Speaking last year, the service's director, Steve Williams, said: "Wildlife biologists and refuge managers have significant concerns about the impacts of growing populations of non-native mute swans on native birds and their habitats. Mute swans can cause extensive habitat degradation in wetland habitats."
In the Chesapeake Bay some 3,600 birds consume 10.5 million pounds (4.8 million kilograms) of submerged aquatic vegetation (SAV) each year, said Jonathan McKnight. McKnight is the Maryland Department of Natural Resources (DNR) biologist responsible for the state's response to invasive species.
He said water plants targeted by the swansincluding eelgrass, wigeongrass, and sago pondweedact as important nursery areas for fish and invertebrates.
"SAV beds are the basis of a complex web of life in Chesapeake Bay," McKnight added. "They are the homes of seahorses and pipefish, of crabs and juvenile fish, of ducks and herons."
These beds have already been affected by declining water quality, and now mute swans threaten those that remain, McKnight said.
Their aggressive behavior has also led to the displacement of native birds from nesting and feeding areas, he said. "Mute swans are responsible for driving the last remaining colony of black skimmers from Chesapeake Bay."
The Great Lakes may face similar pressures, with Canada-based scientists suggesting mute swan populations are growing by up to 30 percent each year on Lake Erie and Lake Ontario. And in Wisconsin conservationists fear efforts to reintroduce the trumpeter swan could be undermined by the presence of its larger, more aggressive cousin.
The U.S. Fish and Wildlife Service is seeking to authorize a controlled cull of mute swans in various states. Maryland plans to kill around 3,000 mute swans over the next two years.
However, all swans are currently protected under the U.S. Migratory Bird Treaty Act (even though the mute swan is non-migratory). But a bill is currently being advanced in the U.S. Congress to remove the mute swan and other invasive species from the list. U.S. animal welfare groups, for their part, argue that mute swans don't pose a significant threat to wetland habitats.
In the case of the Chesapeake Bay, environmental pollutionsuch as from runoff from intensive chicken farmingis to blame for lost wildlife, according to the Fund for Animals, based in New York.
The fund's president, Michael Markarian, said: "[Maryland's] complaint against the birds is not that they are causing problems now, but that they might cause problems in the future. The DNR is offering a solution in search of a problem, and it is managing wildlife by trying to predict the future. It's not scientificit's voodoo management."
In England factors such as climate change and nutrient enrichment via sewage treatment plants may also harm river systems that support mute swans. But that shouldn't divert attention from the problems swans cause, Robin Mulholland said.
"Someone's got to stand up and say, 'This is how many [swans] we want, and we've got to keep populations at that level,'" he added. "It sounds hopelessly optimistic, but we live in a man-made environment, and we've got to begin to talk about controlling populations of certain protected species, otherwise they get out of hand."
The mute swan's soaring success could also signal its swan song.
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