Help - runt of the nest advice needed

Hello,

We have a pigeon nest by out front door and mum and dad have sucessfully had 2 chicks. Issue is....the big one has flown the nest leaving the less developed one behind who is still a little fluffy. I think the parents have stopped coming to feed it too. Is it usual for parents to abandon the younger chick if they have had one successful fledge?

I feel like I am quite literally watching its slow demise at the moment and not sure whether to intervene or not :-(

Thanks so much in advance for any advice anyone can provide :-)