Until recently, 90 percent of Pacific brant wintered in Mexico, but now as many as to 30 percent are opting to spend their winters in Alaska instead, the research shows.
"This increase in wintering numbers of brant in Alaska coincides with a general warming of temperatures in the North Pacific and Bering Sea," said David Ward, the lead author of the study and a USGS researcher at the Alaska Science Center. "This suggests that environmental conditions have changed for one of the northernmost-wintering populations of geese."
Although records are sparse, fewer than 3,000 brant were detected wintering in Alaska before 1977, a number that has jumped to as many as 40,000 birds now.
The species is "of federal management concern" because its numbers have been declining steadily across its entire range since the early 1960s.
The shift in climate to a warmer phase after 1976 had a well-documented effect on the abundance and distribution of marine species, including walleye pollock, Pacific cod, northern fur seals, and seabirds known as thick-billed murres.
Pacific brant fly over California's Morro Bay. (Photo by Teddy Llovet)
But the effects on species such as brant, which are restricted to estuarine ecosystems, had not been investigated.
"Our study suggests that the growth in the brant population wintering on the Alaska Peninsula is linked to this same climate change," Ward said.
The shift, said Ward, appears related to changes in the availability and abundance of eelgrass, the primary food of brant in their nonbreeding season. For this species, coastal environmental conditions have become more favorable with higher air and water temperatures, which have led to a reduction of coastal sea ice and more eelgrass accessible to brant.
"Undisturbed access to sufficient amounts of eelgrass is likely crucial to the winter survival of this species," Ward observed.
Brant are "short-stopping" on their southward migration and remaining north of their traditional wintering grounds, he said.
Ward and his co-authors suspect that Pacific brant numbers will continue to increase in Alaska during winter, given climate predictions for ever-warming temperatures and less ice at higher latitudes.
But sudden and severe cold snaps with more extensive shoreline ice, like that of 1991-1992, could put more of the entire brant population at risk with so many of the birds now wintering in Alaska, Ward said.
In addition, said Ward, a changing wind regime is also affecting brant migration. Traditionally, the flow of southerly winds from the Aleutian Islands favored southward-migrating brant and assisted their migration out of Alaska.
Now, there are fewer days each fall where brant have a favorable tail-wind to speed them on their 3,000 mile-long migration to Mexico. Ward and his team found that the increase in the number of brant wintering in Alaska was linked to the fewer number of days with favorable southward wind flow.
"Alaska now has the greatest concentration of Pacific brant outside of Mexico," said Ward. "Because of this, threats to the Alaska wintering population can affect the entire Pacific Flyway population."
The article, "Change in abundance of Pacific brant wintering in Alaska: evidence of a climate warming effect?," is published in the current issue of "Arctic" magazine.
The U.S. Fish and Wildlife Service designates the brant as a "Bird of Management Concern."
The National Audubon Society warns that the brant's preference for Alaska could be damaging to the species' health. The Izembek Lagoon on the Alaska Peninsula, a key staging area for brant migration, is in line for oil and gas exploration. Bristol Bay, immediately adjacent to the Izembek National Wildlife Refuge, has been excluded from offshore drilling since 1989. But Congress lifted the drilling moratorium in November 2003, and the federal Minerals Management Service is now considering Bristol Bay in its five-year Offshore Oil and Gas Lease Sale Program (2007-2012).
"An oil spill, coupled with a year of bad weather or mismanaged hunting, could be devastating for migrating brant," the Audubon Society says.
Copyright Environment News Service, ENS, 2009. All rights reserved.