One of my hen did not come back home. She has a squab. After 1 day cock abandoned the squab. It is only 2 days old now. I put it under another pair with 4 days old 2 squabs. But they are not feeding it much.
Rotate the bubs, take one of the older ones OUT and bring inside in the warmth in a box and wait until they feed the little one then rotate back. Otherwise u have too hand feed. Dont put more than two chicks under at a time and keep rotating and when a few more days older, they will probably feed all three. I have seen pairs rear 4 at a time. If smaller and with the bigger chicks in nest they will feed the one that begs the most so the older ones will be more aggressive for begging and get the food, if you do as I say and rotate, they will feed baby. Otherwise you will have to hand feed and if you DO have to hand feed, leave the small one with fosters and take an older one and feed IT.
If you buy ANY baby bird hand feeding formula and use it according to the instructions.
I use both Kaytee Exact and Zupreem, but I have also gotten Roudybush Squab formula from Quebec Canada. It is the closest to the fat and nutrience they need at this age.
The other two u can get from any Petsmart, the kaytee anyway.
When u do mix it up, careful NOT to be too hot for crop burn and google now some videos on hand feeding baby pigeons, you will see many ways they do it. I use a instant read gun thermometer from canadian tire to gauge the temp of the formula, ALSO not too cold or can cause crop stasis and thats a whole other problem.
First off, you should never allow parents birds to fly.
Secondly a baby that age needs baby bird formula and follow directions carefully. I would still try to allow the other parents birds to feed it, since it is so important to get that first crop milk, but supplement when needed.
Only feed baby when it has been warmed and only when crop is empty.