Ask Marilyn ® by Marilyn vos Savant is a column in Parade Magazine, published by PARADE, 711 Third Avenue, New York, NY 10017, USA. According to Parade, Marilyn vos Savant is listed in the "Guinness Book of World Records Hall of Fame" for "Highest IQ."
In her Parade Magazine column of April 19, 1998, Marilyn claimed that no one really knows why birds deliberately engage in anting.
I'm sure you'll be overloaded with mail on this one, first, because Marilyn doesn't appear to answer the question fully, and second, because of the Discovery Channel's Saturday Sci-Trek show, entitled 'Wild Style', which aired the night before, Saturday April 18, 1998. The most remarkable part of this show was a demonstration of 'anting', in which a bird (species of jay) approached an anthill and allowed itself to be attacked by the ants guarding the anthill.
The ants' defensive mechanism, of course, is formic acid, which they spray through their abdomens at whomever they are attacking. The jay, being attacked, loved it! - spreading its wings as if taking a shower. The formic acid had no poisonous effect on the jay (it would on a human), but it did kill the feather mites embedded in the jay's wings and other feathers.
As an aside, American bee keepers have been plagued with bee mites which were introduced in the 80's from an introduction into this country of Asian bees. Formic acid will also kill these bee mites with evidently no effect on the bees. I'm not sure whether the EPA has approved any method of using formic acid to stop the bee mite plague.
Barzeski Family (Richard, Sharon, and Karlye) <email@example.com> wrote the following letter to Marilyn:
In the Sunday April 19, edition of Parade you said that nobody really knows why birds participate in the activity of "anting". Part of the answer is grooming but it seems an odd way of interpreting the word grooming. A particular ant, when the nest is threatened, rallies its troops and the ants will then squirt, I believe, formic acid on its attacker. Birds have been documented to purposely agitate these ants and actually bathe in the tiny streams of acid being emitted by the ants. The reason seems to be that while the acid has no effect on the bird, it will kill any ticks or mites that might be infesting the birds feathers. The use of this acid has also found use in recent years in beekeeping. An Asian mite is starting to infest American beehives and the domestic bees haven't had enough years of exposure to create a defense against these mites. Beekeepers will use formic acid to kill the mites before the beehive becomes so infested that its inhabitants die. Passive "anting" has been noted by birds putting ants under their wings where they join the body. The reason for this is suspected to be relief from the itching of new feathers. Birds have also been seen doing the same thing without ants utilizing various items including cigarette butts.