Birds of a feather don't always flock together in fact, they have may have barely mixed with each other for hundreds of years, research by New Zealand scientists has found.

Seabirds [File photo]

Populations of migrating seabirds were genetically distinct and had been for centuries, despite the birds covering thousands of kilometers across the Pacific Ocean each year, researchers at the National Institute of Water and Atmospheric Research (NIWA) and the University of Auckland found.

The research, published by the journal, Nature Communications, studied the migratory behavior and genetics of two populations of Cook's petrel, a small 200-gram seabird that breeds only in New Zealand, said a statement from the university.

It revealed the populations were not interbreeding despite the fact they could easily visit each others' colonies during breeding seasons, said the statement.

Migratory seabirds were some of the most mobile in the world and could travel more than 1,000 km in a day.

"You'd think that interbreeding between populations would be very common where birds can easily visit other breeding locations for their species," said Dr Matt Rayner, a NIWA scientist and University of Auckland research associate.

"What we found is that migrating to different locations contributes to genetic differences between seabird populations as it impacts populations breeding timetables."

Until recently, it had been impossible to track many small seabirds at sea over long periods, but the study conducted from 2008 to 2010 used new geolocators lightweight tracking devices attached to the birds' legs and weighing only 2 grams.

Information was retrieved from the geolocators when the birds returned to New Zealand.

"It works just like a sextant used on Captain Cook's voyage through the Pacific. You get latitude from day length and longitude via the timing of sunrise or sunset," said Rayner.

"We found that seabirds from one Cook's petrel population breeding on Little Barrier Island migrated across the equator to the North Pacific Ocean, whereas birds from Codfish Island stayed within the South Pacific, and migrated to the waters off South America," said Rayner.

The scientists looked at DNA from tissue samples of old Cook's petrel skins, collected from the North Pacific and South Pacific destinations of the tracked birds more than 100 years ago, which were housed in U.S. museums.

They found that the DNA from the old skins matched perfectly the DNA of the modern populations, confirming the populations had been migrating and adapting to the different locations for a long time.

"Many New Zealand seabirds survive by experiencing an endless summer, living and breeding in productive waters north and south of the equator," said Rayner.

The scientists were planning to study the implications of specific seabird populations migrating to destinations that might be affected by climate change, or recent disasters like Fukushima, in Japan. Many New Zealand seabirds forage off the eastern coast of Japan during the New Zealand winter.

The study was funded by New Zealand's Ministry of Science and Innovation and the ASB Bank Community Trust.

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