What species are these baby birds?
If you are in Europe they are juvenile common swift (Apus apus).

There is a website dedicated to them, with a page on how to raise them: http://www.commonswift.org/Hand_rearing_Swifts.html

They are very fragile and with the wrong diet they might develop feather deformations. They also require a continuous effort because they eat very often. I raised them a couple of times and when they take off is magnificent but it is easy to do something wrong. If you have some recovery center or expert nearby  I suggest to hand them over.