As there is diversity in bird species, there is also diversity in birds’ nests. The simplest nest is called a scrape, a depression in the ground that may be lined with stones, shells or vegetation. Australasian birds classified as megapodes build what are, by far, the largest bird nests in the world. The eggs are buried in branch-and-vegetation mounds that can contain as much as 130 cubic yards of material, with an estimated maximum weight of 50 tons. However, most birds build far smaller nests. The flamingo, for example, makes a mud mound just large enough for a single egg. Smaller nests may comprise grasses, twigs, bird saliva or manmade materials, like string. Cups are the smallest of nests, with some hummingbirds producing examples about an inch across and an inch high. The platform nest is favored by birds of prey and is found atop cliffs and tall trees. This flat-topped structure is used by the same birds for years, with new vegetation added every breeding season. Some birds forego nest construction entirely—burrow nests are bored horizontally into the side of dirt cliffs, with a long, narrow tunnel terminating in a chamber for the eggs. Another version is the cavity nest, a natural or manufactured void created in trees or cacti. Woodpeckers, using their chisel-like bills, make them from scratch, a process that takes about two weeks.