By Jennifer Kross, Biologist
The unmistakable honk of a Canada goose as it flies overhead and the raspy decrescendo call of a hen mallard are two familiar waterfowl vocalizations that humans use to identify these birds. But how do waterfowl use these vocalizations, and what do they mean? Why does each species have its own intrinsic calls? Why do males and females of the same species make different sounds?
Vocalizations of waterfowl are considered calls rather than songs because they are short and instinctive in nature. Waterfowl use these vocalizations in a variety of situations as a primary means of conveying information. The vocal organs of waterfowl consist of simple membranes located in a structure called the syrinx. The syrinx is located in the throat, at the bottom of the trachea near the junction of the bronchial tubes. Calls are produced as air passes over the membranes of the syrinx, causing them to vibrate. The shape of the syrinx and the muscles that control membrane tension dictate the different calls within and between species.
Males and females of most species of waterfowl have distinctly different calls because of physical differences in the trachea and the syrinx. Many of us are familiar with the high-intensity squealing call the female wood duck makes when it is disturbed. Male wood ducks are physically incapable of producing this call. Similarly, a female mallard does not produce the grunt-whistle vocalization used by the male mallard during courtship displays.
The call repertoire of waterfowl is somewhat limited; often the same call is used in a variety of circumstances. For example, the slow raehb-raehb-raehb call is used by the drake mallard to draw attention to himself for the purpose of attracting a mate, but this same call is also used to alert other mallards of the presence of a predator.
Communication between members of a species is crucial to survival. Therefore, voice and hearing development in waterfowl begins early. As a hen sits on her nest, incubating her eggs, she exposes the embryos to her maternal call. Two days before hatching, the young are fully capable of hearing this call and begin to make their own vocalizations, which can be heard by the other unhatched ducklings. This is the pipping stage of egg development, and, in fact, it is this vocal communication among un- hatched siblings that enables their synchronized hatching (see sidebar). At this early stage, ducklings learn to identify the voices of their siblings, the specific call of their mother, and the repertoire of their species in general.
The ability of the ducklings to recognize and respond to the hen's call is essential to their survival during this vulnerable period in their life cycle. The first crucial test of their hearing and recognition capability-when the female calls to her young, encouraging them to follow her to food and water-occurs when the ducklings leave their nest. Response to this type of maternal call is best exemplified in wood ducks. The female wood duck calls to her ducklings from outside the nest cavity. This lets the ducklings know it is time to leave, and they must climb out of the nest to join their mother.
After leaving the nest, it is imperative that the hen keep in contact with her young. A series of assembly and maternal calls from the hen and responding vocalizations from the ducklings keep the family unit together. While feeding and exploring in thick vegetation, the family may not always have visual contact, but they do maintain vocal contact. Poised to protect her brood, the ever watchful hen alerts her young to predators by using an alarm call, to which the ducklings instinctively respond, most commonly by freezing their movements.
Vocalizations are important to the social signaling systems of waterfowl. Individuals have calls that work to draw attention to themselves. For example, when a female mallard is sitting on a pond and spots other mallards flying overhead, she will typically hail them by projecting a loud decrescendo-the familiar "hail"-call that encourages them to join her. Within a flock, vocal signals from an individual function to benefit all members of the flock. In the case of Canada geese, their grazing habit exposes them to terrestrial as well as aerial predators. A warning call given by a single goose alerts the entire flock to danger, and the group is able to take precautions in order to avoid predation.
Additionally, vocalization is used to coordinate flight when preparing for migration, leaving a roost for feeding grounds, or escaping predators. Waterfowl often give a pre-flight call to signal their intent to change location. This vocal synchronization of their takeoff functions to keep mates, family members, and flocks together.
Some of the most obvious functions of vocalization are to facilitate mating. The intricate courtship displays of male waterfowl involve the pairing of distinct postures and expressive calls. The female reciprocates to her chosen mate with an inciting call and posture of her own. Once established, pair bonds are reinforced through vocalizations between the drake and hen. Pairs learn to identify each other by the individuality of their mate's voice. This allows them to find one another in a flock, while in flight, and after the female returns from laying or nest searching. Waterfowl pairs recognize their mate's call and respond with a call of their own. Male mallards, for example, respond to the female mallard decrescendo call with a slow raehb.
Territorial waterfowl species such as northern shovelers and Canada geese use vocalizations in concert with threatening postures to defend and advertise their breeding territory. Canada geese can be heard clearly from a distance when challenging an intruder. Mallard, blue-winged teal, and northern shoveler drakes also use boisterous calls and threatening postures to defend their mate against harassment by other males.
Many species of female dabbling ducks including the green-winged teal, northern pintail, mallard, and northern shoveler advertise their location while on the breeding grounds and while searching for nest sites by persistent quacking. One theory is that this behavior functions, in part, to draw out predators within the breeding home range. By doing this, the female is aware of potential threats and is better able to avoid a surprise attack. Predator presence is an important factor in determining a nesting site because the hen is vulnerable during incubation, and the young are highly susceptible to predation while on the journey from the nest to water.
Vocalizing is not just a way for waterfowl to exercise their syrinx. From hatching to breeding, waterfowl vocalizations perform functions vital to survival. A large part of the behavioral strategies of waterfowl involve the use and understanding of calls. Just as humans use speech to communicate, waterfowl use calls to enhance the meaning of distinct movements and to convey information to others of their species.