Thank you for the opportunity to help you with your question!
it is directional selection. the types are stabilizing, directional, and disruptive. it's directional because if you think about it, the lighter the bones are the better adapted the bird will be, so natural selection will just keep selecting for the lightest bones out of every generation. this will cause the average bone weight to get progressively lighter over generations.
Please let me know if you need any clarification. I'm always happy to answer your questions.