Efforts to control the exploding population of mute swans around the Bay have been put on hold after a court ruling that the foreign swans are protected by federal law.

The U.S. Court of Appeals for the District of Columbia ruled Dec. 29 that the mute swan is covered by the Migratory Bird Treaty Act, even though it is not a native species.

In its ruling, the court noted that the act specifically protects “wild ducks, geese and swans.” Although the mute swan may be a nonnative, it was clearly a swan and therefore protected by the act, the court said.

The ruling was a victory for Joyce Hill, a Dorchester County resident, and Kathryn Burton, the Connecticut-based founder and president of Save Our Swans USA, who have opposed any lethal means to control the swans.

While control efforts may be needed at some point, they said the population is too low now to require control efforts.

Mute swans, native to Eurasia, were brought to North America in the late 1800s to adorn ponds. In the early 1960s, five swans escaped to the Bay.

They have no natural predators around the Bay, allowing their population to mushroom to about 4,000, most1y in Maryland. Biologists say that number could double as early as next year.

The birds do not migrate and therefore feed on the Bay’s valuable underwater grass beds during the critical summer growing season, when most other waterfowl are gone. Biologists estimate the swans eat about 9 million pounds of underwater grass beds annually, and say flocks of swans can eliminate grass beds from some areas.

Flocks of mute swans have also overrun the nesting sites of least terns and skimmers, both rare species in the region, squashing their eggs and killing the young.

As a result, state and federal biologists have taken to treating mute swan eggs on state and federal lands so they will not hatch.

A special Maryland task force has been developing a strategy to control the swans. Early versions of its strategy called for designating “swan-free zones” around sensitive resources where troublesome birds could be shot as a last resort. The strategy has not been finalized.

The court ruling does not necessarily preclude efforts to control the mute swans. But management actions are on hold until the U.S. Fish and Wildlife Service interprets the ruling and offers direction.