Recently I've been hearing a very shrill high-pitched call that seemed different than the usual twittering and pipping notes. I wondered if it was a fledgling begging from the adults. But the only way to tell for sure was to follow the sound, find the bird that was making it, and look for either behavioral or plumage clues to confirm that it was a juvenile.
This morning I did that and after a few tries, my ears and eyes landed on this bird.
Note the posture — leaning forward, with wings held slightly out to the side. And the yellow coloration at the gape and at the base of the lower bill.
Below is the same bird, just a few seconds later as it turned to the side. It was quivering its wings and calling.
In general, juvenile birds often have paler colors near the base of their bills. To be sure, I looked this up and confirmed this was true for Pygmy Nuthatches, too. (Adults have all gray bills.)
So the calls I've been hearing are juveniles begging from adults. There isn't too much written about these begging calls, although the Birds of North America account says that incubating females make similar calls when being fed by males.
Here's one more photo of a fledgling Pygmy Nuthatch.
If you'd like to hear an example of this begging call, I've included a short sound clip below. Unfortunately, I've been having some technical difficulties with this audio file tonight, so I hope it works. (If it doesn't, I'll try again another night.) Listen for the high-pitched begging calls at about 6 seconds, 12 seconds, and 16 seconds.
I read that fledglings may be fed by adults for up to 2 months after they leave the nest, so you'll probably be able to hear these calls for a while.