19 November 2009

Population of Trumpeter Swans in High Plains Flock Largest Ever

A late-summer aerial survey of Trumpeter Swans indicates the highest population ever known for the High Plains Flock of this species reintroduced to the region in the early 1960s.

"It is great that these swans have reached these numbers," said Shilo Comeau, a biologist with the U.S. Fish and Wildlife Service and coauthor of a report on the annual survey. This was achieved "without little intensive management from humans.

"The swans are recovering well."

A "record-high" 523 swans were counted during a survey of northwest Nebraska and southwest South Dakota carried out from August 31 to September 3, by biologists with the U.S. Fish and Wildlife Service and Nebraska Game and Parks Commission, according to a report just issued by the federal agency.

There was an "increase of 22% from the 2008 estimate, and was primarily the result of a higher number of cygnets and breeding pairs observed. The number of breeding pairs increased, and correspondingly so did the number of broods and average brood size." The number of non-breeding pairs, did however, remain stable, according to the report.

"The flock continues to experience a positive growth rate of 4.8% annually from 1990 to 2009," according to the survey report.

High Plains Flock Trumpeter Swan Production Survey Results 1995-2009.

The aerial survey was conducted via a Cessna 182 airplane. An area in northwest Wyoming was not included this year, though it had been previously surveyed. There have not been any swans sighted here during the past seven years, according to the government report.

Additional birds in the eastern sandhills were counted via ground-based surveys by personnel of the Nebraska Game and Parks Commission.

The number of young raised was readily indicated by the survey results, which indicated an increase in the number of breeding pairs and average brood size. The increase in cygnets "could be attributed to the coinciding factors of a large number of white [adult] birds becoming reproductively active and an improvement in habitat quality," according to the report. "This year the majority of wetlands in the survey area contained some water, cover, and the subaquatic food resources appeared abundant."

Conditions continued to also be favorable in the central and southeastern portions of the route, and swans have been expanding their range eastward, according to Nebraska Game and Parks Personnel. Three years ago survey efforts were extended further east in the sandhills. Biologists counted an additional 50 swans in this region during the recent autumn survey.

The birds in this area are a portion of the High Plains Flock, said Vrtiska, a coauthor of the swan survey report, and not from the increasing population in Iowa and Minnesota. "Right now, those groups tend to have distinct breeding and wintering areas," he said. There may be starting some overlap in eastern Nebraska, but without neck collars that could provide a certain identification, it is not known if these swans are from the sandhills flock.

Management Objective Achieved

Results of the survey denote the first time that the population objective for managing the Trumpeter Swan has been achieved.

The objective is "to develop a dispersed population consisting of at least 500 total birds counted during the production survey and 50 successful breeding pairs by 2010."

Results from this year, can be attributed to "improvement of breeding habitat coinciding with the maturity of the 2004 hatch year class." In 2004, "a relatively high number of cygnets were produced (107), making these birds reproductively mature."

The future for the swans looks good, according to the government report: "As long as habitat conditions remain favorable and no major stochastic event occurs, it is likely these parameters will continue to be achieved."

Improved habitat conditions for these waterfowl were a result of increased precipitation levels, the report indicated. Localities not suitable for the swans during recent drought conditions, have been used by these birds as wetland conditions improved with an increase in precipitation.

"Wetland that provided marginal breeding for newly established pairs during the drought may now be adequate for production," the report indicated. "Still the number of the pairs observed had no cygnets, but the number of breeding pairs did increase 43% from last year."

Sand hill "ranchers have also been a tremendous benefit for the swans," Comeau said, by their leaving wetlands intact and the relative isolation of many of the places where the swans breed, which limits any disturbance.

Wildlife officials are currently modeling the availability of habitat in the sandhills, and the results are expected to indicate if there are enough wetlands for the swan population to continue at the levels documented by the 2009 autumn survey.

"With decent precipitation," Comeau said, an ongoing "population of 450-550" swans could be expected.

Annual fall surveys to determine the number of Trumpeter Swans have been conducted for at least two decades in order to "track abundance trends in the flock and condition of the wetlands" where this swan - the largest species of waterfowl in North America - occurs.

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