Antarctic birds are breeding later.

As global temperatures rise, Arctic birds are breeding earlier than they did in previous decades. However, the reverse is true in Antarctica, new research shows.

"In the Arctic, spring basically comes earlier, and most species [of migratory birds] respond by arriving and laying eggs sooner," says Christophe Barbrand, an ornithologist at the National Center for Scientific Research in Villiers en Bois, France. But there have been few studies of the phenomenon in the southern hemisphere.

Since 1950, ornithologists in Terre Adelie, Antarctica, have kept careful track of when nine species of seabirds arrive and lay their first eggs. Using those data, Barbrand and his colleague Henri Weimerskirch calculated that the Antarctic birds migrated to the region an average of 9 days later in 2004 than they had in the early 1950s, and they laid their eggs an average of 2 days later.

Temperatures don t appear to be increasing in all of Antarctica, as they are in the Arctic. But temperature changes per se don't explain the Antarctic breeding schedules. Instead, changes to Antarctica's surrounding layer of sea ice may be at least partially to blame, they say.

Since the 1950s, total sea ice around eastern Antarctica has decreased by 12 to 20 percent. At the same time, winters are lasting longer. That combination could be a one-two punch, Barbraud says.

Sea ice shelters krill and other marine organisms that the birds feed on, he says. Less sea ice means less krill, so the birds may need to fatten up before they arrive to breed. And as the sea ice breaks up later because of delayed yearly springs, the birds may also have to wait longer to reach their colonies, the researchers report in the April 18 Proceedings of the National Academy of Sciences.