By Ian Gregg, PGC Wildlife Biologist –
At first glance, mute swans have often inspired positive thoughts from Americans who know little about the consequences of having them in the wild. But given the real-world ecological problems this exotic (non-native) species has caused for native waterfowl, it's apparent to most wildlife enthusiasts that America would be better off without it.
Native to Eurasia, mute swans were first introduced in North America in the 1800s, usually as adornments to parks, estates and zoos. Over time, though, as some of these captive swans escaped or were intentionally released, they became established in the wild along the East coast. These feral populations became established because of the species' high reproductive rate, a lack of natural predators and an abundance of suitable wetland habitat. Today, the Atlantic Coast harbors the largest numbers of free-ranging mute swans, but smaller populations are present in the Great Lakes and Pacific Northwest, with localized inland concentrations scattered throughout the Northeast and Midwest.
Feral mute swans were first observed in Pennsylvania in the 1930s, and the state's wild mute swan population has gradually increased since then by dispersing from larger populations in adjacent states; reproducing from established feral swans; and through escapes or releases of captive swans.
In addition to those currently in the wild, many mute swans continue to be kept in captivity for their aesthetic appeal.
Adult mute swans typically weigh 18 to 25 pounds (males are somewhat larger than females) and have wingspans of six to seven feet. They are intermediate in size between the two swan species native to North America, the smaller tundra swan and larger trumpeter swan, and like those species, mute swans have long necks and all-white plumage. However, most of the similarities end there.
While tundra swans are fairly numerous in southeastern Pennsylvania during the winter, and trumpeter swans can be seen on rare occasions as a result of ongoing reintroduction efforts in Midwestern states and Ontario, these native species nest only in remote areas of the continent, migrating vast distances between breeding and wintering areas each year.
In contrast, mute swans do nest in Pennsylvania, and are essentially non-migratory, at most exhibiting only short-distance movements from breeding areas that freeze up during the winter to nearby bodies of water that remain open. Consequently, in almost all cases any swan seen in Pennsylvania between April and October is a mute swan.
During times of the year when other swan species may be present, several physical characteristics make mute swans easy to distinguish. First, the mute swan usually holds its neck in an elegant "S" curve with its bill pointed downward, while the other two species hold their necks straight up and their bills straight out. In addition, the mute swan's bill is reddish-orange with a black knob at the base; the bill of a trumpeter or tundra swan is all black without a basal knob (Swan identification guide). Finally, although the mute swan actually is not completely silent, as its name suggests, its vocalizations are limited to grunts, whistles and hisses that are much less audible and less melodious than the distinctive calls of native swans.
Mute swans are found in a wide variety of habitats in Pennsylvania, from large lakes and rivers to municipal and farm ponds. In coastal states, they also occur in salt marshes and brackish estuaries. Mute swans primarily eat aquatic plants. They feed by uprooting entire plants, and may consume up to eight pounds of vegetation daily. In addition, they readily accept artificial feeding, and handouts from people may constitute a large part of their diet in some places. In fact, this species does so well in close association with civilization that it's sometimes difficult to distinguish between captive and feral individuals.
Mute swan pairs, which generally establish life-long pair bonds, select nesting territories of one to 10 acres, and typically use them year after year. They also tend to be extremely territorial, especially during the nesting season. Brood sizes average three to five cygnets, and family groups remain together from hatching until the adults abandon or drive away the young prior to the next breeding season.
Like many introduced species, mute swans are causing several ecological problems. One of the greatest concerns is that mute swans can and do severely disrupt natural food chains, because their large appetites and year-round presence put tremendous pressure on aquatic vegetation resources. Several studies have documented large reductions in aquatic vegetation, and even the complete disappearance of certain plant species in some locals, because of heavy grazing by mute swans. In some areas of the Chesapeake Bay, efforts to restore native vegetation have been seriously hampered by foraging mute swans. In turn, there is a negative impact on native wildlife and fish species that depend on these plants for food and cover.
A second problem with mute swans is that their aggressive territorial behavior can negatively impact many native species of waterfowl and other wetland birds. Mute swans have been observed driving tundra swans from preferred roosting and feeding areas; causing nest abandonment by terns and skimmers; and killing mallard ducklings and Canada goose goslings.
Mute swan aggressiveness also may be directed against other animals and even humans, posing at least a nuisance, or even a danger, to pets and people.
Mute swan populations in the Atlantic Flyway states (Maine to Florida) have been monitored since 1986 by cooperative surveys conducted periodically - generally every third year - during the birds' summer molt period. Many states count mute swans by aerial surveys, but for the relatively low, widely dispersed mute swan population in Pennsylvania, the most efficient survey method is for each of the Game Commission's WCOs to record all mute swans encountered in his or her district during the survey period.
