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Posted on 07/05/2021 by Phillip Island Nature Parks

SHORT-TAILED SHEARWATER STUDY SOLVES LATE MIGRATION MYSTERY


New location tracking data has solved the mystery of where hundreds of thousands Short-tailed Shearwaters spent several weeks before making a late return to Phillip Island in 2019.

Ahead of World Migratory Bird Day this Saturday 8 May, Phillip Island Nature Parks is also excited to reveal that the birds have just had their most successful breeding season in 11 years.

Around 1.4 million birds migrate back to Phillip Island from the waters around Alaska in September every year to breed. Normally, their return is very predictable.

At the start of the 2019 breeding season, most of the birds were several weeks late and did not arrive to the island until late October or early November – a large-scale delay that had never been seen by local researchers.

While the breeding season returned to normal and the birds resumed their strict schedule, researchers were concerned by the unexplained delay.

“These birds run on a very tight schedule and it was getting close to when they would normally mate and go on their two-week ‘honeymoon trip’ before returning to lay their egg – so they were cutting it very fine,” said Phillip Island Nature Parks Deputy Research Director, Dr Duncan Sutherland.

“It was a significant delay for a migratory bird that has a very tight schedule to keep.”

New light has now been shed thanks to long-term research being undertaken in partnership between Phillip Island Nature Parks and the Victorian Ornithological Research Group, where geolocators are attached to birds to track their movements.

Geolocators have been recovered from 20 birds who returned to Phillip Island this season, including some that were part of the delayed 2019 migration. The tracking data shows that instead of heading south back toward the Phillip Island colony as they normally would, those birds instead turned north and spent a few weeks in the Arctic Circle first.

While longer-term research will help to answer why this happened, it likely relates to their search for food to fuel up for the 16,000km journey home.

“We don’t know the answer yet, but we do know that the birds are always looking for the most productive waters so they may have had to go further north to find enough food,” said Dr Sutherland.

“The waters around Alaska have been getting warmer, while at the same time the sea ice around the North Pole is receding so the birds can access those waters more than ever.”

This year’s breeding season is the best recorded since monitoring began in 2010.

Of the 180 nesting boxes monitored each year, 116 chicks have made it to fledgling stage this season – compared to 72 last year, 54 in 2019 and 96 in 2018.

These fledgling birds have already commenced their northern migration, and it’s likely this will continue into early May, with strong winds needed to help them take flight.

Phillip Island Nature Parks is implementing its Shearwater Rescue Program, once again partnering with Bass Coast Shire Council, Regional Roads Victoria, SP Ausnet and the local community to assist the shearwaters’ safe departure.