Bird-watchers in Paris have for years noticed something odd about the city's pigeons: Many of the omnipresent avians are missing one or more toes.

Now, scientists think they know why, and it's a bit of a head-scratcher: A new study suggests that human hair might be the culprit.

Previous research had hinted that pigeons could incur foot damage from bacterial infections caused by standing in their own poop. But a closer look later revealed remnants of strings and often human hair caught between the digits, according to research published in 2018 in the journal Natures Sciences Socits.

By observing 1,250 pigeons along 46 blocks in Paris, researchers at the Center for Ecology and Conservation Science in Paris found that 20% of the birds were missing at least one toe. Matching these numbers with data on human activity and pollution at the level of city blocks, the researchers found more missing-toe pigeons in areas with high concentrations of hairdressers as well as densely populated blocks with high air and noise pollution.

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Traffic movement in these blocks could transport hair strands and plastic strings used to tie garbage bags to larger areas where pigeons encounter those objects, said Frdric Jiguet, an ecologist at the Center for Ecology and Conservation Science and lead author of this study, published in the December issue of the journal Biological Conservation.

As pigeons strut across pavements and cobbled streets, their feet can get tangled in human hair. "It's not easy for them to take it off with their beaks," Jiguet told Live Science, referring to the strands of hair. "The more they try to take it off, the tighter it gets around the toe."

The strangling hairs restrict blood flow, potentially causing the pigeon's toe to fall off.

Such foot deformities could affect pigeons' movement and access to food in urban spaces. Researchers said the injuries could also affect reproduction in the species, as missing toes could cause males to lose their balance while on top of female birds during copulation.

In future studies, the research team hopes to place sticky mats across the city to get an actual measure of how much hair these urban birds encounter, the scientists said. They also want to see if foot damage in pigeons across other large cities is similarly linked to humans.

Originally published on Live Science.

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In the Case of the Missing Pigeon Toes, Human Hair May Be to Blame - Livescience.com