Probable molt migration of a wild Trumpeter Swan documented by GPS technology

By Tyler M. Harms1 and Stephen J. Dinsmore2

1Iowa Department of Natural Resources, Boone Wildlife Research Station, 1436 255th Street, Boone, Iowa 50036

2Department of Natural Resource Ecology and Management, Iowa State University, 339 Science Hall II, 2310 Pammel Drive, Ames, Iowa 50011

Once widespread throughout North America, the Trumpeter Swan Cygnus buccinator (hereafter “swan”) was extirpated from much of its historic range by the turn of the 20th Century due largely to overhunting and habitat loss. This was especially true of the Interior Population, which historically spanned across much of the Midwest United States. Thanks to efforts of several Midwest states, starting with Minnesota in 1966, to reintroduce swans and “trumpet the cause” for wetland conservation, the Interior Population now comprises nearly 40% of North America’s swans and continues to increase (Trumpeter Swan Society 2022). 

Tyler M. Harms with hatch-year Trumpeter Swan (Cygnus buccinator) fitted with GPS-GSM collar at Hanlontown Slough Waterfowl Production Area, Worth County, Iowa, USA, September 10, 2017. Photo courtesy of Margaret Smith, The Trumpeter Swan Society.

The rapid increase of swans in the Interior Population prompted a need to better understand their ecology and annual movement dynamics. In 2017, Iowa State University and the Iowa Department of Natural Resources captured nine hatch-year swans (4 males and 5 females) on wetlands in central and north-central Iowa, USA, and marked each with a GPS-GSM collar (WT-300 Swan, KoEco) to monitor year-round movements and habitat use. We captured and marked an additional four hatch-year swans (1 male and 3 females) in 2018. We programmed GPS-GSM collars to record locations every hour from 0900 to 2100 hours in 2017 and each hour from 0500 to 2100 hours in 2018 as well as a single point at 2400 hours both years to capture a roost location. Collars transmitted data to an online data portal using 3G cellular networks twice daily.

Capture crew with Trumpeter Swan (Cygnus buccinator) family group captured at Cardinal Marsh Wildlife Management Area, Winneshiek County, Iowa, USA, August 24, 2020. Photo courtesy of David Hoffman.

In 2019, we observed a probable molt migration of a sub-adult, non-breeding male swan that was captured as a hatch-year bird at Hanlontown Slough Waterfowl Production Area in Worth County, Iowa, USA in 2018. On June 1, 2019, this swan departed an area near its natal wetland, where it spent much of its first spring, flying northwest and eventually stopping in Becker County, Minnesota, USA, on June 3, a distance of approximately 450 km. On July 4, 2019, the swan was located at Great Slave Lake in the Northwest Territories, Canada, approximately 1,970 km from its previous location in northwest Minnesota. Unfortunately, we don’t know how long the swan stayed in Minnesota or when it arrived in the Northwest Territories due to data loss from the collar. This remarkable movement coincides with the approximate timing of the second prebasic molt in swans (Mitchell and Eichholz 2020).

From July 4 to September 14, 2019, the swan limited its movements to two small basins connected by a channel near Great Slave Lake. Distances between points during this time period averaged approximately 126 m from July 4 to August 19 and 583 m from August 20 to September 14, suggesting the bird was likely flightless early and later able to make short flights. The length of this time period matches the approximate flightless period for the second prebasic molt (Mitchell and Eichholz 2020). Then, on September 14, 2019, the swan departed Great Slave Lake to the south making a series of stops in Alberta and Saskatchewan, Canada, as well as Minnesota, USA (very near to where it stopped on its northward journey) before arriving back in Iowa, USA at Muskrat Slough Wildlife Management Area in Jones County.

Apparent molt migration of a nonbreeding male Trumpeter Swan (Cygnus buccinator) captured as a hatch-year bird on Hanlontown Slough Waterfowl Production Area, Worth County, Iowa, USA, on August 24, 2018. Dots represent dates when the bird was at a single location, solid lines represent movements between known start and end points, and dashed line represents movement between unknown start and end points.

Although molt migration has been previously documented in other large-bodied waterfowl such as the Canada Goose (Zicus 1981, Davis et al. 1985, Abraham et al. 1999, Sheaffer et al. 2007, Luukkonen et al. 2008, Dieter and Anderson 2009), little is known about such movements in swans (Mitchell and Eichholz 2020). The swan described in our study made a remarkable 2,420-km movement to the northwest where it spent most of the late-summer months, a movement not exhibited by any other swan in our study. As the Interior Population of swans becomes more established, we speculate such long-distance movements for molting will become more frequent and encourage additional research to monitor swan movements throughout the annual cycle.

Trumpeter Swan (Cygnus buccinator) family group marked with GPS-GSM collar (adult) and alphanumeric neck collars (juveniles) at Anderson Lake, Hamilton County, Iowa, USA, October 7, 2020. Photo courtesy of David Hoffman.

This discovery would not have been possible without financial support from the Iowa State University Trumpeter Swan Restoration Committee, Trumpeter Swan Society, Blank Park Zoo, the Friends of Union Slough National Wildlife Refuge, Chickasaw County (Iowa) Conservation Board, and Carolyn J. Fischer. Logistical support was provided by staff from the Iowa Department of Natural Resources and U.S. Fish and Wildlife Service at Union Slough National Wildlife Refuge as well as C. Lundy, J. Moore, K.T. Murphy, and R. A. Vanausdall.

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