Seabirds that eat plastic have smaller body sizes, raised cholesterol levels, slow wing growth, and impaired kidney function, according to a study published in Environmental Science & Technology.
The toll of plastic pollution is pervasive, with patches of garbage, wildlife deaths, and microplastics in the food we fork into our mouths all well known. However, less explored is that which we cannot see, including the lasting harm plastic inflicts on wildlife that survives consumption.
"Unfortunately, the shearwaters on Lord Howe Island are some of the most affected in terms of plastic ingestion," said study author Dr Alex Bond, senior curator in charge of birds at the Natural History Museum, to IFLScience. "In a given year, up to 80-90 percent of chicks will have some plastic in them, which was fed by their parents during the 90 days they remain in their burrows."
The team took blood samples from 53 flesh-footed shearwaters (Ardenna carneipes) on Lord Howe Island, a tropical UNESCO World Heritage Site off the coast of Australia. It is a destination not only for tourists but tens of thousands of seabirds. The idyllic landscape, however, is marred by the scourge of plastic sickening seabird populations on the island, with shearwaters being some of the most contaminated birds on Earth.
"What was interesting was that for certain blood chemistry values we did find some significant effects. For example, birds with plastic had higher cholesterol, they had lower dissolved calcium and they also tended to have more uric acid and more amylase in their blood. Just a single piece of plastic is enough to cause this change," said Bond in a statement.
Plastic is toxic and increasingly hazardous as pollutants accumulate from the surrounding environment over time, note the authors. If ingested, the toxins can leach into the creature’s bloodstream. Although the effects of higher cholesterol levels for seabirds is unknown at this time, in humans it can cause circulatory issues. For now, it's uncertain whether plastic is causing this harm or, for example, it is the bacteria on the surface.
"We don’t yet know what these differences mean for the birds themselves; a 'normal' range for blood chemistry values just hasn’t been established for many wild species, and we can’t use analogies from domestic chickens or humans, for example," said Bond to IFLScience. "What they point to is that even in outwardly apparently healthy birds, plastic may be having hidden impacts that have gone undetected until now."
In birds, increased uric acid production can occur during the final phase of fasting, as well as result in renal failure or kidney stones. The increased levels of uric acid are thus "potentially problematic". The slow wing growth is of concern too.
"We know from other studies that chicks that leave their breeding colony underweight and with shorter wings tend to have lower survival during their first years at sea," said Bond. "Flesh-footed shearwaters don’t start breeding until they’re 5-7 years old, so these 'delayed mortalities' are often not observed, or simply result in a chick not returning to a colony when it’s mature. Shorter wings means it takes more energy to fly, meaning the birds have to find more food, which is something they have to learn to do themselves when they fledge."
Flesh-footed shearwater populations are in decline with previous studies finding a link between plastic consumption and chick growth. Bond has observed seabirds on this island feeding their younglings plastic toy car wheels, balloon clips, and bottle tops. The IUCN Red List lists flesh-footed shearwaters as "near-threatened".
"You have to wonder how long this species can keep this up," added Bond.