Zebra Finch Nestling

[Gigi_lg]

I found a nestling huddled in the corner of my cage yesterday. Its sibling is a fledgling and doing well. The Nestling has closed eyes, breathing erratically and a large protruding mass on the side of its neck.
I’m wondering what the mass is, tumor, egg sac? I think the nestling is two or three days out of the egg.
I have seen the Dad feeding the fledgling but not the nestling. Dad doesn’t seem to want anything to do with the nestling.
I have bought the nestling in with me this evening for me to feed. I’m going to see if the mass on the side of the neck goes down overnight. I’m really hoping it’s the yolk sac and it will be absorbed soon.
Please see the photographs. If you are able to help with the mass identification, I would appreciate it. Also, if you can help with how much I should feed it and when.

 

[CCsGarden]

Almost looks like the crop! Is it hard? Yes, I can help you with the feeding since I've raised birds like this.
Keep it in a warm place!
Is it pooping?

It may have canker.

 

[Gigi_lg]

Almost looks like the crop! Is it hard? Yes, I can help you with the feeding since I've raised birds like this.
Keep it in a warm place!
Is it pooping?

It may have canker.

No, the mass is not hard. It has gone down a small amount today. I haven’t seen any poop. The nestling is in a small box, so I would see poop if there was any.
It’s in a nice warm kitchen.
I have just fed it a tiny bit of hard-boiled egg watered down. Try again in an hour and a half, but I really need to know how much to give.
Thanks for your reply.