By Bruce Batt, Ph.D.
All waterfowlers enjoy seeing large numbers of waterfowl, especially from a blind during the hunting season. Indeed, one aspect of waterfowl behavior that people find most compelling is the birds' propensity to gather in spectacular concentrations on staging, migration, and wintering areas.
While ducks and geese are drawn together socially throughout most of the year, the opposite is true during the breeding season. At this time, breeding pairs seek isolation and can be downright unsociable. Visit the famous Prairie Pothole Region at the height of breeding season, and you will see pairs of various waterfowl species scattered here and there on wetlands, but few large flocks. In fact, after hens begin incubating their eggs, it can be surprisingly quiet on the breeding grounds.
Breeding waterfowl isolate themselves from rival pairs in many ways. As an example, let's look at the mallard, which can be observed just about anywhere there is suitable habitat. After selecting a mate on the wintering grounds, a female mallard will typically lead her chosen drake back to the area where she was raised—often to the same pond. The male stays close to her side and is diligent about defending her from the advances of other males. Males greatly outnumber females in many duck populations, so competition is often fierce among drakes for prospective mates.
Soon after a mallard pair arrives on the breeding grounds, the birds take up residence on a small wetland or a portion of a larger one. After the hen has made a nest—usually in adjacent upland cover—she begins laying eggs, usually at the rate of one egg a day. While the hen is away, the drake stays behind to defend their wetland. This is where the female spends most of her time feeding and where the pair copulates frequently to help assure that each egg is fertilized.
When unmated drake mallards intrude on a pair's space in search of potential mates or breeding opportunities with a paired hen, the resident drake will stay close by the hen's side and try to prevent the intruders from copulating with his mate. He is not always successful, as mallard broods often include ducklings "fathered" by different males.
Conflict also arises when an intruding mallard pair attempts to settle on a pond that is already occupied by another pair. In these instances, the resident male quickly pursues the intruding female to drive her away. This often leads to a twisting and towering "three-bird flight," where the intruding female flees from the determined resident drake. The intruding hen's mate trails helplessly behind during the chase without making much of an attempt to defend her. These flights may last for several minutes and extend up to a half mile or more away from the pond where the chase began. Once the intruding hen has been driven far enough away, the resident male turns back, setting his wings and making a long glide back to the pond and his waiting mate. He can't be away too long or other males may take advantage of his absence. These "three-bird flights" are a common sight on the breeding grounds during spring.
A paired male mallard will remain intolerant of other males and pairs until his mate has finished laying her eggs and begins incubation. Only then will he abandon the hen and pond that he had previously defended so vigorously. At this point, he may attempt to breed with other paired females in the area that are still in the process of laying eggs, or he may pair with another hen whose first nesting attempt has failed. In the latter case, the drake will once again actively defend the hen and the pond chosen by his new mate.
The male's strong defense of his mate helps ensure that he will "sire" a higher number of ducklings in the next generation. This can be thought of as a type of paternity defense. For the female, the likely advantage is she breeds with a male that is capable of defending her and the breeding pond and is therefore a good choice to fertilize her eggs. Her mate also helps ensure that she will be relatively undisturbed while feeding during the energy-demanding laying period.
There are other apparent reasons why waterfowl disperse widely in breeding habitats. For one, hatched ducklings are not able to fly and initially depend largely on food resources in the area previously defended by their mother's mate. By keeping other pairs away from the pond, resident males help ensure that, in the future, the ducklings will not have to compete with other broods of the same species for limited food supplies.
Indeed, biologists believe that one of the main reasons that fewer birds attempt to breed during dry years is there simply isn't enough habitat to support breeding hens and, later, their broods. Females with mates that are capable of defending their chosen breeding ponds occupy all the suitable habitat, forcing other pairs to move to other regions or to forgo breeding altogether.
Another reason breeding waterfowl disperse widely across the landscape is to limit their vulnerability to predators. Large numbers of breeding birds and nests concentrated in a small area are more visible and produce more scent, both of which could attract predators. Conversely, isolation of pairs and nests in the best breeding cover possible can significantly reduce their odds of being discovered by predators.
Regardless of why breeding waterfowl seek isolation, spring is a fascinating time of year to visit wetlands and watch the birds demonstrate a variety of behaviors in defense of their mates and habitat. If mallards breed on a pond near your home, you will probably be able to watch the drama unfold right in front of you. And if other waterfowl species are present, you can observe interesting variations on the same basic theme.
Dr. Bruce Batt retired after a long tenure as chief biologist at DU's national headquarters in Memphis.