Eastern gray squirrels (Sciurus carolinensis) could be quite vocal at times, but if you are not familiar with their vocalizations, you may think that you are hearing a bird calling. Until yesterday that's what I used to think. But I finally realized that a peculiar "bird" sound I often hear around here was in fact one of the alarm or warning sounds of squirrels.
Here is the subject squirrel on one of the high branches of the oak tree next to my driveway. Yes, I know it is hard to notice the squirrel and that's why I put an arrow pointing at its tail. This was the best shot I could get of it with my iPhone camera.
And here is a recording of the same squirrel's calls made, again, with the iPhone. You need to turn up your speakers to hear it loudly enough. (Towards the end you will also hear the voice of my son in the background.)
The squirrel was uttering a series of short calls followed by a long screech. In Mammalian Species No. 480, John L. Koprowski described the 4 notes used in alarm vocalizations of Eastern gray squirrels as "buzz, kuk, moan and quaa". The calls heard in my recording could perhaps be a series of "kuks" followed by one or more "quaaaaa". Doesn't that sound like a bird?