Mourning Doves are very lazy nest builders.  They often will find an abandoned nest that was created by another type of bird or they build their own very poorly constructed nest.  Often they will reuse their nest for 4 to 5 sets of eggs.  With the large size of their hatchlings, the nest often gives out causing the baby birds to tumble to the ground.  If you ever see young doves on the ground don't pick them up!  Their mother and father are sure to be close by watching and feeding them.