Behold, birds who have lost the ability to can!
Just kidding, guys. These birds are just trolling the hell out of ants. I really, really wanted to show you this clip of a Galapagos finch or something harassing the shit out of formica ants and then being all “Yes, yes, bathe me in your fury! Your chemical defenses are now my own! Mwahahahaha!”, but the closest thing I could find is this video of David Attenborough pissing off some wood ants. It was basically like that, only instead of an Englishman with a stick, it was a bird stomping around with its wings spread just being an absolute asshole about everything.
This behavior is actually called anting, and there are two types of anting that birds can engage in. One is just anting, where birds will rub ants all over themselves to get that precious, precious formic acid all up in their feathers. They’ll also do it with mothballs, cigarette butts, and certain sorts of beetles and millipedes. The other one is passive anting, where a particularly lazy bird will find an anthill and just flop down on it with all their feathers spread and puffed and annoy the ants until they hop to and try to make them leave, at which point the bird rubs its wings together and goes “Yeeeeeess.”
They do this to get rid of external parasites, because external parasites are annoying. Ant-eating birds who do this are getting a two-for deal out of it, because they get the ants to empty their acid sacs in a beneficial location (the bird’s feathers) and then get to eat them without having to deal with the acid in their crops, so it’s basically like if your bug-spray or deoderant came in a bacon bottle.
Formica ants get the brunt of this, because they’re super-common and quite frequently spray the acid instead of trying to inject it, so the bird can get itself doused and then preen it into its feathers. Considering the spraying of acid is like the ant way of saying “Oh my god go away you dickhead I hate you we all hate you why are you still here jesus christ what is wrong with you,” we can be reasonably sure that they’re not super-thrilled by this bird behavior. Since the birds keep doing it, we can be reasonably sure that they don’t care about the ants’ feelings.
Birds are the actual best thing.