I always thought the way crows seem to examine humans was a little creepy. As it turns out, I was right; they're looking at you so they can tell if you have wronged them in the past.
A 2011 study revealed that crows memorize the faces of people who have wronged them and they may even bring in mobs of other crows to attack the person if they see them later on. The crows in the mobs will also remember the person's face, so even if you never harmed that specific crow, it may still organize a mob against you. The bottom line is: don't mess with crows.
This gave me a whole new perspective on Hitchcock's The Birds.
[Image courtesy of aigle dore's Flickr stream.]