The risk depends on the bird , the type of plane and other variables. A 2 seater Cessna can be easily downed by a red breasted nuthatch. A 747 by a flock of sandhill cranes. A ruby throated hummingbird could take out an ultralight, but only during the mating season. The greatest risk would be a skydiver colliding in a vulnerable manner with a a harpy eagle. Trains are safer. There are no reports of derailments from collisions with anything larger than a finch.
Sallan1 on CBC.ca commenting on bird/aircraft collisions