Collision course

Our apartment here in Philly has a single, albeit large, window in the living room. Early this morning, I was awoken by a loud THUMP from the direction of the living room. "Eh," I thought sleepily, "someone must've thrown a snowball at our window." And I promptly fell back asleep. Later, after getting up, I realised that (a) there is no snow on the ground, and (b) our apartment is on the 4th floor and thus not readily accessible to snowball throwing. Curious as to the source of the thump, I started to look closely at the window. In one of the panes, I found a bit of a greasy spot about the size of my palm, in the middle of which was stuck ... a couple of small feathers. I think an unfortunate bird flew right into our window, and pretty hard judging from the sound it made. I hope it was okay.