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Many birders have encountered what can be a weird-looking sight: a bird on the ground or perched, wings spread wide, basking in the warm sunlight. If the temperature is hot enough, the bird might even have its mouth open, panting like a dog.

This behavior is called sunbathing, or “sunning,” and has been noted by ornithologists since at least 1831, when John James ̽����ѡ described a Great White Heron that “will sometimes drop its wings several inches as if they were dislocated.” The bird was extending its feathers in the sun’s warmth, he understood, but he wasn’t sure why. 

Today, scientists know that birds from more than  sunbathe. These families include birds of prey, rails, doves, larks, swallows, thrushes, finches, buntings, and more, though they are all liable to sun at different times and for different reasons. On cold mornings, for instance, Turkey Vultures might fly up into a perch and spread their wings to the sun, allowing the day’s first rays to blast away the chill of night. And when mousebirds, native to sub-Saharan Africa, are looking to dry their cold, damp plumage after rain or heavy dew, they will often sun themselves communally, like wet swimmers lying poolside. 

While birds often sun for these practical reasons of warmth and dryness, a growing body of research now points to one largely understudied purpose: to rid themselves of pesky parasites living on their skin and feathers.

“Sunning seems like a pretty valid way of controlling ectoparasites,” says biologist Jennifer Koop of the University of Northern Illinois. “We've come a long way in at least looking at it from ‘this is a rare and strange behavior’ to ‘you know, this is actually a somewhat common behavior that probably has a lot of different functions.’” 

Birds devote about to so-called maintenance behaviors. They use their bills to pick dirt, mud, and other impurities out of their feathers, and also use them as weapons to hunt for unwanted hitchhikers—parasites, such as feather lice. A feather louse is only about 1 millimeter long, and made of keratin, the same material found in bird feathers as well as human hair and nails. Feather lice are so skilled at hiding amid a bird's feathers that they can be hard to remove by preening alone. The presence of lice isn't just a hygiene issue—it can make it harder for birds to , possibly due to duller plumage or the need for more frequent preening.  

Scientists only started studying  how sunning dovetails with parasite prevention relatively recently. In 1993, researchers at Virginia Commonwealth University sprayed a sampling of wild with a pesticide used to treat mite and louse infestations in caged birds. Then, they waited to see how much time the birds spent at their favorite sunbathing spot—the shiny, aluminum roof of the research laboratory. They found that the swallows sprayed with pesticide spent less time sunning themselves than those that were clean, suggesting that the birds probably sunbathe to control their lice.

Exactly how sunlight might kill lice remains unclear, but scientists now believe that short blasts of heat, UV radiation, or some combination of both from the sun's rays is likely the cause. , a pair of scientists tested this idea out by building a pair of model bird wings, outfitted with real Black Noddy feathers, and infested them with a single louse. The researchers found that even a relatively short time in direct sunlight—around 10 minutes—heated the wings to 140 to 160 degrees Fahrenheit. That far exceeds the temperatures required to kill bedbugs, for example, which die at around 120 degrees. Of the 12 trials, the louse died in six, and it died in six of seven trials where feather temperature surpassed 140 degrees.

This finding—that short bursts of sunlight can work as a non-chemical pesticide—was bolstered by . A group of Spanish researchers were in Guinea-Bissau, a country on West Africa’s Atlantic coast, when they saw Hooded Vultures panting in a clearing with their wings spread wide, almost touching the scorching sand underneath. They weren’t sure what to make of this behavior, but closer inspection revealed the scruffy birds to be sunning. 

After the researchers captured four Hooded Vultures, they discovered lice in the birds’ feathers. “They were full of them, they had hundreds of lice, especially under the wing covers,” says lead author Jorge Gutiérrez, an avian ecologist from the University of Extremadura in Badajoz, Spain. “They were full of eggs, too. That was something completely unexpected.” The discovery of so many lice and eggs made the team curious how the hot sun might be helping the birds keep the parasites under control.

To investigate, they harvested 41 lice from the captured vultures, and placed them on feathers in petri dishes. To start, the scientists placed 10 lice in the shade for 20 hours, where only one died. Then, the rest of the lice were exposed to direct sunlight that reached 140 degrees Fahrenheit for three minutes—a treatment that killed 26, or 84 percent, of the parasites. The lice that survived were subsequently subjected to 158 degree heat, at which point they finally perished. These findings, according to Gutiérrez, strongly suggest that short periods of sunning can effectively kill lice.

In addition to killing the lice, high temperatures might cause individual lice to move from their hiding spots before death, making them easier for birds to preen. But further research is needed. 

For Gutiérrez, better understandng sunning could be beneficial beyond untangling an unusual avian behavior—it could also aid in conservation efforts. A bird that suns often might have more lice, for example, and ectoparasites have been linked to a decrease in host fitness and the transmission of infectious diseases. “We know how birds respond to parasite and environmental conditions,” he says, “and by looking at how birds change their behavior, we can also understand the health of the birds.” In Hooded Vultures, a species particularly vulnerable to forms of avian flu from eating dead poultry, knowing which birds are more susceptible to spreading disease—or dying from it—is valuable information for researchers. 

Next up in Gutiérrez's research is figuring out exactly how hot birds get—not just the ground around them, but their actual feathers—when sunning. To do that, he plans to use thermal imaging to measure the body heat of Hooded Vultures as they sun in the wild. Considering the temperatures required to kill off the lice, the intense heat of the West African savannas, and the Hooded Vulture's dark plumage, chances are good the birds will surprise Gutiérrez and his team yet again.