Baby Zebra Finches Dying?

My zebra finches had hatched a baby on the floor of the aviary. I understand that it died either because it was squashed by the mother, sitting on an egg next to it, or of coldness. That was my second baby zebra finch to die, but with both, a few hours after they died, there was no sign of them. The other one that died was in a hanging nesting box. Why have they disappeared?