SIDEWAYS‐THROWING AND SIDEWAYS‐BUILDING IN BIRDS
Authors: Harrison C. J. O. 
Published: April 2008

Birds which build the simplest and scantiest nests use a single movement, sideways‐throwing, in assembling material; and a modified form of it, sideways‐building, in arranging material on the site. Sideways‐throwing is usually performed by a standing or walking bird which picks up a small object in front of it, and throws it back, either to one side or beneath it, with a sharp sideways movement of the head. Sideways‐building is normally performed by a sitting bird which takes material and deliberately places it to either side or in towards its breast. Sideways‐throwing and sideways‐building occur in the Struthionidae, Rheidae, Dromaiidae, Gaviformes, Procellariformes, Anseriformes, Meleagrididae, Phasianidae, Gruidae and Charadrüformes. Sideways‐building only is recorded in the Sulidae ahd, for Platalea leucorodia, in the Threskiornithidae. There are no records of nest‐building in the Apterigidae, Tinamidae, Otidae, and Pteroclididae.

Sideways‐throwing occurs within the vicinity of the nest‐site, often in conjunction with conflict behaviour; and typically occurs at nest‐relief, performed by the bird leaving the nest. This results in material accumulating at the nest‐site, and this may be pulled in by the sitting bird during sideways‐building, The amount and type of material depends on its availability around the nest‐site. In species where only one sex incubates the sitting bird may leave the nest at intervals and perform sideways‐throwing.

Sideways‐throwing appears to occur frequently as a form of irrelevant behaviour, usually in agonistic situations; sideways‐building often occurs when a sitting bird is disturbed. Since sideways‐throwing is a relatively ineffectual form of nest‐building, the creation of a nest appears to depend on the extent to which the bird finds itself in a conflict situation and the amount of material near the nest. It seems possible that it may have evolved as Irrelevant Behaviour rather than true nest‐building. It is suggested that the movements might be derived from false‐feeding movements involving the picking up of inedible objects, and from the flick of the head, with which a bird removes an unwanted object adhering to its bill. Such behaviour performed by a bird walking repeatedly from the nest could result in the accumulation of material at a nest‐scrape. Relatively little modification would be needed to create a form of nest‐building suitable for very simple nests.

Chance sideways‐throwing seems to occur in many species. Nests in damp situations may be larger than those in dryer ones and, although this might be accidental, the behaviour of some species during floods suggests purposeful building. Some species appear to align their throwing in relation to the position of the nest. In some groups within the Charadrüformes sideways‐throwing appears to have been replaced by the carrying of material to the nest. In such species relict sideways‐throwing often occurs when material is deposited on the nest‐site; and sideways‐building is used in the arrangement of the material. It is possible that movements similar to sideways‐building used in nest construction by birds in other groups might be homologous with it and represent the persistence of an earlier behaviour pattern.