Some Displacement Activities of the Black-Headed Gull1)

A series of unusual activities are provoked when a Black-headed Gull is unable to incubate its eggs properly. The most conspicuous of these activities are the subject of this study. Among these activities are various nest-building movements and preening movements. Both of these are certainly displacement activities; and both of these are certainly provoked by the thwarting of the incubation drive. A list of the situations in which displacements occur is given in the text. These situations differ, at first sight, from those in which previously described displacement activities occur. Displacements usually appear when a strongly motivated animal is unable to perform the final act. In this case, it is obvious that the gull can, and often does, perform the action of sitting on nest and eggs; but this may not be enough. In this case, the final act really consists of sitting on a full clutch of properly arranged eggs. These various displacement movements are apparently unusual, in that they are complete and do not differ significantly from their autochthonous examples. Nest-building movements are the most common of the displacement movements provoked by a thwarted incubation drive. It seems probable that, although they are not ritualized, they have considerable survival value and have been selected for in the course of evolution. These displacements were quantified by counting, over a standard period of time, the movements performed by birds from whose nests different numbers of eggs had been removed. In general, all displacement activities became significantly more frequent as more and more eggs were removed. The various nest-building movements, however, increased at significantly different rates. Preening movements increased markedly only when all three eggs were removed. It is suggested that nest-building and preening serve as outlets for great surpluses of incubation drive. A comparison is made with displacement activities in other species. True displacement activities should be distinguished from those apparent alternative displacement activities, where the surplus motivation has sparked-over to one center only, but where the exact form that the resultant activity takes may vary with the external situation. Fighting behavior also increases when all three eggs are removed from a nest. There are, however, reasons for believing that it is not a displacement activity. It is the more or less expected result of the unusual activities, of all kinds, that follow removal of the eggs.