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New research shows how the body condition of fledglings in a colony of shearwaters has rapidly declined in just over a decade. Human impacts are believed to be the most likely cause.

Researchers have collected data on how body condition, which includes mass and length of wings, head, and bill, changed in Flesh-footed Shearwaters on Lord Howe Island between 2010 and 2022. This new study reveals that the body condition of young birds has significantly decreased in the colony over the last decade. Lord Howe Island lies 600 kilometers off the east coast of Australia and houses the largest colony of Flesh-footed Shearwaters in the world.

‘We were speechless’

Alex Bond, curator in charge of birds at the Natural History Museum of London and co-author of the study published in the ICES Journal of Marine Science, 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 realized 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.”

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 after hatching being fed by their parents. After this time, the parents will abandon their young, leaving them to fend for themselves, flying to Japan and not returning to land for another five to eight years until they are old enough to breed. As the chicks now have to forage for themselves, they must be well-fed enough to survive on their own.

Scientists captured the fledglings at the colony and on adjacent beaches, weighed them and measured their wing, bill, and head length. Using this data collected from the past 13 years, they could see how their body condition has changed over a decade.

“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,” Bond says.

“Imagine your parents providing for you your whole life, and then you get 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 learning period in your first year while you learn how to survive on your own.”

Young Flesh-footed Shearwaters weigh 42 percent less

At the beginning of the study in 2010, most birds weighed around 690 grams, but scientists have found that in the last few years, less than half the birds weighed more than 400g, a critical threshold for survival in their first year. That amounts to a 42 percent decline in body weight in 13 years.

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

The most plastic-contaminated birds in the world

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 candidate for reduced body condition.

When the stomach is full of plastic, it alters the blood and causes scarring. 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.” Bond says. “We have been studying this colony for 15 years, and we can see in the data the changes that are taking place.

“Chicks that fledge at smaller mass and with shorter wings have lower survival, and we know that from across a bunch of 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 earlier 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.”

Thanks to the Natural History Museum of London for providing this news.

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