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In cities around the world, anti-bird spikes are used to protect statues and balconies from unwanted birds - but now, it appears the birds are getting their own back.

Dutch researchers have found that some birds use the spikes as weapons around their nests - using them to keep pests away in the same way that humans do.

It shows amazing adaptability, biologist Auke-Florian Hiemstra says.

"They are incredible fortresses - like a bunker for birds," he told the ³ÉÈË¿ìÊÖ.

Human-made objects being used in bird nests is nothing new - there is evidence of species around the world using everything from barbed wire to knitting needles.

However, this research by Naturalis Biodiversity Center and the Natural History Museum Rotterdam that says birds appear to be positioning the sharp spikes outwards, maximising protection.

Mr Hiemstra's research started in the courtyard of a hospital in Antwerp, Belgium, where an enormous magpie nest was found containing some 1,500 spikes.

"For the first few minutes, I just stared at it - this strange, beautiful, weird nest," Mr Hiemstra explained.

He says the spikes were pointing outwards, creating a perfect armour around the nest.

A trip to the hospital roof confirmed it - about 50m (164 ft) of anti-bird spike strips had been ripped off the building - all that remained was the trail of glue.

One unfinished nest is at the museum in Rotterdam - and a larger, finished nest is in the collection of Naturalis Biodiversity Center.

Mr Hiemstra says many more need to be found to further prove his theory, but there are several aspects to the nest architecture that suggest the birds are using the spikes as protection.

One is their placement - the spikes are on the roof of the nests, he says, "so they aren't just making a roof - it's a roof with thorny material for protection".

Birds often use thorny branches to protect their nests, but humans aren't fans of these kinds of bushes and trees, so birds living in built up areas go for the next best thing, Mr Hiemstra says.

It shows a remarkable adaptability to their environment, he adds, and also a determination to protect their nests, as the glue used to attach the spikes to buildings is strong and the spikes not easy to remove.

There have been many instances of birds taking matters into their own talons - like the cheeky cockatoo ripping away spikes on a building near Sydney in Australia, or that went viral for building its nest right on top of them.

And while this may be an annoyance for the humans who paid for the spikes in the first place, Mr Hiemstra sees it as a "beautiful revenge".

"They are using the material that we made to keep them away, to make a nest to make more birds."