What is my cat saying?

Does she look calm? Perhaps she is warning the birds and squirrels. I would video her and then, take it to her vet. See what the vet says. I hope your kitten is not being stressed with those birds and squirrels. Or perhaps she is trying to talk to them. Does she do this to anyone else?

Perhaps she is trying to imitate the birds. Is she around any other kittens or cats? Perhaps she needs to hear other cats so she will know how to purr and meow.

In other words, I really don't know. It does sound funny. But I would check with your vet to make sure she is not being stressed out.