I'd just have a float as the X speed of each bird, then increment the position based on the speed, and increment the speed each loop as well. That way, you could start the takeoff and set the speed to a small value, randomly - so the birds take off in a random X velocity, then smoothly start flying to whatever direction, just by incrementing the speed and limiting it.
You could have the birds head towards a direction, but you probably don't need that, just have them take off and fly to the right, out of screen. You could use Atanfull, and distance checks, or vectors to have the birds head towards a target location, but that might only be relevant if they do more than just fly away. For example, you could have birds attacking other birds, or swooping at the player.
It can also depend on how you handle your birds. If you have a particle system, then I'd just have a bird as a unique particle, and let the particle system take care of it. I've used particle systems for this sort of thing before - last time it was a moth that circles around light sources, then if the player destroys the light source, the moth chases the player instead. It was real easy to bolt the moths onto the particle system, and saved having to control all that seperately.
Health, Ammo, and bacon and eggs!
