Hey even Jesus watched bird in some sense. First of all, for fun.
Birds are funny animals.
Secondly, to gain some knowledge. Ever since I started watching birds, I've not only gained knowledge in ornithology, but also got some common sense about ecology, geography, plants, oceanology, interaction between birds and human beings, and the vague clues through which migration birds get oriented when they travel. Finally, birds fly. Man, being able to fly makes birds so cool and so beautiful . I think Leonardo da Vinci would have agreed.