The Feather Postures of Birds and the Problem of the Origin of Social Signals
Authors: Morris Desmond 
Published: April 2008

AbstractThe primary thermoregulatory function of body feathers and their movements is discussed and four general feather postures are distinguished : 1. S l e e k e d : The feathers are fully depressed against the body. This is characteristic of active birds, or birds ready for action, and streamlines them in flight and also reduces the insulating effect of the plumage. 2. R e l a x e d: The feathers are neither depressed nor erected. This is the usual state of birds when only moderately active, or inactive. 3. F l u f f e d : The feathers are partially erected giving the body a rounded appearance with smooth outlines. This increases the thickness of the insulating layer and helps to keep the bird warm. 4. R u f f l e d : The feathers are fully erected giving the bird a round but ragged appearance. The separating out of the individual feathers tends to destroy the insulating layer and a cooling effect results. It is observed in very hot birds which are inactive. The secondary signal function of body feathers is investigated. It is shown that in non-thwarting situations the mood of a bird may under certain conditions be conveyed to its fellows by its unspecialised feather postures, which therefore act as simple signals. In thwarting situations, the autonomic discharge, which accompanies the primary somatic response, is known to produce dramatic pilomotoric effects. It is suggested that this physiological relationship between thwarting and feather erection is the basis of all feather-posture displays (with the exception of the simple ones mentioned above which occur in non-thwarting situations). Not only generalised body fluffing or ruffling, but also the highly specialised body-feather displays are explicable in this way. Examples given include the evolution of crests, ruffs, chin-growths, throat-plumes, eartufts, flank-plumes, etc. It is stressed that a detailed study of the physio-ethology of thwarting would greatly assist in advancing the study of social signals in animals. The general problem of the evolution as signals of responses to thwarting situations is next considered. Three conditions of thwarting are recognised, namely: The absence of indispensible stimuli, following arousal; simple physical obstruction; and simultaneous arousal of incompatible tendencies. The Primary Response to thwarting is subdivided into a Somatic Response, which tends to adjust the animal to its external environment, and an Autonomic Response, which tends to adjust the internal environment of the animal to the requirements of this Somatic Response. Five types of Somatic Response are conceived: "Perseverance", "Snap Decision", Thwarted Intention Movements, Ambivalent Posturing and Alternating Ambivalent Movements. Five types of Autonomic Response are also mentioned as being relevant to ethology, and the signals which have probably evolved from them are discussed. The most important are as follows : Alimentary changes leading to urination and defecation, which have evolved into territorial marking systems. Circulatory changes leading to flushings which have evolved into bare-skin-flushing displays. Respiratory changes leading to alterations in the breathing rate, amplitude and regularity, which have evolved into vocalisation on the one hand, and inflation displays, on the other. Thermoregulatory changes leading to sweating and pilomotoric activity, which have evolved into scent signals and hair and feather erection displays, respectively. The Secondary Responses which appear as alternative somatic responses, when the unsuccessful Primary Somatic Responses are abandoned, are categorised as follows : Displacement Activities, Redirection Activities, Regressive Activities, and "Neurotic Inactivity". Only the former is discussed in any detail and it is pointed out that the autonomic changes accompanying thwarting may be important in determining the nature of the Displacement Activities which follow. The question of whether or not Displacement Activities arising in this way can really be considered as irrelevant activities is left open. In conclusion it is pointed out that three, and not two, fundamental types of signal must be considered as arising from thwarting situations. Not only are there P r i m a r y S o m a t i c S i g n a l s arising from Thwarted Intention Movements and Ambivalent Activities and S e c o n d a r y S o m a t i c S i g n a 1 s arising from Displacement Activities, but there are also many A u t o n o m i c S i g n a 1 s, the origin of which has hitherto been ignored.