delaying animation by 1 second for 500 bats out of 2,000

Hi,

I have 2,000 Bats flying in a cave tagged to one ACTOR. I move the ACTOR and bats move in the same pathway. Unfortunately, they all flap their wings at the same time. How can I make it that the first 200 start flapping their wings .5 second later. Then the next 200 start flapping them .75 second later. I just want to make it that not all of them the wings at the same time. How can I do that?

Thank you so much for the help.