baskets. Wooden twigs form the greater proportion of materials used in the nests of large birds. Plants and trees not only provide resources but also sites. Branches provide support in the form of cantilevered beams while leaves and green twigs provide flexible but strong supports. Structures formed from plant material include beaver dams, which are constructed by foraged branches and sticks. The dam is a wall of sticks constructed on a moving water source, which forces the water to collect in one area and to stop flowing. Beavers begin to build a dam in an area where rocks and other debris slow the flow of the water. The beavers then form a small platform of sticks stretching across the water source. More sticks and branches are added to build the dam up over time. The structure in the center of the dam, known as the lodge, serves as a home for the beavers and protects them from predators. The primary reason behind the construction of beaver dams is to surround the lodge with deep water, which protects the beaver from land-dwelling predators. The entrance of the dam is underwater to prevent predators such as bears and wolves from entering, and the sticks at the top of the lodge are not packed tightly, which allows air into the structure. Mud and stones Mud is used by a few species of a wide variety of families including wasps and birds. Mud is plastic when wet and provides compressive strength when dried. Amongst birds, 5% of all birds use mud and stones in their nest for toughness and compressive strength. Males in some species of crab will construct structures out of mud to attract mates and avoid predators. Uca musica, also known as fiddler crabs, will build short, wide “hoods” out of sand. Another species of crab, Uca beebei, will build tall, thin pillars out of mud. These structures attract female crabs to male crab burrows and provide a hiding place for both males and females when predators are nearby. Beavers will often seal their dams and lodges with mud for extra support. Self-secreted materials The majority of self-secreted materials are produced by insects and selection acts on this characteristic of production of self-secreting materials and increases the fitness of the animal. In some cases, the self-secreted material is directly applied, as in the case of ecribellate silk, spun by ecribellate spiders, to