The ozone hole over the Antarctic could be to blame for changes in the breeding patterns of Ross Sea adelie penguins.

United States ecologist David Ainley and Landcare Research, Lincoln, ecologist Phil Lyver have been on the ice this summer studying the Ross Sea's smallest and most abundant penguins.

Ainley, of HT Harvey and Associates in San Francisco, is the driving force behind efforts to make the Ross Sea a marine-protected area.

In Christchurch yesterday, on his way home from the Antarctic, he said adelie penguins were breeding later in their lives than in the past.

Comprehensive surveying of the birds at three Ross Sea sites - Cape Royds, Cape Bird and Cape Crozier - over 16 summers had showed some of their vital dates were changing.

The first serious study of adelie penguin demographics was in early 1959.

"The average age of first breeding has increased from around 4 to 5 years old to 6 to 7 years. It's very interesting. My hypothesis is that it has to do with increasing amounts of sea ice in the Ross Sea region, which has been under way for the last 20 years or so because of the ozone hole."

More ice meant the penguins ended up drifting further away from their traditional breeding grounds, Ainley said.

"During the winter, the birds are finding themselves further and further north, so in spring they have to get back south.

"There's more ice, and ice is not smooth, and penguins are hesitant to cross cracks or jump from one ice floe to the next, because of the fear of leopard seals. So they can end up being delayed long enough that they miss out on that breeding season."

Despite that, the later first breeding did not seem to be having an impact on penguin numbers.

"The [Antarctic] toothfish industry is removing a major competitor of the penguins - both feed mostly on the Antarctic silverfish - so it's getting easier to successfully raise chicks to a larger size."

Ten years ago it had been incorrectly assumed that the adelie penguin population would decline as the ice grew, Ainley said.