Do fledglings die if they are not fed constantly?
Depends on the species and their age. All birds have fat reserves that can keep them alive for a relatively long time. It's when fledglings are left for many hours or days that one should start worrying about their survival.
The following link explains more detail on how frequently birds must be fed, even if in your case I suppose you aren't the one feeding them:
https://birdwatchingbuzz.com/how-often-should-you-feed-a-baby-bird/
In my experience some species of birds might become less afraid if you leave food or water stations for them.