Ecological Impacts of Introduced Mute Swans in the Hudson River

New York Bioscape Initiative Studies Invasive Species
by Susan Elbin, Ph.D

Nearly 24 million people - eight percent of the U.S. population - make their homes in the New York Bioscape, a region defined geographically within a 100-mile radius from Columbus Circle in Manhattan; ecologically by a mosaic of land use practices along an urban to rural gradient; and socio-politically as the greater New York Metropolitan region.

Suburban sprawl - uncontrolled population growth and accompanying development - within the region have dictated three important areas of conservation research carried out by the New York Bioscape Initiative:

•  Invasive and over-abundant species;
•  ecotoxicants and animals as sentinels of ecosystem health; and
•  human-wildlife conflict.

The Hudson River Mute Swan study is a New York Bioscape Initiative project that addresses invasive species.

The Hudson River Mute Swan Project

Mute Swans are typically seen swimming in the moats of ancient English castles...and on the Hudson River, bringing great pleasure to Hudson Valley residents. But the sight of these graceful, large-bodied birds potentially comes at a high ecological cost.

Mute Swans were introduced into the eastern United States from England in the late 1800s for their ornamental value. The population expanded slowly at first, but today the swans are found not only in their primary range along the Atlantic Coast, but also in the Great Lakes and Pacific Northwest. A combination of factors, including a high reproductive rate, a lack of natural predators and abundant habitat, has contributed to the success of the introduction by people of this non-native species.

By the late 1960s, the Mute Swan population had begun its rapid growth. Between 1986 and 2002, Mute Swan numbers increased by 143% in the Atlantic Flyway, with a population of nearly 14,344 birds, from the lower Great Lakes to Florida.

The New York State Department of Environmental Conservation (NY DEC) has followed Mute Swan population growth in New York as part of its larger waterfowl effort, and has conducted periodic surveys and censuses in the Hudson River region since the 1950s. State wide summer counts in 1986 were 1,815, whereas the count in 2002 was 2,848; the Mute Swan population is growing by about 9% a year.

As their numbers increase, Mute Swans become a growing concern for wildlife managers. In the Chesapeake Bay region, Mute Swans have been found to consume, disturb or destroy large amounts of submerged aquatic vegetation (SAV) and proved aggressive toward native waterfowl.

Based on these studies from other regions, Wildlife Trust began in 2003 to investigate the ecology of Mute Swans living in the lower Hudson River Valley, first focusing on their use of the river.

During the past two years, with support from the NY DEC's Hudson River Estuary Program, we have conducted aerial surveys along the Hudson River, from the Tappan Zee Bridge to Troy Dam, to count and map swan locations with the aid of a GPS and digital maps. On our flights, zero to 175 birds were counted, depending upon the season.

During the breeding season, in March and early April, birds form pairs and disperse to their nesting territories to rear their young along the tributaries and lakes of the Hudson Valley. After the young hatch in June and July, flocks reform and numbers increase throughout the Fall as the birds return to feed on the river. Numbers continue to increase on the river from May through October, when non-breeding birds are joined by successful families of swans. November and December continue to be peak times for swans in the Hudson River, and then their numbers slowly start to decrease.

The largest recorded flock on the Hudson River included 175 birds, seen in October, whereas flocks of 600 to 1,000 birds have been recorded in the Chesapeake Bay.

Very few birds remain on the river in January and February (0 - 20 birds). When the river freezes, the birds move south (although they aren't true migrants) . We do not know exactly where they go. In fact, not much is known about their behavior or ecological requirements in the Hudson Valley.

Last year NY DEC Biologist Bryan Swift created a team led by Robin Holevinski and Kevin Clarke to study travel patterns of Mute Swans. They have banded and tagged Mute Swans with white neck collars, etched with a unique combination of four large, black numbers and letters. Five birds that were caught, marked, and released last August were observed this winter at the Jersey Shore (Belmar and Manahawkin). If you should see one of these collared birds, please report the unique ID, date, time, and location of the sighting to the NY DEC at (518) 402-8875.

While on the Hudson River, Mute Swans congregate in fairly large numbers in predictable locations: Croton Point, Annesville Creek and Roundout Creek. These areas are rich in SAV, including water celery ( Vallisneria americana ). Rooted in the river bed, SAV plants play a vital in maintaining a healthy river by increasing oxygen levels and providing habitat for macroinvertebrates and fish.

An adult Mute Swan can consume 4 - 8 pounds of plant material per day, about 82% of which is SAV. They also eat immature seeds and new growth, and uproot whole plants, which exacerbates the total impact of their consumption on the SAV beds.

Betsy Blair and Chuck Nieder of the NY DEC, along with scientists from Cornell University and the Institute of Ecosystem Studies are mapping these beds in the Hudson River and studying their ecological role. If large numbers of swans continue to disturb and destroy SAV beds, irreparable ecological damage will be done to the river.