My squirrels will sometimes perch themselves up high and chirp like a bird. What does this mean?

Brodi started chirping like crazy then ran and hid. He was on our screened lanai in his tree while I was standing beside him. Soon after some raccoons in the preserve behind our home started fighting. Is the chirp a warning cry?

Does one chirp mean something different than a series of chirps?

Still trying to get to know their lingo. :wott