When I asked Leo about this a long time ago, he said that often animals in the wild eat birds (for example) and that's why a variety of animals can drop bird feathers. Honestly it seems unlikely that a bird would eat another bird in nature but if you apply that thinking here, the randomness of some of the drops makes sense (kinda).
To Feuer- the only actual bird monsters I can think of are the Sikuku Gargoyles tho there are several flying type monsters like the bees, wasps, Warbiz, Eudy and Orpe, Frozen Sentinels on Lona and the ghosts since they kind of hover rather than walk. There are some flying things in the temple on Luna too but I don't remember their names.
@ the topic starter- When a clannie of mine was farming for dragon feathers, he fought around the big skeleton and did alright but it does take time. If you are looking to sell those bird feathers tho, let me know since I can always use them.