Maunder

From a few months ago, I found this bright Townsend's Warbler bill-rubbing on a sycamore branch - apparently essentially all species of bird are known to bill-rub, and the most accepted hypothesis for why is known as the "napkin principle;" that is, they're wiping clean after a meal. Finches seem to be particularly messy eaters, and are the birds I see bill-rubbing the most often.