Plastic ingested by young seabirds can drastically reduce their chances of surviving to adulthood and returning to breed, threatening entire populations. The study’s findings have been published in the Proceedings of the National Academy of Sciences.
Scientists have demonstrated for the first time that plastic pollution reduces wild animal populations, rather than merely harming individual animals.
Flesh-footed shearwater chicks that ingest as little as half a gram of plastic are 12% less likely to survive and return home to breed as adults, while chicks carrying more than five grams of plastic almost never return.
The study was conducted by Jennifer Lavers (Charles Sturt University and the Natural History Museum), Hannah Worthington (University of St Andrews), and Alexander Bond (Natural History Museum). The team utilized data collected over 15 years on Lord Howe Island, a remote Australian island that hosts the world’s largest colony of flesh-footed shearwaters.
Approximately 50 million tons of plastic enter the environment annually, and a significant portion of this waste impacts marine wildlife.
Flesh-footed shearwaters have become one of the most plastic-contaminated animal populations on the planet, primarily because parents mistake floating debris for food and feed it directly to their chicks.
Earlier work by the same research team had already documented the development of a fibrotic disease known as “plasticosis” in the chicks, as well as the appearance of symptoms resembling dementia, even before they left the nest. It had not previously been proven whether all of this actually affects the number of surviving birds needed to maintain the population. To find out, researchers combined 15 years of banding and recapture data with a statistical model designed to estimate “recruitment”—the probability that a chick would survive its years at sea and return to Lord Howe Island to breed.
Between 2010 and 2024, they captured shearwater chicks on the island, measured the amount of plastic each had ingested using a safe stomach-flushing method, and then tracked which of these birds reappeared as breeding adults starting in 2020.
The pattern that emerged was striking. Chicks that fledged without plastic in their stomachs had a roughly 31% probability of eventually returning to breed.
This probability declined steadily as the amount of ingested plastic increased. With half a gram in their systems, the probability dropped by about 12%. At two grams, it was cut roughly in half.
At a weight of around two grams, the estimated probability of recruitment falls to near zero, meaning the chances of ever seeing that bird again become negligible.
This threshold is significant because the scenario is not uncommon. Roughly 10–20% of the chicks fledging from nests on Lord Howe Island each year carry more than a gram of plastic, and some are found to have more than ten grams.
Perhaps the most alarming aspect of the findings is what they reveal about how seabird health is typically measured in conservation studies. Fledging success—a standard metric used to assess a colony’s health—primarily measures whether chicks survive long enough to leave the nest. On Lord Howe Island, this rate is around 60%, which is generally considered normal for this species of seabird.
However, this figure fails to account for almost anything that happens afterward. Many chicks, stuffed with plastic, leave the island in relatively good condition but perish at sea years before they would have returned to breed.
Researchers describe these birds as essentially “the walking dead”: alive and countable when they fledge, yet statistically unlikely to ever return home.
“For a long time, people claimed that plastic doesn’t cause changes in wild population numbers. That simply isn’t true,” says Bond. “For the first time, using data from 15 years of research, we’ve been able to show that the more plastic seabirds ingest as chicks, the less likely they are to return to the colony and raise their own chicks as adults.”
The flesh-footed shearwater population on Lord Howe Island has been declining by approximately 1.3% annually for at least two decades; as of 2019, it numbered around 22,600 breeding pairs.
Because these birds are long-lived, the population decline changes in numbers have occurred gradually so far, aided by the fact that adult survival generally has a greater impact on population trends than recruitment in any single year.
However, researchers warn that even a slight but sustained decline in the number of young birds could worsen over time, especially if combined with other factors already affecting adult birds.
Their findings suggest that the problem of plastic pollution is not stabilizing; on the contrary, it could lead to a much sharper population decline.
Now, a team of researchers is urging seabird scientists to look beyond fledgling success and adult survival—the two most common health indicators. They want more attention paid to what happens to young birds during the years they spend at sea, remaining largely out of sight.
“Although we demonstrated this using Sooty Shearwaters, plastic affects all seabirds—we simply aren’t studying most of them closely enough to detect it,” explains Bond. “Without immediate action to stem the flow of plastic into our oceans, this problem will only get worse.”
About the Author
