The highest reported bird strike occurred at 37,000 ft, with the highest reported sighting at 54,000 ft!
A common misconception is that only large birds cause significant damage to aircraft and that small birds are a mere nuisance. This may be true for slow moving aircraft like helicopters, but this is hardly true for fast-moving aircraft. If an 8-oz bird hits an aircraft traveling at 90 kts, the bird strikes the aircraft with a force of 2 tons. With the same 8-oz bird at double the speed (180 kts), the force of the impact is quadrupled to 8 tons of force!
Gratuitously lifted from
http://www.propilotmag.com/March/avhazmar.html