>“During the metabolic stresses of flight, some of the bat DNA is released into cells, where it doesn’t belong, and normally this would trigger a powerful immune response, however [the researchers] have shown that during evolution bats have mutated a key protein in this response called STING.”

This doesn't make sense to me. What is so special about the metabolic stress of flight? Is it different than the stress cheetah cells experience when it runs 100 kph?

Flying needs a lot more energy than running. All flying animals have a lot more muscles compared to body weight than others.

And bats fly all night, while Cheetahs only run at best a few minutes per day

Makes sense, but is the same also true for birds then?

They don't know. It's bleeding edge science and so far the study was only done for bats. Likely other scientists will look into birds now.

Birds are only very distantly related to bats (hundreds of millions of years away in evolutionary time) and their last common ancestors didn't fly.

There's something called convergent evolution when you have similar problems you come up independently with similar solutions, but then birds are in many ways very different from bats. So it could be that they have a similar mechanism, or they get around the problem in some other way.

This raises the question of why this would not be an optimal strategy for other mammals, or if it would be, why it has not arisen independently in many other lineages.

The headline states that hosting viruses is a cost of flying, but it does not really say what the downside of doing so is, given that the viruses do not seem to be harming the bats. Perhaps it leaves them more vulnerable when dangerous pathogens come along, such as the fungus causing White-nose Syndrome in North American bats. The article says that bats are relatively long-lived, suggesting that this adaptation is not a compromise, though it may be that the advantages of being a bat (presumably coming mostly from flying) compensate for any compromise that this trait requires.

The article mentions a consequence of flight, stress-induced DNA leakage into cells that could trigger an auto-immune response, that might have increased the selective pressure for this adaptation in bats alone (among mammals). I imagine someone will be looking into whether birds have a similar adaptation.