Castiel hates birds.
Any size, any color, any species. He knew it was unreasonable of him. Birds had never actively done anything to earn his animosity. It was what they could do that he hated them for. They could fly. They had wings, not nearly as nice as his used to be, but wings nevertheless. Everytime he saw one of the little fearthered creatures he felt an envy and hatred that he couldn't deny.
Why should they have the freedom of flight that he so longed for while he could only watch from below? Being stranded on the ground like some leaden weight was worse than anything he had ever endured in his milleniums long existence.
Sam and Dean would never really be able to understand his feelings. They had never flown to the top of a mountain just to feel the cold air play over their wings. Or over a volcano, catching the hot updraft to rise so far so fast that it was entoxicating.
Even so, it was not his proudest moment when they caught him yelling at a hummingbird to "Get the fuck away from me!". He was very embarassed, but the damn thing had been diving at him for several minutes.
Okay, maybe at the time he had been standing near a flowering bush that might have attracted it. Still, though he didn't actually hurt the little winged vermin.
If he never saw another bird as long as his human life lasted, it would be just fine with him.
And don't even get him started about the damn bees!