Building nests is busy work (pigeons are famously terrible at it) and in a competitive environment, it pays to get creative when sourcing materials for your impending young. While many of us associate nests with twigs and leaves, a new study has shone a light on a less passive form of asset collection practiced by some bold birds: stealing hair from living animals. That’s right, in a new paper (ingeniously titled “What the pluck? Theft of mammal hair by birds is an overlooked but common behavior with fitness implications”) that’s been accepted for publication in the journal Ecology, the authors explore the use of mammalian fur in birds’ nests. Specifically, they wanted to review examples of a behavior they term “kleptotrichy” (the Greek for “steal” (klepto) and “hair” (trich)), whereby birds pluck fur from living animals.

The researchers on the paper became interested in the behavior after watching a tufted titmouse (Baeolophus bicolor) approach a sleeping raccoon before relieving the unsuspecting animal of some of its fur for around four minutes. It did cross their minds that the raccoon might have actually been dead, but closer inspection confirmed it was alive, just snoozing. While they believe this to be the only example of such behavior in peer-reviewed literature, a quick search on YouTube demonstrates that it's not the first time a raccoon has unwittingly facilitated the creation of a cozy nest.

Curious to see if this criminal behavior was widespread, they searched academic literature for several keywords pertaining to birds, mammals, and the theft of hair. Their results turned up 11 occurrences across six species among the peer-reviewed literature. More than half of the hairy heists were carried out by tufted titmice who stole hair from several mammals including humans.

Given the titmouse’s seeming affinity for kleptotrichy, the researchers took to YouTube to see if they could find further evidence of the behavior among these birds. Sure enough, their search turfed up 99 examples from Paridae species, the bird family to which titmice belong. Unbothered victims included dogs, cats, humans, raccoons, and even a porcupine, and revealed that while there wasn’t much to be found in the world of published academia the behavior seemed quite familiar to the general public.

While kleptotrichy might seem a little risky considering most mammals involved could probably kill a bird if they wanted to, the review found that in most instances the involuntary groomee either didn’t really care or didn't even notice what was going on. In that respect, harvesting from a live donor rather than foraging for shed hair in the environment makes energetic sense in the face of so little risk.

“Overall, we show that most species of Paridae incorporate hair in their nests and hair theft has been much more commonly documented in the popular literature than the scientific literature,” wrote the paper’s authors. “Our results also suggest that the kleptotrichy is a largely commensal ecological interaction; the mammal species remained inactive or ignored the hair plucking by birds… and were likely unaffected by the loss of a relatively small amount of hair."

“We speculate that there may be higher search costs associated with gathering patchily distributed hair shed into the environment, in comparison with extracting hair directly from live mammals, since hair is much more concentrated and potentially easier to locate in the latter case.”