Flock lifeFlock life is very simple. When not off delivering messages or with their chosen earthbound family (if they're a familiar), the birds flock together and fly together, find food together, share their resources - all very casually. Youngsters are taught by all the flock to fly in loose formation, what the shapes of predators look like above, what food is good and how to use their instincts/smarts to find their destination.
Males win their mates as is customary to their species, be it brief bouts or by showy feathers. They might mate for life or the female might just pick whoever looks to make a good father! Fights are fine, as is stealing food and such.
The only thing that is outlawed is killing a rival when they're clearly defeated (unless they're suicidally attacking, because then your talons are forced), killing eggchicks or stealing a nest, or taking away anothers power of flight ie. damaging the wings beyond hope of healing.
CartelWhat's less casual is their cartel. This is when all the various birds co-operate on how much they'll charge to take a message, and distributing any large goods evenly. It also deals with any injustices that may happen, such as unlawful fights or complaints of an eggstealer. A bad offender is likely to be mobbed or have their flight feathers pulled out. All birds are of equal rank (except of course, Hestia, but she's a myth to them for now).
The general attitude of the flock is this - birds rule. Respect the birds, damnit! Beyond that, they dont care if you adore a lion and become a familiar out of love or what. Just stick with the flock and don't let anyone diss the birds.
The cartel will also meet if there's any...important messages. If a bird was hired to have a message flown from one pride to another declaring war, or something. They're not above tampering with the mail if it is advantageous to the flock.
Messages and packagesAll birds should carry some sort or pouch or 'quiver' (message-tube?) so they can take small packages. Vines can be used for several birds to carry something a bit heavier, but they're birds...they are limited in what they can take! Depending on how big/heavy a package, it could be one or two 'shinies'. No bird can take too many messages either.
The birds in general have a sort of magpie tendancy to horde things, shiny things, good nesting material, and the like. They'll trade amongst themselves for these and one way for a less...erm, advantageous male to woo a female is to present her with something sparkley!
These are what they generally take in exchange for taking a message (though food, protection or herbs might do as well). For 50 words, one 'shiny', for 100 words, two - they have to remember the message and speak it (parrot it ;D ) to the intended party. All the flock who wish to take messages must take three before a single bird can take a fourth. This keeps their rough little economy pretty even.
If a bird is a familiar, they can take messages for their bonded lion/hyena/etc. at no charge. A bird also doesn't have to charge for their offical flights, but most would.
Own language
Yep, nicked from Dinotopia, which inspired the idea anyway ^^;
Toying with the idea they also can carry bird-written notes to remind themselves or maybe have their name on their pouches/quivers.
Spoken language is pretty normal, but I'm thinking of something like Redwall's sparrow language. So anything they look down upon becomes a worm, chicks are called eggchicks and when making an oath they swear upon their own eggshell. Oh and desirable items are shinies!