Cornell:
“Along with Wood Ducks and other cavity-nesting ducks, Hooded Mergansers often lay their eggs in other females’ nests. This is called “brood parasitism” and is similar to the practice of Brown-headed Cowbirds, except that the ducks only lay eggs in nests of their own species. Female Hooded Mergansers can lay up to about 13 eggs in a clutch, but nests have been found with up to 44 eggs in them.