Birds are cool additions to the mod, with their annoying thievery and endless chirping. However, the majority of them don't really do much. For this suggestion, I'll be proposing an improvement/addition of several types of birds (and bats).
Behaviour
Gorcrows, Crebain, Bats and Vultures (See below), etc, would all exclusively steal meat, whether it's rotten, raw, or salted. They'd also do a very slight amount of damage while doing so, say 1 health point, to represent them violently attacking people for food. Gorcrows would prefer raw or rotting flesh to cooked, and do slightly more damage. All birds would flee a short distance if hit. Crebain and Gorcrows would also attack vanilla animals (small ones, pigs, sheep, chickens, etc). Crows would spawn in larger flocks around the Misty Mountains foothills, and around Dale and Erebor. All would have hideous, grating cries.
Taming
Crebain and Bats could both be tamed with raw meat. Not much would be needed, maybe 2-4 pieces. For Crebain, 25+ Isengard alignment would be required, and Gundabad for Bats. Taming one would spawn about 5 others, also tamed. These tamed units would function like cheap air units, would attack enemies and nearby animals if nothing else is nearby.
The real use of these would be eliminating the debuffs of Orcs in the daylight. Any non-Uruk Orc within a 4 block radius of a hired bird would lose their weakened state. If this sounds random, it comes from Bolg's armies bringing huge flocks of Bats with them, creating a darkness around them. Both mobs can also be assigned to specific hired units by right-clicking them and the mob you want them to protect.
Vultures
Vultures would be a new bird added. They'd spawn in the deserts and Mordor in small numbers and follow mobs who have lost health or are at low hunger (low enough that the ability to sprint has vanished). Like the other birds mentioned, they would steal food from players. The most unique feature about them would be that if a player reaches 4 health points or has completely empties their hunger bar, they will begin attacking. Vultures would be weak, having about as much health and damage as swans.
Vampire Bats
Yes, yes, I know these are in the planned features list. Anyway, here's my rendition of them.
They'd spawn at night in the Misty Mountains, Dale and its surrounding biomes, and especially caves. These Bats would be larger than the vanilla mobs (which should be removed anyway), and attack all good-aligned mobs, vanilla animals, and players with less than 50+ Gundabad alignment. They would have 8 points of health and deal about 3 damage to enemies. If possible, red particles would drip down from their heads after dealing damage for a few seconds.
And that's it for my suggestion. This would add a few more features to birds and evil players, and I think it would add a new element of danger to certain areas of Middle-earth. Feel free to leave criticism in the comments.