1.  You could do several things.  Make a simple bird with flapping wings in Vue.  Link the wings to the body.  Duplicate it, then use time splines to make the wings flap differently for each one.  You can also make the speed, and direction change the same way.  A complicated flock of birs can be made this way.

2.  If the birds are far in the distance or up high, they don't need to be 3D or color.  Use the animation you made in #1 and render it as a mask.  Use this animation as an animated texture on an alpha plane in the background.  To make them color, render the animation twice once in color and again as a mask.  Combine then in the animated texture on the alpha plane.  

Remember, the animation needs to be rendered as an uncompressed AVI.  It will be a big file, but it will save your CPU from having to animate 3D when it isn't necessary.     

There are many free simple bird models to download.  

I do love animated textures and time splines.  I feel they are some of the most useful things in VUE.