I don’t think it was the flu they would all be dead they share water feed and home
The vets are kinda like the Dr when a pandemic is going around they forget all other issues can happen they focus only on the pandemic.
I truly believe it was your heat set up
The bins and the heat lamp you are using are going to cause high heat
Can you please get a smaller watt bulb or find a way to move them up higher
They should be at 80-85 at this age
It’s only going to happen again as they grow more feathers they need less heat
I know we all came at you but not in a negative way we all love birds here and our first priority when help is asked is to figure it out and save them if we can
Unfortunately that can cause the OP hurt feelings as they may feel blamed
That’s not the intention at all
We have all made mistakes and had losses that we learn from
Nobody is prefect and google is the worst to get advise from
This is a great community and so much info of actual bird owners
I truly hope to see updated pictures of these sweet little ones in the future as they grow