INTRODUCTIONNest building has usually evolved to a point where the resultant structure can be recognized as the work of a particular species. Within the avian class, however, nests range from being completely absent to complex and highly stylized structures such as those found in species of Ploceidae or Remizinae. It seems possible, indeed likely, that the movements used during nest-building have their origin in movements used during the normal maintenance activities of the birds, which have become modified and stereotyped for this new purpose. In the present paper one nest-building movement, which is present in even the simplest and scantiest form of nesting, has been examined, and its probable origin speculated upon. This is the movement known in its simplest form as " sidewaysthrowing " and in its more evolved manifestations as " sideways-building " (Lind 1961).In making this study, which is based on avian orders in which little, or relatively simple, nest-building occurs it has been necessary to assume that the taxa concerned show a stage in nest-building which has been arrested in its evolution at a point where the species had reached the limit of its capability, or where it most adequately fulfilled the needs of the species concerned. It has also been assumed that the behaviour and movements involved are those which existed at an earlier stage in the general evolution of nest-building. Since the taxa concerned consist of species which appear to have been in existence for a relatively long period, it might possibly be argued that a secondary loss of nest-building ability has occurred, and that the movements now existing are relics of more complex building behaviour. While this possibility is recognized, I have decided on the evidence available to reject it in favour of the first assumption.For a bird to build a nest a series of specialized actions are required.
SIDEWAYS-THROWING AND SIDEWAYS-BUILDINGThe rather simple action of sideways-throwing occurs in a number of species but is most easily studied in the wading birds of the order Charadriiformes. Sidewaysthrowing is usually performed by a standing or walking bird which reaches down and picks up a small object, often a plant-stem or a leaf, but sometimes a stick, stone, shell, or any small fragment of movable debris that is within easy reach. This picking-up is immediately followed by a sharp sideways movement of the head or a flick of the bill, the object being thrown backwards on one side of the bird or beneath it. In the Lapwing V. vanellus, this is usually an abrupt flick of the head which casts a small object back, often to fall about a length behind the bird. Lind (1961), illustrating the Blacktailed Godwit L. limsa in one of the few good photographs of this behaviour, shows the bird swinging the head well along the side in casting back a grass-stem that will fall a little behind.