Why are some seabirds better at avoiding eating plastic?
A 13-year study has found that some shearwaters in Australia appear to be better than others at avoiding feeding plastic to their chicks.
Earth's oceans are awash with plastic, from tiny pellets to plastic bottles, and animals all over the globe are suffering from the pollution flowing steadily into their habitats.
In Australia, more than 77 marine species are known to be affected by plastic pollution, including birds and seals.
Plastic can entangle animals, block their airways or digestive tracks and damage their internal organs. Eating plastic can also have other, non-fatal effects that are less studied. For birds, these can include smaller wings and lower body mass.
Dr Alex Bond, a Senior Curator of Birds at the Museum, has for years been part of a team studying seabirds in and around Australia, investigating just how much damage plastic does to several species. His focus has been on two species of shearwaters, long-winged seabirds that can be found all over the world.
His research has come up with some puzzling findings. A new paper published in Marine Pollution Bulletin suggests that while Australian wedge-tailed shearwaters do eat plastic, they eat far less of it than other species, and it doesn't appear to have a huge negative impact on their body sizes.
Over 13 years, Alex and his colleagues measured the body mass, wing length, head size and bill length of 224 wedge-tailed shearwaters. They also measured how much plastic was in each bird's stomach.
Alex says, 'We didn't find that plastic pollution affected the shearwaters' body size.
'This is good news, but it is interesting because this isn't true for all seabirds living in Australia. Wedge-tailed shearwaters are closely related to flesh-footed shearwaters - and that species is among the worst-contaminated bird species in the world.
'Both species eat broadly the same food, breed at roughly the same time of year, and feed in or around the same place, yet one is so much more badly affected than the other, and we have no idea why.'
A plastic diet
The team studied wedge-tailed shearwaters on Lord Howe Island, off New South Wales.
Fledglings around 80 days old were examined, in order to find out if plastic consumption affects how young birds grow.
Some of them were found already dead on the island's beaches, and others were examined while still alive before being released.
It's more than a decade's work, as the team took samples during 2005, then further samples between 2013 and 2018.
Of the birds they looked at after 2013, 35.2% had between one and 15 pieces of plastic inside them.
That's actually marginally less than the group studied in 2005, in which about 40% had plastic in their stomachs.
The chicks were also fed far more white plastic than any other colour every year during the study, possibly due to there being more white plastic in the ocean.
But despite being found with pellets, bottle caps and balloon fragments inside their bellies, the young wedge-footed shearwaters' health did not seem to suffer significantly.
Alex says, 'These findings are fascinating. Why are these birds doing so much better than their relatives? What selection is going on? They could be better at figuring out what is plastic and what is prey. What is it about this animal that makes them different?'
Monitoring the ocean's future
Wedge-tailed shearwaters live all over the Indian and Pacific oceans, meaning they could be a useful species for monitoring levels of plastic in the water in the future.
Alex says, 'Monitoring plastic inside seabirds is one of the best ways to measure plastic pollution on an international scale.
'If you really want to get an understanding of what's going on at a global scale, you need to work with widespread species over a number of years.
'Many studies on ocean plastic are based on one species during one year at one site, which may not be representative over a longer period of time. The great thing about this study is that it has given us a dataset over a longer term.'