In 1962, five mute swans escaped from an estate in Talbot County, Maryland. More than four decades later, their descendants had grown to nearly 4,000 in the Chesapeake Bay region. As their numbers rose across the Chesapeake Bay region, so did concerns about how much underwater vegetation they were consuming and how they were affecting native birds.The Maryland Invasive Species Council said in 2005 that the population had more than doubled nine times in 40 years and was approaching another doubling. The US Geological Survey, in a 2002 publication, put the Chesapeake Bay population at about 4,500.Mute swans are native to Europe and Asia and are considered an invasive species in the Chesapeake Bay. They are large white waterfowl, with adults growing up to five feet long, having a wingspan of seven to eight feet and weighing as much as 25 pounds.These swans feed heavily on submerged aquatic vegetation, also known as SAV, which includes underwater grasses such as eelgrass and widgeon grass. The Maryland Invasive Species Council estimated that the region’s mute swans were eating about 10.5 million pounds of aquatic grasses every year.
A fast-growing population
The five swans that escaped in 1962 became the starting point of a population that grew rapidly in the Chesapeake Bay region. By the late 1990s, the population had increased sharply. A USGS report said the number had risen by 1,200% between 1986 and 1999.The report also noted that the Chesapeake Bay population represented about 30% of the total Atlantic Flyway mute swan population at the time. Unlike tundra swans, which visit the Chesapeake Bay during winter, mute swans remain in the region throughout the year.That means the birds continue feeding on the Bay’s aquatic vegetation during spring and summer. Native waterfowl also eat these grasses, but they are only present in the region for part of the year.The Maryland Invasive Species Council said mute swans could also interfere with efforts to restore aquatic grass beds. The birds can consume new plants that are important for the recovery of these underwater habitats. According to the council, they could wipe out a restoration planting in only a few days.
Info source: Wildlife Nomad
Why underwater grasses matter
Aquatic grasses are an important part of the Chesapeake Bay ecosystem. The Maryland Invasive Species Council described them as the base of many food chains in the Chesapeake estuary. A range of plants and animals depend on these grasses, including species that are important to commercial activities.Mute swans feed by putting their heads and long necks underwater. They can pull out whole plants, including their roots and rhizomes. The Chesapeake Bay Program says adult mute swans eat more than eight pounds of underwater grasses each day.Their diet includes eelgrass and widgeon grass. The USGS report found that widgeon grass made up 56% of the food found in the birds’ gullets, while eelgrass accounted for 43%. Other submerged aquatic vegetation and invertebrates made up a much smaller share.The birds also change their diet at different times of the year. During winter, they can eat wheat and other grains from farm fields. But underwater grasses remain an important food source, particularly during spring and summer.
Pressure on native birds
Mute swans are known for aggressive behaviour, and the Maryland Invasive Species Council said this had led to native birds being displaced from nesting and feeding areas.The USGS also reported that populations of black skimmers and least terns nesting on beaches and oyster shell bars had been eliminated by molting mute swans.Mute swans can also be highly territorial. The Chesapeake Bay Program says a male becomes particularly protective while the female is incubating eggs and when the cygnets are young. It may hiss at and chase away people and other birds that come too close.
Difficult management
The rapid growth of the population led Maryland resource managers to consider ways to control mute swan numbers. The issue became controversial, with animal rights groups opposing control measures.The Maryland Invasive Species Council said animal rights groups had argued that state and federal wildlife agencies were using mute swans as a scapegoat for wider environmental problems.The USGS reported that proposed control measures included addling eggs, which prevents eggs from hatching, as well as killing adult swans. The issue also became contentious among some residents of the Chesapeake Bay area.Legal challenges also affected efforts to control the birds. However, by 2005, new legislation passed by Congress had made clear that the invasive species had no federal protection, according to the council. However, animal rights activists said they planned to continue their court challenges.
