Calling card

Eating breakfast this morning I was startled by a great bang at the
window a metre behind my head. Turning, I saw a dozen and a half
birds, two flickers, three or four bluejays, some juncos and finches,
flying en masse into the tree in the yard. I immediately thought a
hawk had been near by, but none of the birds seemed worried. The two
flickers swooped around and went back whence they came. I couldn't
see which bird had hit the window and none was on the snow below.
Whichever bird had hit seemed to have startled the lot of them.

A few hours later, I saw this left behind, stuck to the window. It's
not much more than a cm across, a half-inch or less. From the yellow
coloration, I'd say it was a flicker that hit.