Human impacts may cause the deterioration in body condition of shearwater chicks

New research shows how the body condition of fledglings in a colony of shearwaters has rapidly declined in just over a decade, with human impacts thought 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.

Dr Alex Bond, Curator in Charge of Birds at the Natural History 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.'

Lord Howe Island lies 600 kilometres off the East coast of Australia and houses 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 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 of environmental change.

Dr Bond continues, '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.

'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.'

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.

As these changes do not appear to be tied to food availability or El Nino or La Nina climate oscillations, researchers believe it points towards a stressor 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 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.’ says Alex.

'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.'

The findings of the study were published in the ICES Journal of Marine Sciences.

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