once the birds have found partners, they find a suitable place to lay eggs. the idea of what is a suitable place differs between species, but most build bird nests. the bird is driven by a hormone (estradiol e2) to prepare a place for the eggs to hatch. birds' nests may be up a tree, in a cliff or on the ground according to species. when filled with eggs they are almost always guarded by one of the pair. in fact it is virtually impossible for the eggs to survive if one of the parents dies.