The thork is a bird from the first edition Fiend Folio that I do not believe was ever converted into any other edition. It is a 9' tall stork with copper feathers and the ability to use boiling water as a breath weapon. Because it has metal feathers, the bird can not fly, but it can swim (slowly). Its boiling water breath weapon is a bit odd- a cone but the bird still has to make an attack roll. Also, even though it can boil water 3 times per day, it has no special resistance or immunity to heat or cold. There is no mention of how it grows metal feathers and the treasure it collects is platinum(?!?).

The basic idea is sound but it needs some fleshing out to make it shine. The bird is a carnivore that feeds on fish, but not any fish. Where it lives, there are fish with copper based blood (instead of iron). People who eat these fish have copper highlights in their hair. Its breath weapon is meant to kill its predators, something possible as the water does 4d8 points of damage. It does take half damage from fire and heat based attacks and is immune to cold. Copper blooded fish are found in all the temperature zones of the world, and thus the thorks are as well. People raise both to get an easy supply of copper, some meat and very strong bird bones as well as a reliable source of hot water. Their predators are magical creatures that survive the boiling water and can make hunting thorks that much more dangerous. That is how I would use the standard thork. There are also several variants.

A pigeon that has gold feathers and is bred for its appearance. Different cities have different requirements for what is the highest standard of gold pigeon and thus different lines have been created (some even used as city flags). [Got the idea after reading http://www.sciencenews.org/view/generic/id/347984/description/Pigeons_prominent_plumage_traces_to_one_gene ] Only the wealthy raise golden pigeons. The more common folk are restricted to silver, copper and iron breeds. Sea elves may have penguins with pearl feathers, bred as living ornaments in much the same way as the pigeons.

A vulture with mithral feathers doesn't feed on corpses, but rather slag from dwarven mines. They were created by elves to clean up after the mines and the result was better than expected- its outer feathers are valuable and its inner feathers (of different metals and minerals) are shed through out the birds' territory. All the slag is eaten and the result is a minimal impact on the environment. Elves and dwarves attract the birds and pluck the most valuable feathers just before the birds go into molt and lose them anyways.

An owl with stone or metal feathers. Created from the burrowing owl, it lives in the underdark and is used by miners to find deposits. They allow the owls to burrow into the stone and when they emerge for food, their feathers will be made of the most common metal or stone in the area.

A falcon with steel feathers and talons. Though some use it to hunt goblins and large game animals (it can take a deer without too much effort), the bird was created for war. It also has a breath weapon, a Stinking Cloud in the form of a 30' long by 10' wide cone.

An eagle bred by necromancers that has feathers of bone. It can be used as a powerful focus for spells (metamagic feats or similar) and possibly act as a familiar.

A starling bred by astrologers. These glow at night and can alter the way the constelations affect magic (obviously only if you use such magic). Huge flocks can produce enough light to drown out the moon, but most are used in much smaller groups to provide little bonuses.