Bread can be used as a cheap filler ingredient and can leave birds feeling full, but it must only be a small component of their diet. This is because bread does not contain the necessary fats and proteins birds need to thrive.
A diet of only bread can cause birds to suffer vitamin deficiencies or even starve. Baby birds in particular require a varied diet to help them grow into healthy fledglings.
If kitchen scraps are your only option, foods like grated cheese and cake crumbs offer more nutritional value.