A flesh-footed shearwater flying over the ocean.

Young flesh-footed shearwaters spend five to eight years at sea, but human impacts are making them less able to survive the trip. Image © Wirestock Creators/Shutterstock.

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Plastic diet could be causing seabird chicks to shrink

Seabird colonies worldwide have shrunk in recent decades as human pressure on the marine environment continues to increase.

New research shows how the body condition of young shearwaters has rapidly deteriorated in just over a decade, with plastic consumption believed to be the most likely cause.

Oceans are under extreme pressure due to increased human activities such as habitat loss, overfishing and plastic pollution.

Seabirds rely on a healthy marine environment as the resources it provides are essential for surviving prolonged periods in the open oceans. However, as the state of the ocean deteriorates, it's likely that these species will also suffer.

A new study published in the ICES Journal of Marine Sciences looks at how body condition has changed in flesh-footed shearwaters on Lord Howe Island, which are on the front line of the Anthropocene.  

Between 2010 and 2022, the mass and length of wings, head and bill of fledglings in the colony has significantly decreased, which will have severe impacts on their survival.

Dr Alex Bond, Curator in Charge of Birds at the Museum and co-author of the study, says, 'When we looked at the data collected over the last 13 years, it was just an idle thought of wondering how chick body condition has changed.'

'We expected it to fluctuate up and down with good and bad years, but we were speechless when we saw the results.'

'That's when we realised we hadn't seen any of the really heavy birds we would often see in the early years. So putting it together and seeing it in the bigger context of the last 13 years was surprising because it was not what we expected.'

Two dead flesh-footed shearwaters lying next to each other on a table. One is a healthy bird and much larger and the second is smaller and still has its down feathers

Fledgling flesh-footed Shearwaters on Lord Howe Island in May 2022, (left) a 'healthy' chick that had been hit by a car and (right) a similar age bird found washed up on the beach with lower body mass, shorter wing length, and downy feathers. © J L Lavers & A L Bond, 2023.

Why does a decline in body condition matter?

Lord Howe Island lies 600 kilometres off the east coast of Australia and supports the largest colony of flesh-footed shearwaters in the world. The island's biodiversity faces many pressures, including habitat loss, invasive species, fishing, and plastic pollution.

Shearwater chicks spend around 90 days in their burrow being fed by their parents. After this time, the parents will abandon their young, leaving them to fend for themselves.  

Usually, the chicks will stay in the colony for a night or two before flying to Japan. After leaving land, they won't return for another five to eight years until they are old enough to breed. As the chicks have to forage for themselves, they must be well-fed enough to survive on their own.

'Even the healthier birds are getting lighter, which is worrying because body mass is probably one of the biggest predictors of survival in the first couple years,' explains Alex.

'Imagine your parents providing for you your whole life, and then being told you now have to run a marathon without any training or knowledge of how to gather food. That's sort of the equivalent of what these birds are facing.' 

'Obviously, the more reserves you have and the more fat you've got on your body, the longer you can withstand that period in your first year while you learn how to survive on your own.'

Aerial shot of lord Howe Island

Lord Howe Island has the largest colony of flesh-footed shearwaters in the world, but human activity threatens the island's wildlife. Image © Juergen_Wallstabe/Shutterstock.

What is causing the decline in condition?

At the beginning of the study in 2010, most birds weighed around 690 grams, which is about the same as a basketball. In the last few years, however, more than half the birds weighed less than 400 grams, a critical threshold for survival in their first year.

As these changes do not appear to be tied to food availability or El Niño or La Niña climate oscillations, researchers believe it points towards something else impacting the population.

The team have previously found that the flesh-footed shearwaters on Lord Howe Island are the most plastic-contaminated birds in the world, as they consume pieces of plastic at sea after mistaking it for food. Therefore, the researchers believe this is the most likely reason for their reduced body condition. 

When the stomach is full of plastic, it alters the blood and causes scarring in the stomach. It also displaces room for more nutritious food, such as squid, that the chicks would normally eat.

'We can't say conclusively that plastic is causing this massive decline, but it's pretty high up the list of candidates.' says Alex. 'We have been studying this colony for 15 years, and we can see in the data the substantial changes that are taking place.'

'Chicks that fledge at smaller mass and with shorter wings have lower survival, and we know that from across many seabird species. Because these birds go to sea for the first few years of their life, we probably won't see the consequences of this for several years.' 

'This is an early indication that the number of burrows and breeding pairs we see in 10 to 15 years will likely decrease, mainly because those birds won't survive the first eight years to return to breed.'