Protecting the eggs
The other disadvantage of egg laying is the need to build a nest, or in some way to safe guard the eggs. This puts both eggs and parents at risk. Vertical cliffs being almost inaccessible represent one safe site, providing the eggs do not roll off. This is minimized by producing eggs that are pointed at the one end which permits them to roll in a circular direction. Other birds, particularly those belong to the plover group (Order Charadriiformes) lay their eggs on open fields and gravel plains, but are usually highly cryptic and not easily found. More commonly birds construct nests to provide some form of protection. Woodpeckers (Picidae) excavate or enlarge holes in trees, kingfishers (Alcedinidae) use holes in river banks. The Tailor bird of India, (Orthotomus sutorius), a warbler, sews together the growing leaves of a tree by piercing holes in their margin and tying them together with strands of plant fibre. The weaver bird weaves plant material together to form an almost basket-like structure which is attached to a thin twig and hangs upside down. Other species of weaver birds collaborate and build elaborate community nests. The oven bird of Argentina (Furnarius rufus; Furnariidae) builds its nest out of mud and against fence posts and bare branches. Hornbills, also nests in holes in trees and incaserates using mud the incubating female and feeds both the female and the young hatchlings through a small hole in the mud wall. Cave Swiftlets (Collocalia inexpectata; Apodidae) in southeast Asia construct artificial nests from glutinous spittle which is attached to the walls of the cave.