Baby storks being strangled by plastic

On a late spring morning in the farmland of southern Portugal, Dr Marta Acácio rested a ladder against a tree and began to climb. Around four metres up, she reached the large white stork nest she was aiming for. Telescopic camera footage had revealed a healthy-looking chick inside, and now she intended to ring it.

However, when Acácio—a French-based ecologist at the University of Montpellier—attempted to lift the chick, it would not budge: it was tethered to the nest by a strand of plastic baler twine. She turned the bird over and recoiled—the underside of its body was crawling with maggots.

“It was being eaten alive from underneath,” says Acácio.

Equipped for such increasingly common scenarios, she took out a pocketknife, cut the twine, placed the chick in a carrier bag, and climbed back down. She and her colleagues cleaned and disinfected the wound before carefully returning the bird to its nest.

“I was hopeful that the chick would survive,” she says. “But unfortunately it did not recover from the wounds.”

This nest was one of 93 monitored weekly by a team of ecologists during the 2023 breeding season. White storks construct enormous nests over many years—some weighing up to 1,000 kilograms. These nests are not only found in trees but also atop manmade structures such as telephone poles. Various smaller bird species, including sparrows, starlings, and kestrels, often inhabit the outer edges of these nests.

“The stork nest is actually a colony of other species. It’s a fantastic species,” says prof Aldina Franco, an ecologist at the University of East Anglia (UEA) and member of the research team.

The researchers were investigating a suspicion shared by ecologists worldwide: that a hidden toll of plastic-related chick deaths is going undetected. Standard nest checks typically occur only around fledging time. However, as stork parents often dispose of deceased chicks promptly, early deaths—particularly those caused by plastic entanglement—may be missed entirely.

Over a four-year period, researchers from the University of East Anglia (UEA) and the University of Lisbon photographed nearly 600 white stork (Ciconia ciconia) nests. During the 2023 breeding season, Dr Acácio and Ursula Heinze of UEA conducted weekly physical inspections of selected nests.

Their findings, published in the journal Ecological Indicators, are cause for concern. Approximately 90% of the nests photographed contained plastic. Of those that were physically examined, over a quarter (27%) held chicks that had become entangled—most of them no older than two weeks.

The principal offender was baler twine, a plastic string widely used to secure hay bales; either the twine itself or its packaging was implicated in the vast majority of entanglements. In a smaller number of cases, chicks were caught in domestic plastic items such as bags or milk containers. Causes of death included strangulation, limb amputation, and infections from wounds.

“They roll and roll and they go around and it’s almost as if they tie the rope around their legs even harder as they move,” Franco says.

Acácio is keen to highlight successful rescues as well. On one occasion, she discovered a pair of three-week-old chicks in a nest atop a cork oak stump, both entangled in coils of blue baler twine.

“I thought the chicks were so badly entangled that neither would survive,” says Acácio, “Unfortunately the smaller chick did not survive but the larger one, which still has the marks of the entanglement, survived and fledged.”

Across every continent, birds have been observed using plastic and other human-made debris in their nests. While the dangers of plastic pollution in marine ecosystems are well-documented—with distressing images of turtles, seabirds, and fish ensnared or harmed—its impact on terrestrial birds has received less attention.

“This is not a Portuguese problem or even a white stork problem,” says Dr Inês Catry, an avian ecologist at the University of Lisbon who led the project. “Baler twine is widespread in many areas in many countries.”

Previous studies in Europe and the Americas have tended to report lower rates of nestling entanglement—ranging from 0.3% to 5.6%—largely due to less frequent nest inspections. By contrast, this study recorded an entanglement rate of 12%.

In Montana, USA, wildlife biologist Marco Restani has been collaborating with volunteers to monitor ospreys nesting along a 600-kilometre stretch of the Yellowstone River. Employed by the utility company NorthWestern Energy, Restani notes that while plastic entanglement has not yet reached population-level significance for ospreys, the individual cases he encounters are “gruesome”.

“It’s a horrible way to die. And it’s horrible for people who are discovering it as well.”

Meanwhile in Argentina, Dr María Soledad Liébana, a raptor specialist at the Institute of Earth and Environmental Sciences of La Pampa, has been examining the impact of plastic entanglement on nestlings of the southern caracara, a species of raptor.

“Plastic entanglement does appear to be a serious and growing threat to a wide range of bird species, across many different regions of the world,” she says.

For bird species already facing pressures from other environmental threats, an entanglement rate of 12% could place them under significant additional strain, says Dr Neil James, an ecologist at the University of the Highlands and Islands in Scotland.

In 2019, James launched the website birdsanddebris.com, which allows individuals to report sightings of entangled birds or human-made debris in nests. To date, reports have identified human debris in the nests of an “alarming” 160 bird species worldwide—two-thirds of which are land-dwelling species, he notes.

According to Heinze’s research team, plastic baler twine is accumulating in the environment at an alarming rate. In 2023, the global baler twine market was worth $300m (£220m) globally, and as of 2019, 80,000 tonnes were being used annually across Europe. However, the volume of twine leaking into the natural environment remains unknown.

Farmers play a key role in preventing plastic pollution—for instance, by ensuring no plastic remnants are left in fields after use. Nevertheless, many aspects of the plastic lifecycle remain beyond their control, such as the availability of local recycling infrastructure or access to biodegradable alternatives.

While collection schemes for agricultural plastics are inconsistent across Europe, research has found that where such programmes are implemented, they tend to be effective. Scientists are also working on alternatives to conventional polypropylene twine, and some biodegradable twines options are already available on the market.

In the meantime, the researchers argue that polypropylene baler twine should be classified as a hazardous material and phased out of use. They are also calling for environmental remediation measures to actively remove existing twine from the landscape.