are often unable to climb out. The antlions may alter these pits based on prey availability. In areas with less available prey, antlions will make wider holes to increase the chance of catching an insect. If prey are able to climb out of the hole, antlions will increase the depth of the hole. Displays Animal structures can serve as a means of communication with other organisms. Animals may construct to attract mates, as seen in species of male fiddler crabs. These crabs may form "pillars" or "hoods" out of sand and mud to gain the attention of nearby females. Bowerbirds (Ptilonorhynchus violaceus) also create display structures to attract mates. During the mating season, male Bowerbirds will collect twigs and colourful objects to create structures known as "bowers", which attract the attention of females. Bowers that are more colourful and well constructed are more attractive to female bowerbirds, as the quality of the constructed bowers reflects the quality of the male bird. Transportation Army Ants (Eciton hamatum) form "living bridges" to assist in transportation. Army Ant colonies may move locations each day in search of food. These bridges provide a path over obstacles and allow for the ants to search for food at an increased speed. The bridges are constructed when the ants join their bodies together, and can vary in size and shape depending on the situation the ants face. Ants are confined to their position when they are forming these bridges, preventing them from moving. The bridges are broken apart when they are no longer needed. Building materials Materials used by animals in building structures need to not only be suitable for the kind of structure to be built but also to be manipulable by the animals. These materials may be organic in nature or mineral. They may also be categorised as "collected material" and "self-secreted material". Collected materials Some animals collect materials with plastic properties which are used to construct and shape the nest. These include resin collected by stingless bees, mud collected by swallows and silk collected by hummingbirds. Some materials in nature act as ready made "building blocks" to the animals in question, such as feathers and leaf petioles for some birds and animal hair for the chaffinch. Other materials need to be "processed". Caddisfly larvae use stone pieces and also cut sections from green leaves for use in construction. The stone pieces are selected as