Why would grackles rub mothballs under their tails and wings?
Grackles are probably interested in mothballs for the insecticidal ingredient found in most. It’s likely that a grackle would investigate mothballs first as a food source. Finding them unpalatable but with a characteristic penetrating taste and odour, they may then have intuited, correctly, that the mothballs would be useful against parasites and fleas.
Similar behaviours are widespread and well documented among icterids (the New World family that includes grackles, blackbirds, orioles, and cowbirds) and among other “backyard” birds like crows and robins. These species have adapted well to living near humans and tend to be quick learners. Many have been observed rubbing themselves all over with ants, presumably because the ants have an acrid taste the birds expect will discourage insect pests.