During the most recent mute swan survey - conducted in the summer of 2002 - about 14,300 were found in the Atlantic Flyway. Based on those figures, the population has more than doubled in just 16 years. Growth has been especially dramatic in the Chesapeake Bay, with a more than tenfold increase over the same time period. In Pennsylvania, about 350 mute swans were observed in 2002, an increase of 44 percent since the previous (1999) survey. And like the flyway trend, there are now more than twice as many mute swans in Pennsylvania than there were in 1986. The 2002 survey also documented record numbers of mute swan broods (24) and cygnets (59) in Pennsylvania, and a wider distribution of mute swans in the state than ever observed; mute swans were present in one-third of WCO districts statewide. Consistent with previous surveys, total numbers of mute swans were highest in southeastern Pennsylvania (161), and the northwest region had the second-highest population (103) and the fastest population growth rate.
The good news is that so far, unlike some other states, Pennsylvania has been spared from explosive growth in the free-ranging component of its mute swan population. The number of mute swans classified as feral in 2002 (94) was only 27 percent higher than in 1986, did not increase from the 1999 survey and has actually remained fairly stable for the last 10 years.
Meanwhile, however, the number of captive and semi-domesticated mute swans has been steadily increasing. Although feral mute swans pose the most acute threats to native wetlands and waterfowl, domestic mute swans are still capable of causing ecological and nuisance problems. Moreover, domestic mute swans serve as a potential reservoir of new feral swans through reproduction, escape and release.
Control and Management
From a strictly biological standpoint, it would be best if there were no mute swans in Pennsylvania. However, because some people enjoy owning or viewing mute swans, their presence in small numbers and under controlled conditions would probably not cause major adverse effects.
Historically, because mute swans are an exotic and non-migratory species, the U.S. Fish and Wildlife Service did not consider them protected under the Migratory Bird Treaty Act (MBTA), an international treaty which gives the federal government primary responsibility for managing migratory birds, including native ducks, geese and swans. Consequently, management of mute swans was the responsibility of individual states. In Pennsylvania, although there was no formal mute swan eradication program, mute swans that turned up on public lands were often destroyed, and state law followed the MBTA in classifying them as unprotected. In effect, they could be taken without a permit at any time, for any reason, by just about anyone. It's likely that this approach was responsible for keeping our feral mute swan population from growing as rapidly as in states where mute swans were protected under state law and/or not consistently removed from public lands.
In December 2001, however, in response to a lawsuit, a federal court ruled that the U.S. Fish and Wildlife Service could not exclude mute swans from MBTA protection. This placed mute swans in the same protected category as native waterfowl, and in effect meant that control measures, including those used in Pennsylvania, had to be put on hold or stopped.
Combined with the rapidly multiplying feral mute swan population in the Chesapeake Bay just to our south, and the increasing numbers of domesticated mute swans already in Pennsylvania that could serve as a source of new feral swans, these constraints on control activities were a major cause for concern.
Following the court ruling, the PGC worked with the Fish and Wildlife Service and other Atlantic Flyway states to develop the Atlantic Flyway Mute Swan Management Plan. This plan established mute swan population goals for the flyway, along with strategies to begin reducing populations to those levels within the parameters of the MBTA. As part of this plan, the PGC established a Pennsylvania population goal of zero feral, free-ranging swans and a maximum population of 250 legally-permitted swans held in captivity.
One of the most important control measures suggested in the management plan was the issuance of federal depredation permits to allow state personnel to resume removing mute swans from public lands and from private lands with landowner permission. However, attempts by the USFWS to issue mute swan depredation permits were obstructed by further lawsuits. The lack of depredation permits severely hampered the ability of wildlife managers in Pennsylvania and elsewhere to maintain mute swan populations at or below the levels specified in the management plan. Fortunately, legislation clarifying that the MBTA does not, and was never intended to, apply to non-native species such as the mute swan was passed by the U.S. Congress and signed by the President in November, 2004. In effect, this action has returned management authority for mute swans to the states, as was the case prior to 2001. This change should make it much easier to keep Pennsylvania's mute swan population under control; of particular benefit is the ability to resume euthanasia of feral mute swans where appropriate. Other management tools may also prove useful in reducing the threat posed by mute swans.
Egg addling (treating eggs to prevent them from hatching) is labor-intensive and much less effective at reducing populations than removal of adult swans, but it has been used with some success on mute swan nests in Maryland, and may be appropriate for some situations in Pennsylvania. Another useful option might be the adoption of regulations to strictly regulate the possession of captive mute swans to ensure they do not cause major conflicts or serve as a source of new feral populations. Regardless of the exact control methods ultimately adopted, it is very important to change public perception that mute swans are a wonderful addition to our outdoors. Just as house sparrows and starlings rob eastern bluebirds of critical cavity-nesting locations, mute swans are displacing native waterfowl and beginning to overuse aquatic food sources. Without our intervention now, while Pennsylvania’s mute swan population is still at a relatively manageable level, the problems mute swans cause for waterfowl and other wetland species will worsen, and addressing these problems will only become more difficult and expensive.
By becoming more informed about the threats posed by mute swans and other introduced invasive species, communicating this information to others, and supporting control efforts, all Pennsylvanians who care about native wetland wildlife and its habitats can play a role in safeguarding these species from biological invasions.