A new study reveals that human impacts could be the cause of the rapid decline in the body condition of fledgling shearwaters on Lord Howe Island. The research, conducted by scientists from the Natural History Museum, found that the body condition of young birds has significantly decreased in the colony over the past decade. This could have long-term effects on the population of flesh-footed shearwaters on the island, as chicks with lower body mass and shorter wings have lower survival rates.
The Study
The researchers collected data on how body condition, including mass and length of wings, head, and bill, changed in flesh-footed shearwaters on Lord Howe Island between 2010 and 2022. The island houses the largest colony of flesh-footed shearwaters in the world, making it an ideal location for the study. Shearwater chicks spend around 90 days in their burrow after hatching being fed by their parents. After this time, the parents abandon their young, leaving them to fend for themselves.
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.
Unexpected Results
The results of the study were unexpected. 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."
"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," Dr. Bond continued.
Potential Causes
The team found that these changes in body condition were not tied to food availability or climate oscillations, leading researchers to believe that human impacts were the most likely cause. Lord Howe Island faces many pressures, including habitat loss, invasive species, fishing, and plastic pollution.
The researchers 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 that plastic pollution 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 Dr. Bond.
Long-Term Effects
The long-term effects of this decline in body condition are still unknown. However, chicks that fledge at smaller mass and with shorter wings have lower survival rates, which could result in a decrease in the number of burrows and breeding pairs in the future.
Dr. Bond explains, "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."
Journal Information: J L Lavers et al, Long-term decline in fledging body condition of Flesh-footed Shearwaters (Ardenna carneipes), ICES Journal of Marine Science (2023). DOI: 10.1093/icesjms/fsad048