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.