Birds' nests are usually comfortable: warm, safe, and lined with soft materials to keep their susceptible babies cozy and protected.
For the tufted titmouse - Baeolophus bicolor, and its closest relatives, the material for this is usually the hair of carnivorous mammals, which researchers observed were stolen from already dead animals, or obtained when the animals shed.
But new observations have discovered that this isn't what usually happens: the feathered filchers usually steal the hair off the backs of predators that are still living.
Jeffrey Brawn, an ecologist from the University of Illinois at Urbana-Champaignan said: "The titmouse I saw was plucking hair from a live animal. This was from a live raccoon with claws and teeth. And the raccoon didn't seem to mind because it didn't even wake up."
Brawn noticed the act quite by chance while carrying out a bird count in Illinois, and was so fascinated that he went in search of an explanation.
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Kleptotrichy
Mark Hauber and Henry Pollock from the University of Illinois Urbana-Champaign led Brawn and his colleagues, and they discovered that hair theft has been hinted at only casually in the scientific literature - but the YouTube videos bird lovers posted eventually became a rich resource indeed.
However, in spite of the fact that the literature may have few records, other resources propose that birds purloining fur from living mammals is somewhat well known among the public. On the Cornell Lab webpage for the species, tufted titmice are regarded as occasional fur thieves and in Australia, yellow-faced honeyeaters thieve fuzz from snoozing koalas.
The researchers gave the behavior a name - kleptotrichy, from Greek for "theft" and "hair".
Why Do These Birds Take the Risk of Stealing the Fur?
This bird risk stealing the fur because animal fur, of course, helps in insulating a nest and keep it warm, but the scientists believe that fur specifically from predators may have more benefits.
Brawn said: "There's a local species called the great crested flycatcher, which, like the titmouse, is a cavity nester, that actually puts shed snakeskins into its nest, possibly to deter predators." Finches in Africa display a behavior similar to this, making use of predator feces as a dissuasion.
There is a possibility that the fur assists in repelling parasites, which can quickly kill little hatchlings. Some birds line use plant materials that can keep such intruders at bay to line their nests, although it's uncertain whether mammal hair possesses similar properties.
There will be a need for further research so as to find out what benefit the birds are deriving from the fruits of their wrongdoing, but preliminary geographic analysis carried out by the team proposes that kleptotrichy is often done in higher latitudes. In turn, this proposes that the hair is collected so as to keep the nests warm first and foremost.
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