If you mean your pigeons have a baby; nothing. The parent will feed it until it can eat byi tself.
If you mean you have a baby pigeon - that fell out of a nest perhaps - you need to guess how old it is. Your better local petshop has a special formula food which can be fed to young pigeons (1 day to 6 weeks old). There is a different formula for different ages.
My firend is an expert on birds and hand rearing, she has just brought up 2 baby pigeons and they are now thriving adults and I recall she fed her baby pigeon on Hovis granary bread soaked in water, take a pea size amount at a time and squeeze out the water and open its beak and gently push in to the back of its throat - it should readily take the mush! LOL!
Hope this helps.