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Tundra
Swan (Cygnus columbianus)
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Skiing to work over a persistent spring snowpack, I looked up
to see a large white bird flapping gracefully over the spruce tops.
A few gentle honks confirmed it was a tundra swan.
After a long winter when all the large birds were black, it
was good to see one of the frontrunners of the billions now winging
to Alaska.
Tundra swans can live to be older than 20. Perhaps this bird,
about 15 pounds with a wingspan of almost six feet, had passed over
the lowlands north of UAF many times. The swan was probably headed
to northwest Alaska or the Yukon-Kuskokwim delta.
That's according to a biologist who was part of a team that
captured and released more than 500 tundra swans in 2007 and 2008.
The birds were temporarily flightless as they shed old feathers
and grew new ones in their favorite Alaska breeding areas: Cold
Bay, King Salmon, the Yukon and Kuskokwim river deltas, Kotzebue
Sound and the Colville River delta.
Craig Ely of the USGS Science Center in Anchorage handled many
of those swans and helped to implant satellite transmitters in the
abdomens of 50 birds. Those transmitters lasted for a few years,
enabling scientists to confirm a split in Alaska tundra swan populations.
Swans that spend their summers in tundra lakes north of the
Brooks Range are East Coast birds in the winter, settling in Maryland,
Virginia and North Carolina.
Swans that breed south of the Brooks Range in the wetlands of
western Alaska are West Coast birds in the winter, feeding in farm
fields of Washington, Oregon and the central valley of California.
Though the East Coast and West Coast swans migrate wingtip to
wingtip and pause in similar barley fields and lakes in Saskatchewan,
the birds almost never follow their neighbors to a new place.
"Tundra swans are extremely site faithful, so only extremely
rarely would a Colville River bird end up on the West Coast," Ely
said.
Both
the East and West Coast tundra swans spend the majority of their
lives migrating. Based on information from the satellite trackers,
the bird I saw might have left the Central Valley of California
in late January. From there, it looped across Oregon and Washington,
maybe northern Utah, on its way to the prairie in Canada. Then it
flew over northern British Columbia and back to Alaska.
After a few weeks in wetlands of the boreal forest, the bird
will be on its way to treeless tundra that will soon be thawed and
rich with aquatic plants.
Unlike other birds now flying direct paths to Alaska, tundra
swans take months to cross the continent.
"All the Alaska populations of tundra swans spend more time
in migration than on breeding or wintering areas," Ely said. "They
truly are birds on the move."
Ely and other scientists think that the East-Coast-wintering
birds that summer on the Colville River may be flying an ancient
ice-free corridor that could have been the only unfrozen pathway
north during the last Ice Age.
Fascinated by the unique, divergent paths of the two populations
of tundra swans, Ely and others studied which group, including lower
Alaska Peninsula tundra swans that don't migrate, might have a survival
advantage. They found all swans were doing well despite how far
they traveled and how little time that might give them on the breeding
grounds.
"The tremendous diversity of migration strategies we identify
in Alaskan tundra swans, without clear impacts on survival, underscores
the ability of this species to adapt to different environments and
climatic regimes," Ely wrote.
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