Birds build nests with rubbish like cigarette butts, plastic bags and fishing nets, scientists say, raising fears for the safety of their chicks.
Research shows 176 bird species have been found to build nests with human litter, including items that hurt them and their offspring.
The study, published on Monday by a team of European scientists, reviewed research covering almost 35,000 nests and found birds had used human-made materials in nests on all continents except Antarctica.
“A wide variety of bird species included anthropogenic materials into their nests,” said Zuzanna Jagiełło, an ornithologist at the University of Warsaw and lead author of the study. “This is worrying because it is becoming increasingly apparent that such materials can harm nestlings and even adult birds.”
The scientists found a range of materials was used by birds from different species: blackbirds have bulked out nests with plastic string; blue tits have garnished them with colourful stuffing; white storks have added cardboard, foil and plastic string.
While such behaviour had been observed as early as the 1830s, the scientists said in recent years it had “undoubtedly” increased.
“We are living together with birds in a very human-altered environment,” said Jagiełło. “Pollution is everywhere.”
Birds, like many creatures, are threatened by the destruction of habitats and contact with people. A landmark assessment in 2019 found that nature is declining faster than scientists have ever seen before.
The scientists highlighted ways birds are at risk from human material in their homes. Twine entangles birds; rubbish exposes offspring to toxins; bright colours attract predators who steal eggs; and chicks choke on material they mistake for prey.
But some of the litter can be helpful. Cigarette butts, for instance, contain nicotine and other compounds that can repel blood-sucking parasites; plastic films provide insulation that can keep chicks warm; and sturdier materials can make nests stronger.
It is too early to say what building nests with human-made materials does to bird populations, said Dr Jim Reynolds, an ornithologist at the University of Birmingham and co-author of the study. “There could be some winners and losers.”
Scientists have come up with several theories to explain the variation in material between different species. The most promising, the study found, is that birds build nests with human objects as signals to potential mates – similar to people owning flashy cars or big houses, said Jagiełło.
The study found mixed support for the theory that birds’ nests reflect the materials in their local area. They found no support for two other theories about the birds’ age and the placement of their nests.
Further research could help scientists learn more about where pollution is and how wildlife uses it.
“Birds are wonderful bio-monitoring tools,” said Reynolds. “We’re asking people to show an interest in this and go and take a photograph of a nest in their garden, wherever they might live.”
