Most people admire the majestic white swans that glide along the surfaces of Rhode Island's coastal ponds-but the swans have their critics, too. Some local wildlife managers consider these large, territorial birds a nuisance, pointing out that swans chase ducks and geese, eat voraciously, sometimes attack people, and are not even native to Rhode Island.
Rhode Island's swans belong to the species Cygnus olor, the mute swan. Mute swans are indigenous to Europe and Asia, where for centuries they have been celebrated in art and legend and prized by the wealthy as status symbols. In 1910, mute swans were imported to the United States to grace ornamental ponds on New York estates. Eventually some of these swans escaped and established a wild population.
In spite of their name, mute swans are not completely silent: They can hiss, grunt, and make snoring sounds. But they are quiet compared to some of their noisy relatives, such as the whooper swan, the whistler swan, and especially the trumpeter swan whose bellowing can be heard for up to two miles.
Weighing 18 to 35 pounds, mute swans are the largest birds in Rhode Island. The average wing spread at full growth is eight feet for the male swan (called a cob) and seven feet for the female (pen). The swans' average lifespan is seven years, although some have lived for as long as 50 years.
By their third spring, mute swans reach sexual maturity and mate for life. The breeding pair builds a large nest, about six feet in diameter, of reeds and grasses. In Rhode Island, the eggs (five to seven per nest) are laid from late March to mid-April. The eggs are large: 4-1/2 inches long by 3 inches wide. They hatch in 32 to 35 days and the brownish-colored cygnets (young swans) grow very rapidly, from a six-inch wingspan to near-adult size in less than six months.
Rhode Island's mute swans are not migratory (that is, they do not fly south for the winter) but they do move about frequently between Rhode Island and neighboring states.
The first sighting of a mute swan in Rhode Island was in 1938, and the first nesting pair was seen here in 1948. The subsequent increase in the swan population (see graph) has led to fears among conservationists that the swans might outcompete native aquatic birds such as ducks and geese for food and nesting space, or reduce the amount of food available for migrating waterfowl. (Note: the swan counts are quite variable from year to year, especially since the swans move frequently between ponds and across state lines, but the overall trend towards increasing population is clear.)
Concerns about Rhode Island's "swan problem" generally focus on two issues: (1) the swans' aggressive behavior, and (2) the amount of vegetation that the swans consume.
AGGRESSIVE BEHAVIOR. Mute swansespecially the cobscan be aggressive, particularly when defending their nesting territories or young cygnets. Swans have different personalities, with some being much more belligerent than others. In general, mute swans behave most aggressively towards other swans, and next most aggressively towards other white birds such as gulls. Swans are least aggressive towards brownish birds like ducks and geese, probably because their own cygnets are brown.
Rhode Island Sea Grant researcher J. Stanley Cobb, who studied the mute swans on Rhode Island's coastal ponds between 1978 and 1981, observed great variation in territory-defending behavior. In one case, a pair of swans vigorously defended a 300-by-600-foot pond in Narragansett, chasing gulls that were as much as 300 feet away. Researchers saw no other waterfowl at the pond. In contrast, another pair of swans frequently allowed ducks to feed within 30 feet of their nest.
Charles Allin, Senior Wildlife Biologist with Rhode Island Department of Environmental Management's Division of Fish and Wildlife, has received numerous complaints about swans chasing and attacking ducks and geese and driving them away from ponds. Since 1970, Allin's department has received 21 reports of swans attacking people in Rhode Island. No one has been seriously injured so far, but in other countries children have had bones broken when they were struck by swans' wings.
EFFECTS OF SWANS ON AQUATIC VEGETATION. Being large and herbivorous, swans consume large quantities of submerged aquatic vegetation that native birds also feed on. Moreover, swans are wasteful feeders, tearing up more vegetation than they actually eat.
Cobb's studies included observations of a traditionally defended area on Briggs Marsh. Two pairs of swans who had nested in the same location for several years, always defending a large area, provided a kind of natural experiment since the rest of the swan flock were prevented from grazing in the defended part of the pond. Cobb found that the defended area contained a significantly greater amount of submerged aquatic vegetation than the heavily grazed area.
In another experiment, Cobb's research group placed exclosures (wire cages) into ponds. No significant difference was found between the amount of vegetation inside the exclosures (where swans could not graze) and the amount outside. Cobb speculated that these experiments failed to demonstrate any effect of swan grazing because the vegetation inside the exclosures did not have time to grow back during the few months that the exclosures were present. Further experiments, covering a longer period of time, are needed.
MANAGEMENT. Because of the possible negative impact of mute swans on native waterfowl, the absence of natural enemies for adult swans, and the potential for rapid expansion of the swan population, Rhode Island's Division of Fish and Wildlife maintains a policy of population control. In the mid-1970s, division personnel captured and killed swans, but this practice was abandoned because of public opposition. Currently, division biologists shake the swans' eggs to kill the embryos, then replace the eggs in the nest where the adults continue to incubate them. (If the eggs were not replaced, the swans might lay more.)
Allin estimates the cost of the egg-shaking program at $6,000 per year, counting the cost of making an aerial survey to locate nests, and the salaries of the biologists who go out in canoes and on foot to actually shake the eggs.
Even with egg shaking, Rhode Island's swan population has increased, but much more slowly than without controls. The goal of Rhode Island's swan management progam is to maintain a swan population large enough for people to enjoy, yet not so large as to constitute a nuisance. In order for this goal to be realized, other New England states must also establish similar control programs.
Additional reading: Mute Swans of the Atlantic Coast. James E. Twining. Dutch Island Press, Wickford, RI. 1987.
Fact sheet published in 1988.
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