The calls of young birds fresh out of the nest seem to be everywhere at this time of year. Yesterday I recorded a family of young Gray Catbirds in my backyard and was surprised by the variety of their calls. They range from a high trill similar to the Golden-crowned Kinglet see call, a tsip-like begging call that speeds up as a parent approaches, clearer see calls, and a louder, more sibilant tssup call. It appears that each fledgling has its own call, and I suspect this helps the parents identify individual fledglings.
Featured here are the tsip begging call, the trill, and some calls that seem to be intermediate.
tsip begging call and trill
Clearer see calls.
The louder tssup call.
An example of how the begging tsip calls increase in rate as fledglings sense food is on the way (June, PA).