Why are the babies dying

What kind of bird?
Was this in a nest box with a camera?

I've had the same problem with Blue tits, no camera. does a camera make a difference?

In reply to kenisa:

This is an old thread from last year. The OP hasn't posted anything other than this, so unlikely to respond.

Re camera making a difference to nest survival, not directly as far as I'm aware. If people are opening the box to readjust the camera, clean it etc etc, while the nestbox is in use, then clearly, that will make a difference and risk nest failure.

From memory, last year was ok blue tit nesting wise on the whole. 2021 was terrible. Reasons for nest failure are most likely lack of food (due to lack of availability or due to death of one or more adults), predation (of adult or nest), disturbance. If the nestbox is in full sun, that can be an issue too.