This is the time of year when young birds are leaving the nest, and they continue to be fed by their parents for several more weeks. Their twittering is, like most bird calls, species-specific, and many fledglings can be identified by ear.
I recently recorded several species in Maine:
Juvenile Yellow Warblers give rapid high chip calls. Here the chips in the first 6 seconds are adult chips, followed by two flight calls and then by the calls of the fledglings (Dark-eyed Junco singing).
Juvenile Black-throated Green Warblers also chip rapidly, but they have a slightly higher metallic sound.
Fledgling Blackburnian Warblers have a rapid, high buzzy call.
Black-throated Green and Blackburnian Warbler fledglings together for comparison.
Yellow-rumped Warbler juveniles have a low staccato call.
Juvenile Chestnut-sided Warblers utter a soft chip, and have a slurred begging call when a parent approaches.
w-shaped begging calls
The softer chips are fledgling calls, the sharper, emphatic chips are by the adult male.
both chips, adult on right
The chips of a Canada Warbler fledgling, with lower chips by the adult in background.
Northern Parula fledglings have very high chip calls. Adult chips can be heard towards the end of this recording. These high calls, apart from being difficult to hear, are ventriloqial and make locating the fledglings difficult. One tip, if you can't locate the direction from which a bird is calling, there's a good chance it is directly above you.
Golden-crowned Kinglet fledglings have a rapid high chipping call.
Calls speed up at the time of feeding.
Brown Creeper fledglings have clear, long, descending calls.
Juvenile Dark-eyed Junco calls are similar to the common call heard by flying adults.
Adult call in flight for comparison (Feb, PA).
With practice, these under-appreciated calls can be learned, making for a new and interesting challenge while out birding in the summer.