ALMOST 1.5 million short-tailed shearwaters have begun arriving on Phillip Island for their annual breeding season, after a 16,000-kilometre journey, as new analysis sheds light on the possible impact of climate change on the migratory birds.

In the 2019-20 breeding season, the birds, which always arrive in late September, did not arrive until October and some even as late as November – a delay never seen before by local researchers.

Location tracking data has now been analysed and solved the mystery of where hundreds of thousands of short-tailed shearwaters spent several weeks before making a late return to the Island.

Scientists have now visually mapped the birds’ flight path, showing their epic journey to find food and return to breed.

Geolocators were recovered from 20 birds when they returned to Phillip Island, and the tracking data showed 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.

Boat-based surveys in the Northern Hemisphere since 2007 have recorded shearwaters concentrating their foraging further and further north.

Seabird die-offs, mostly of short-tailed shearwaters, have been recorded in the Bering and Chukchi Seas since 2017.

Phillip Island Nature Parks deputy research director Dr Duncan Sutherland said the changes in bird migration could well be caused by climate change.

“We are seeing significant changes in the sea temperature and extent of sea ice in the northern Bering and Chukchi Seas, near Alaska. These are changes driven by climate change,” Dr Sutherland said.

“While the 2019-20 breeding season did return to normal after their delayed arrival, and the birds resumed their strict schedule, we were concerned by the unexplained delay.

“In seasons to come, climate change could see shearwaters pushed further north looking for food in colder, more productive waters, and further delaying their return to breed, with the possibility of missing the breeding window.”

Shearwaters run on a tight breeding schedule and only lay one egg each season, so any delay can impact the colony’s population.

Long-term research will help to answer why the birds were delayed and if it will happen more frequently, but researchers have found the delay likely relates to their search for food to fuel up for the 16,000km journey home.

“We don’t have all the answers 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,” Dr Sutherland said.

“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.”

The 2020-21 breeding season is the best recorded since monitoring began in 2010. Of the 180 nesting boxes monitored each year, 116 chicks made it to fledgling stage – compared to 72 last year, 54 in 2018-19 and 96 in 2017-18.

Many of the fledgling birds will remain in northern waters for their first couple of winters, and only arrive back on Phillip Island when they are a couple of years old.

Researchers hope the returning birds will have another bumper breeding season. Short-tailed shearwaters, known as biyadin to the island’s Traditional Owners, are a culturally significant species to the Bunurong, who have lived alongside these birds for many thousands of years as they care for Country.

The research was undertaken in partnership between Phillip Island Nature Parks and the Victorian Ornithological Research Group.