Your kitty does what I call "chattering" to the birds. Every cat I've had does this when they see a bird, it's very cute isn't it?
Check out this site, especially "bi-lingual", Annie does a pretty good chatter!!
http://www.zefrank.com/annie/navigation.html
As far as the flock of birds by the window - I have no idea! I did have ONE bird come and sit by our window almost daily and I swear he was TAUNTING my cat!!