Auke-Florian Hiemstra, a biologist who studies how wild animals repurpose human materials, thought he had seen everything. In his research on the common coot, a water bird often found in Dutch canals, he had discovered nests containing windshield wipers, sunglasses, plastic carnations, condoms and envelopes used to package cocaine.
“So my definition of what is nesting material was already quite a broad one,” said Mr. Hiemstra, a doctoral student at the Naturalis Biodiversity Center in the Netherlands. “Almost anything can become part of a bird nest.”
Still, he was not prepared for what he found when he went to investigate a strange nest that had been spotted outside a hospital in Antwerp, Belgium, in July 2021. Nestled near the top of a sugar maple tree was a Eurasian magpie nest that resembled a cyberpunk porcupine, with thin metal rods sticking out in every direction.
“I couldn’t believe my eyes,” he recalled. “These are birds making a nest with anti-bird spikes.”
Rows of these sharp metal pins have become a common feature of the urban environment, installed on rooftops and ledges to discourage birds from perching or nesting on buildings. But outside the Antwerp hospital — where, as it happened, many of the rooftop spikes had gone missing — the magpies had managed to convert hostile architecture into a home.
“They’re outsmarting us,” Mr. Hiemstra said. “We’re trying to get rid of birds, the birds are collecting our metal spikes and actually making more birds in these nests. I think it’s just a brilliant comeback.”
And the Antwerp magpies were not alone. Over the two years that followed, Mr. Hiemstra and his colleagues discovered several other nests, built by Eurasian magpies and carrion crows, that contained anti-bird spikes. They described their findings this week in a paper published in the journal Deinsea.
“It’s absolutely fascinating,” said Mark Mainwaring, an expert on bird nests at Bangor University in Wales, who was not involved in the new study. “It shows just how intuitive these birds are, and it shows a certain amount of flexibility to go out and find these new materials and use them.”
Magpies and crows are both members of the corvid family, a group of birds renowned for their intelligence and problem-solving skills. Magpies often build domed nests, assembling thorny branches into roofs designed to protect against predators. In the nests that Mr. Hiemstra and his colleagues found, the magpies seemed to use the anti-bird spikes for the same purpose, turning them into a spiky nest cover.
“The Antwerp nest is really like a bunker for birds,” said Mr. Hiemstra, who calculated that it contained roughly 50 meters’ worth of anti-bird strips and 1,500 visible spikes. “It must feel really safe sitting in the middle knowing that there are 1,500 metal shards or pins defending you.”
Although the researchers did not catch the magpies in the act of tearing the strips from the hospital roof, spikes had disappeared from the area near the birds’ nest, and other birds have been observed ripping such spikes from buildings. And sharp, human materials, including barbed wire and knitting needles, have previously been found in magpie domes, the scientists noted. (“That must be such a happy magpie coming home to the nest with this big knitting needle in its beak,” Mr. Hiemstra mused.)
The crows seemed to use the spikes differently, turning the sharp pins toward the interior of the nest. Although the idea remains unproven, positioning the spikes this way might provide the nests with more structural support, Mr. Hiemstra speculated.
It is not entirely clear whether the birds are simply using the spikes because they are available — in the urban wild, they might be easier to come by than thorny branches — or whether they might be even better suited for the job than natural materials are.
But the use of artificial nesting materials is common across the avian universe, according to a new review of the scientific literature by Dr. Mainwaring and his colleagues, which was published in the journal Philosophical Transactions of the Royal Society B on Monday. They found reports of tens of thousands of nests — built by 176 different bird species, on every continent except for Antarctica — that contained artificial materials, including plastic bags, cloth straps, fishing line, paper towels, dental floss, rubber bands and cigarette butts.
“Where there’s the opportunity to incorporate anthropogenic materials, human-made materials, into your nest, you’re probably going to do it as a bird,” said Jim Reynolds, an ornithologist at the University of Birmingham, in England, and an author of the new review. “Some of it causes furrowed brows among we field ornithologists, because you think, Really?”
The findings reflect just how much garbage humans leave behind, Dr. Reynolds said, and research suggests that the use of artificial nesting materials is becoming more common.
The long-term consequences are unknown. Shiny or colorful materials could help a bird attract a mate — or catch the attention of predators. Research suggests that the chemicals in cigarette butts can help protect nests from parasites — but also can be toxic to birds. And there are many reports of chicks becoming entangled in plastic string or twine that made its way into a nest.
As for the use of the anti-bird spikes, Dr. Mainwaring was curious to see “if the behavior spreads, if other magpies see their neighbors using these spikes in nests and think, That’s how you build a nest,” he said. “And the offspring raised in those nests are also going to grow up thinking it’s perfectly normal and natural.”
Mr. Hiemstra suspects that there are more spike nests out there waiting to be found. He certainly hopes there are.
“I’m definitely rooting for the birds, cheering for the birds and actually enjoying that the birds are fighting back a little bit,” he said. “Because they deserve a place in the city just like us.”