By Mike Brasher, Ph.D.
When researchers study reproduction in ducks and geese, females usually receive most of the attention. That is understandable, given that females are largely responsible for egg laying, incubation, and brood rearing. But male ducks and geese also play many important roles in waterfowl reproduction, some of which are surprisingly complex.
In North America, nearly all duck populations consist of a higher proportion of males than females, with drakes constituting 55 percent of mallard populations and almost 70 percent of populations of diving ducks such as scaup. This gender imbalance is mostly caused by higher rates of mortality suffered by females while nesting and brood rearing.
Most of this continent's ducks are seasonally monogamous, meaning that one male and one female form a pair bond that lasts for only one year. Due to monogamous mating systems and male-biased sex ratios, some drakes remain unpaired during the breeding season. Understanding the different reproductive strategies of paired and unpaired males has been of particular interest to biologists charged with surveying and estimating the size of annual waterfowl breeding populations.
A male duck that successfully acquires and follows a mate back to the breeding grounds is primarily concerned with protecting her from harassment by other males of the same species. The paired male provides this protection by aggressively chasing away other males that intrude on his territory. The degree of territoriality varies among species, but in all cases serves two primary functions. First, the male's territorial behavior allows the female to feed and rest undisturbed when not on the nest, which enables her to acquire the necessary resources for egg laying and incubation. Second, and most important from the male's perspective, this behavior prevents other males from breeding with his mate.
Although monogamy is the primary mating system in waterfowl, males will attempt to forcibly mate with other females to increase the chances of passing on their genes. But a paired male that attempts to mate with additional females faces a dilemma: by engaging in such pursuits he must leave his own female unattended and vulnerable to other males. Thus paired males must carefully choose when to pursue additional females. Interestingly, forced mating attempts are directed primarily at females in the egg-laying phase, suggesting that males assess the reproductive condition of females and pursue those that offer them the greatest odds of fathering additional ducklings.
Forced mating is fairly common among many species of ducks, sometimes accounting for 20 percent of all mating activity, yet genetic analyses reveal that only 2 to 5 percent of ducklings are the result of these interactions. Why? Researchers have discovered that females possess a fascinating array of anatomical features and pairs adopt interesting behaviors that reduce the chance of eggs being fertilized by other drakes. Once females lay their eggs and begin incubation, paired males start roaming more widely in search of additional breeding opportunities until they eventually depart breeding areas in late summer to undergo their annual wing molt.
Male ducks that arrive on the breeding grounds without a mate were once thought to be responsible for most forced mating attempts. But studies have shown that paired males are primarily responsible for this behavior and that unpaired drakes spend most of their time courting and attempting to establish pair bonds with females. Although rare, some of these males successfully establish pair bonds on the breeding grounds, presumably with females that have lost or abandoned their original mate.
Unpaired males were once thought to also play an important role in reproduction by breeding with renesting females that had lost their first nests. Yet this too has proved rare, as renesting females usually breed with their original mate or another previously paired male. In reality, unpaired males are likely young or small birds that are unable to effectively compete for and attract available females. It appears that the best strategy for unpaired males is to simply associate with and gain experience from existing pairs to better prepare for courtship and pairing opportunities the following year.
The breeding behaviors of geese provide an interesting contrast to those of ducks. Geese are perennially monogamous, meaning that one male and one female form a lifelong pair bond. Like ducks, paired male geese protect their mates from harassment by intruders, but their larger size allows ganders to defend against not only other males but also many predators. While male ducks wait on nearby ponds to rejoin their respective mates following egg laying and during incubation breaks, ganders stand guard in the immediate vicinity of the nest. In further contrast to ducks, male geese remain intensely territorial until the eggs hatch and then assist the female with brood-rearing duties. Paired ganders will attempt to forcibly mate with other females, but this behavior is less common in geese and primarily restricted to colonial-nesting species such as snow and Ross's geese. Moreover, most geese do not breed until they are two or three years of age, which results in a fairly large segment of the population being nonbreeding birds. Although these nonbreeders have no role in goose reproduction, they may begin courting and forming pair bonds during breeding seasons in advance of becoming sexually mature.
Over the past several decades, researchers have made great strides in unraveling the mysteries of waterfowl mating strategies. The fascinating breeding behaviors observed today among ducks and geese developed over thousands of years to give the fittest individuals the greatest chance of contributing to the fall population. It is our responsibility as hunters and conservationists to ensure that sufficient habitat is available to allow these same behaviors to continue producing healthy populations of ducks and geese for generations to come.
Based in Lafayette, Louisiana, Dr. Mike Brasher is biological team leader of the Gulf Coast Joint Venture.