Burrow nesters use their feet to remove soil and push it out of the excavation site. Tree hole dwellers use their beaks to gain entrance and hollow out a site in a tree. The typical passerine nest, cup shaped, are built from the bottom up. Hanging nests are built by wrapping material around a supporting twig or branch and then looping material from side to side to form a framework. The primary cause for nesting failure in general is predation.
Safety depends upon site selection, camouflage, and inaccessibility.
Ground nesting species are more likely to fall prey to mammalian predators. Cavity nesters have a lower predation rate than both ground and open nesters. Some birds nest near protective insects or animals to reduce their risks of predation. Many species nest very near to wasp, bees, or hornets nests to thwart predators. Some species of African shorebirds nest in close proximity to crocodiles for the same reason.