The birds, which have wing spans up to 11 feet, are breeding earlier in the season compared with 30 years ago, the British Antarctic Survey reported Monday.
Studying the birds on the sub-Antarctic island of South Georgia, researchers found the birds laying eggs an average of 2.2 days earlier than in the past.
"Our results are surprising. Every year we can determine when the birds return to the island after migration, and the exact day they lay their egg," Sue Lewis of the University of Edinburgh's School of Biological Sciences said.
"We knew that some birds were laying earlier -- those who were older or had recently changed partner -- but now we see that those which haven't bred successfully in the past are also laying earlier, and these birds are effectively driving this trend in earlier laying."
Scientists say they are unsure what is causing the earlier breeding.
"It is possible that earlier breeding in some females at South Georgia is a consequence of environmental change, but at the moment we are not sure if this is related to weather, a change in oceanographic conditions or food availability to which only some birds are responding," British Antarctic Survey bird ecologist Richard Phillips said,
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