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Humans have made the world less hospitable for birds in many ways. One obvious and intentional example of this can be found in towns and cities worldwide: anti-bird spikes. The pointy wires you might see attached to roofs, ledges, and light poles are meant to deter urban species like pigeons from landing, pooping, and even nesting where people don’t want them to. But in an avian act of poetic justice, a handful of European birds have struck back.

Apparently Carrion Crows and Eurasian Magpies are stealing and repurposing the spikes as a nest-building material. Nests featuring the deterrent were documented in  published Tuesday in the Dutch journal Deinsea, an online periodical from the Natural History Museum Rotterdam. 

Many birds are known to use human-made elements in their nests. In fact, 176 different species have been documented nest building with synthetic materials, according to published this week in the journal Philosophical Transactions of the Royal Society B. Yet the birds in the Dutch study are exceptional for having taken something so purposefully built to minimize their presence and using it to rear the next generation.

“It feels very rebellious,” says Auke-Florian Hiemstra, lead study author and a biologist researching animal architecture at the Naturalis Biodiversity Center in Leiden, Netherlands. Kevin McGowan, an ornithologist who studies corvid behavior at the Cornell Lab of Ornithology and wasn’t involved in the new research, agrees. The behavior conveys a “sort of a Bugs Bunny attitude,” McGowan says—though he wasn’t particularly surprised to see magpies or crows using the spikes in this way. As he explains, both are smart species and other studies have demonstrated corvids’ abilities to recognize the function and use of different materials.

In the Tuesday publication, Hiemstra and his co-researchers describe three Eurasian Magpie nests and one Carrion Crow nest that heavily incorporated anti-bird spikes into their construction. These four nests were found between 2021 and 2023, each in a different European city: Rotterdam and Enschede in The Netherlands, Glasgow, Scotland, and Antwerp, Belgium. In addition, the researchers also came across one previously documented of a Carrion Crow nest building with roost-deterrent wires—one of many other names for the spikes—in their literature review.

When Cornelis Moeliker, director of the Natural History Museum Rotterdam and one of the study authors, saw the nests, he was “totally baffled,” he says. “I’ve been watching wildlife, and especially urban nature, for about half a century, and I’ve never seen something like this. It’s almost unbelievable that it actually happens–I mean, the sheer irony of it.”

One of the recently discovered magpie nests now lives at the Leiden museum and includes about 1,500 visible metal spikes. “It’s quite solid,” Hiemstra says, though still light enough to easily pick up. He describes the nest as “magnificent and weird looking, but also so well-built and creative.” The nest was first noticed by a patient looking out of his Antwerp hospital window in 2021. Soon after, Hiemstra received an email from the man imploring him to come see it.

Meanwhile, in Rotterdam, maintenance workers had found an odd, unfinished crow’s nest in a tree and contacted the local natural history museum. From there, the researchers teamed up and tracked down other similar nest observations by scouring the scientific literature, local news reports, and even social media posts.

Based on the the scientists' research, the two corvid species appear to use the anti-bird spikes in slightly different ways. For the crows, the spikes seem purely structural, a material used to fashion a solid foundation. In both crow nests, the wires were incorporated into the base–interwoven with the points facing inward, below where a softer nest cup would sit. But for the magpies, there was an additional layer of intrigue; not only were the birds using the spikes to build nests, but it’s possible they were also employing the devices for their intended purpose—to ward off other birds. “It’s a very natural behavior,” Hiemstra says. “We think these spikes are for nest defense."

Magpies fabricate domed nests that and bramble canes into their construction, explains the biologist. The building strategy is thought to help protect their nests from would-be interlopers. Usually, the birds use natural materials, but in at least a few cases, human-made components like knitting needles and barbed wire have been observed in magpie domes.

It also appears that the birds specifically sought out the spikes. Two of the newly documented nests, collected by the researchers and brought back to the museum for preservation and display, still had sticky adhesive lingering on the hardware, implying that the animals tore the spikes off of buildings. Although magpies and crows haven’t been documented doing this, other birds have. In 2019, a video of a Sulphur-Crested Cockatoo pulling a spiky strip from the ledge of a building in Australia was online. For the magpies and crows, the spikes are apparently valuable, Hiemstra notes. They must be “worth putting in the extra effort.”

The idea that the birds could favor these spikes for nesting raises questions the study couldn't answer. Are they a necessary substitute, made in the absence of sufficient, thorny plants, or is this an adaptive choice? Do the synthetic spikes work better than the natural materials? Addressing those questions, Hiemstra says, would require many more data points and long-term study comparing the nest success of various breeding pairs.

Even if the use of the spikes is simply urban adaptation at play, neither Hiemstra nor McGowan are convinced it’s good for the corvids. Both scientists point out that anti-bird spikes can impale birds and other wildlife. Other research, too, that human-made materials in bird nests can cause injuries. In the crows’ nest, now at the Rotterdam museum, the researchers found a piece of another bird deterrent: anti-bird netting, often placed over garden plots. The netting is known to be dangerous for animals, who often end up entangled in it. To see it in a nest is similarly worrying, notes Moeliker.

For Hiemstra, while the new discoveries are fascinating and inherently humorous, they also highlight a broader issue: the unfortunate prevalence of anti-bird spikes in the first place. “I just think it’s kind of sad to keep fighting against nature, instead of embracing it as being part of the city.”