The longer I drive the more convinced I am that squirrels and birds have similar "rites of passage" to perform before being confirmed as an adult squirrel or bird. It goes like this: if the squirrel or bird can skitter across the path of a moving vehicle and make it to the other side unharmed, then he/she is a confirmed adult squirrel or bird. It's a test of courage and spatial reasoning: both of which are necessary to survive for small creatures.
For squirrels there seem to be two approaches. The first goes halfway across the street, stops, sits and considers the next course of action (meanwhile the car is coming closer and ever faster), makes a decision about the time that the vibrations from the approaching car start to really hurt its tushy and then either contines the dart across the street and into adulthood or scampers back to the safety of the original side. We've all seen what happens when neither situation occurs, and it's not pretty.
For birds I imagine the ritual has to be a bit more clearly defined within the bird community. Clearly, flying right over the car is not nearly as thrilling or worthy of adult-status as flying directly in the path of said auto. So, the lower a bird flies the more honor is bestowed. On the other hand, the approach cannot be as deliberate as the squirrel. There is no time to contemplate mid-flight whether to continue on the chosen course; any hesitation and chances are the owner of the oncoming traffic will need to get a new car grill or a gruesome windshield cleaning. Once a bird commits, it's on. The options left are death, chickenin' out (pun intended) or adulthood. Their decision-making process has to happen quickly, not sitting in the middle of the street weighing the pros and cons.
In case there were any questions regarding the reasons for these creatures' roadside behavior, that's my theory. You weren't pondering this? Well, now you will the next time one darts in front of you. Cheer 'em on to adulthood!