecting potential nest locations in the surrounding pasture. Now she is ready to nest. The hen pintail lands at her chosen site, scrapes out a shallow depression in the soil next to a clump of little bluestem grass, and lays her first egg. If she has chosen a good nest site and luck is with her, her eggs will hatch, and she will lead a brood of ducklings to a nearby wetland in three to four weeks.
Selecting a nest site is an important decision in the annual cycle of ducks. Nest site choice can influence whether the female survives the nesting season and her eggs survive to hatch. A poor choice might expose the nesting female and her eggs to predators, destruction by machinery, or flooding. This aspect of nesting behavior is shaped over time by natural selection, reflecting strategies that have been effective for the species.
Ducks generally use three strategies for nest site selection. Notwithstanding the occasional mallard that nests in a backyard apple tree, most ducks can be categorized as either upland nesters, overwater nesters, or cavity nesters. Upland nesters include the familiar puddle ducks like mallards and pintails as well as some divers like white-winged scoters and lesser scaup. Overwater nesters include many diving ducks including canvasbacks, ruddy ducks, redheads, greater scaup, and ring-necked ducks. Cavity nesters are wood ducks, buffleheads, goldeneyes, and mergansers (although the common and red-breasted mergansers will also nest on the ground). These nesting strategies are best understood in the context of each species' life history and habitat.
Pairs of upland-nesting ducks begin arriving in the Prairie Pothole Region just after ice out. They settle on shallow, seasonal wetlands, many of which are less than one acre in size and are embedded in cropland, pasture, hay meadows, and perennial grassland. The females feed heavily on protein-rich invertebrates in these shallow wetlands, and pairs of breeding ducks establish wetland territories that are defended by drakes to ensure their mates have sufficient food resources to recover from migration and begin nesting. Most mallards and pintails establish breeding territories on wetlands from mid-April to early May. Once the birds have established territories and acquired adequate nutrient reserves, upland-nesting ducks begin prospecting for potential nest sites. During this period, it's common to see pairs of ducks walking along the edge of nesting cover.
Where upland-nesting ducks ultimately nest varies among species. Blue-winged teal, which have a smaller home range than other species, build their nests close to wetlands. Bluewings are also willing to nest in rather sparse cover compared to many other species. In contrast, mallards have large home ranges, and hens sometimes nest more than a mile from their wetland territory. In addition, mallards typically nest in some of the thickest nesting cover available.
Overwater nesters make their nests on floating mats of emergent vegetation like cattails and bulrush. Their preferred nesting sites are semipermanent wetlands with relatively deep water and dense stands of emergent vegetation. Ideally, females select nest sites that provide cover and protection as well as an easy escape route if they have to flee the nest. Once a suitable overwater nest site is located, females create a platform of nesting material on which to safely deposit their eggs.
Redheads and canvasbacks, which largely breeding in the Prairie Pothole Region, typically establish breeding territories on relatively deep, semipermanent wetlands ringed by a thick wall of tall cattails. The bottom third of these six- to eight-foot plants is often underwater. Female redheads and canvasbacks make floating nest bowls by folding cattails down into a cup-shaped raft.
Not surprisingly, nesting sites are more limited for cavity nesters than they are for upland- and overwater-nesting ducks. Nesting cavities are often made in trees by the excavations of pileated woodpeckers or are created by decay caused by old age or damage from wind or lightning. Although tree cavities are relatively safe from most predators, female ducks must select a nest site that has an entry hole large enough for the birds to enter and a cavity roomy enough to hold a clutch of eggs. As a result, cavity nesters must carefully explore and scout for suitable cavities before making a decision about a nest site.
Perhaps the most well known cavity-nesting species is the wood duck. In areas where suitable nesting cavities are limited, female wood ducks that can't find a suitable nest site often pursue a strategy known as nest parasitism or dump nesting, where the birds lay their eggs in the nests of other female wood ducks. Hooded mergansers, which are also cavity nesters and share many of the same habitats with wood ducks, will also lay eggs in wood ducks nests. In some cases, as many as 50 eggs can be deposited in a single wood duck nest, forcing the residing female to abandon the nest. Hence, it's advantageous for female wood ducks to select secluded nesting sites that are less likely to be discovered by other female wood ducks or hooded mergansers.
The development of "life history characteristics" like nest site selection behavior in waterfowl is influenced by survival and successful reproduction of many generations of nesting females. Over time, the process of natural selection leads to consistent patterns of behavior that maximizes individual reproductive success on average. To determine the past effects of natural selection on present nest site selection behavior in ducks, waterfowl biologists compare where ducks nest in a particular area to mathematical projections of where the birds would nest if they selected nest sites randomly. For example, mallards will typically nest in some of the densest vegetation in a particular field and will generally avoid nesting in surrounding sparse vegetation.
But natural selection is a never-ending process. Waterfowl biologists can identify the current effects of natural selection on duck nest site selection by measuring and comparing the survival of nests in different cover types. Recent research has confirmed that mallard hens tend to hatch more nests in dense vegetation than in sparse vegetation. Thus, waterfowl researchers infer that this behavior among mallards is the product of both past and ongoing natural selection.
Natural selection also helps explain why the three basic nest site selection behaviors developed over time among waterfowl. The ground-nesting dabbling ducks tend to nest in areas where the birds are most vulnerable to predators. Consequently, these species renest persistently and disperse farther between successive nesting attempts.
Overwater-nesting ducks build their nests in areas that are well-protected from many types of mammalian and avian predators. Hence, they are generally less persistent renesters and do not disperse as far between nesting attempts. They will also defer breeding entirely in dry years.
Cavity-nesting species like wood ducks also nest in relatively secure places, so they tend to lay large clutches, which typically result in higher production. However, throughout much of the wood duck's breeding range, the growing season is long and in many cases predation on ducklings is high, so the birds have retained the ability to renest. Where the birds attempt to renest is determined by the availability of suitable nesting cavities.
By pursuing different nest site-selection behavior, ducks are able to occupy different habitats and use different resources. Collectively, different species occupy diverse ecological niches, helping ducks become one of the most common groups of birds on Earth.
Pintails and Cropland: An Ecological Trap
Nest site selection behavior that once had a beneficial or neutral effect on waterfowl reproductive success can be detrimental in a changing landscape or environment. The nesting behavior of pintails in Prairie Canada is a prime example. Compared to other upland-nesting ducks, pintails aren't particular about the type of cover in which they nest. They seem to be much more concerned about getting an early start on the nesting season on landscapes where there are large numbers of highly productive, shallow wetland basins holding water.
Unfortunately, the same landscapes that are attractive to nesting pintails are also well suited for cultivation, and today there is very little perennial grassland left in many of these areas. Grain stubble is often the predominant residual vegetative cover on the landscape when breeding pintails return in spring. In the past, a significant proportion of the cropland in Prairie Canada was left idle during the growing season in a practice known as "summer fallowing." At that time, pintails nesting in crop stubble had comparable nest success as pintails nesting in grassland. Today, almost all the cropland in Prairie Canada is cultivated every spring, and pintails nesting in spring-cultivated cropland generally have poor nesting success and are at high risk of being killed by machinery. As a result, DU is working with prairie landowners to switch from spring wheat to winter wheat, which isn't cultivated in the spring and provides more secure upland nesting cover for pintails and other ducks.