322 SOME EFFECTS PRODUCED BY THE HOODING OF BIRDS BY DOROTHY M. PATRICK, M.B., CH.B. (EDINB.). (From the Physiology Department of the University of Birmingham.) (Received 17 October 1926.) (With Plate XIII.) A BLACK velvet hood was placed over the head of a pigeon and tied round the neck with a piece of cotton, sufficiently tight to prevent the bird removing it with its claws, which it at once attempted to do. Presently some movements of progression were made along the floor, which seemed to suggest that disturbances of equilibration had supervened; then the bird rapidly became quiescent and assumed a peculiar and unusual position, in which the head was bent over until the crown rested upon its back, with the tip of the beak pointing towards the tail. In this condition the bird can perch quite well, but tends to fall over backwards; it can progress along the floor for a few paces if disturbed, but usually walks backwards ; it is not disturbed by loud noises and when 'thrown into the air flies quite well, but quickly alights upon the floor where it rapidly regains its quiescent state and peculiar posture. Hoods of various materials and of divers colours, some with holes in them for one or both eyes, or for one or both ears and so* on were used, to ascertain whether the condition was due to pressure upon or loss of function by these structures. All constrictions round the neck and beak were dispensed with in case the thread might, by constricting the vessels in the neck, produce an anaemic condition of the head. By the kind permission of the Zoological Committee of the Birmingham Botanical Gardens birds of various kinds were used for experiments, the results of which are detailed below. THE HOMER PIGEON. Most of the experiments were performed on pigeons. A typical case is here detailed. Plate XIII, fig. 1 is a photograph of the hood used; the beak protruded through the one hole, the other hole fell over the right eye so that the left eye alone was covered. At the beginning of the experiment the bird resisted handling, but as the hood was in position it became tranquil and lay down on its breast head extended in a straight line with the body. When placed on its feet it managed
Some Effects Produced by the Hooding of Birds 323 its balance, and made some attempt to rid itself of the hood by shaking. If made to walk it did so with a very ataxic gait and usually in a backward direction. This ataxic gait at once suggested cerebellar or semicircular canal effects. After a few minutes the bird became stationary and sank down on its breast, breathing somewhat heavily. The head was then slowly bent backwards. After a few minutes the head twisted to one side though still in the dorsi-flexed position. During this period the exposed eye was not shut, but remained wide open, winking only at long intervals. Again the head was slowly brought down to the ground only to be replaced in the dorsi-flexed position after one or two side-to-side shakes. When placed on a perch it had at first great difficulty in maintaining its balance, but once that had been secured the head very slowly dorsi-flexed, reaching the extreme position after 60 seconds (Plate XIII, fig. 2). Its complete passivity was very striking, no emotion whatever being shown. Loud noises such as clapping the hands close to its ears failed to disturb it or cause fear. The sight of food aroused no interest. When made to fly it would do so but alighted immediately. During this time the heart beat and respiration remained normal both in rate and force. The pigeon remained on its perch in the described position for two hours maintaining complete apathy to all its surroundings in the laboratory where other work was going on. Immediately the hood was removed the pigeon shook itself two or three times and then began to feed. It was now absolutely normal. During these experiments it was noticed that a slight touch on the back caused the bird to dorsi-flex its head to the utmost; or if the head was in the downward position with the beak resting on the floor, as sometimes happens, the slightest touch on the back brought it into the dorsi-flexed position. Possibly this increases the sense of weight of the hood on the head. Various factors were suggested as the cause of this phenomenon. It was thought that the thread holding the hood in position might have caused constriction of the vessels of the neck and hence cerebral anaemia. But a constricting elastic band placed around the neck in absence of a hood had no effect. Dorsi-flexion also took place when the hood was merely placed over the head without the thread being fastened. Therefore cerebral anaemia is not the cause. Various types of hood were used with equally good results, whether they rested on the top of the head or not. As time went on the pigeon seemed to know what was expected of it and assumed the dorsi-flexed position almost as soon as the hood touched its head. The oftener the hood was placed in position the more immediately were the results obtained. Secondly, it was thought that visual disturbances might be the cause, but dorsiflexion of the head and passivity were present if one or both eyes were exposed to light or if completely kept in the dark by the hood. Therefore disturbances of vision do not account for the condition. Pressure of the hood on the cere or on any sense organs lying in or near the was prevented by enlarging the hole in the hood until the cloth rested on the plumage in front of the eyes, yet dorsi-flexion took place as before. As disturbance of the semicircular canals might be a cause, the hood was
324 DOROTHY M. PATRICK discarded and the external auditory meatus occluded, first on "one side and then on both, by means of plasticine. This produced no effect upon the bird. Fu^fc hoods with holes large enough to expose both ears to air and sound were employe and dorsi-flexion of the head still took place. Thus it would seem that the semicircular canals are not the causal factor. Slight pressure, equal to the weight of the hood, was next applied to the vertex of the head by placing upon it a small piece of plasticine and on another occasion a small strip of sticking plaster. Neither of these had any effect upon the bird's behaviour. The exhibition of an anaesthetic (chloroform or ether) produced at once a commencement of the dorsi-flexed condition, which became more marked as the anaesthesia deepened and remained so until death supervened. A pigeon also died of natural causes with its head in the dorsi-flexed position. DOMESTIC FOWLS. A brooding hen which pecked and objected vigorously to being handled, became passive immediately the hood was placed over its head and remained stationary with its beak resting on the ground (Plate XIII, fig. 3). This was the position assumed by pigeons before dorsi-flexion of the head took place. The almost sudden transition from a state of rebellion without the hood to the passive condition with the hood was remarkable. OTHER BIRDS. At the Birmingham Zoo, in almost every case, from the largest bird available down to the smallest, hooding produced the dorsi-flexed position of the head. Among the birds used were bullfinches, various kinds of pigeon, i.e. Homers, Fantails, Jacobins, ring-doves, Tumblers, etc., partridges, magpies, hawks of various kinds, owls, etc. In the majority of cases, as soon as the hood was on, the bird sank down on its legs into the roosting attitude. The head then slowly moved backwards into the dorsi-flexed position. They were all passive yet never went to sleep, their eyes remaining open all the time with occasional winking. They appeared to be watching one's movements. In the normal condition the partridge is very sensitive and takes to flight at the slightest disturbance or noise. Yet with the hood on it became passive, sinking down on its legs and then falling over to one side with its head in the described position. It remained thus breathing deeply and made no attempt to free itself or fly. It was so passive that it could be pushed about the floor like an inanimate object, without offering the slightest resistance. Yet the moment the hood was removed it flew away. A.hooded Magpie remained in the dorsi-flexed position for half-an-hour took to flight on being disturbed. When recaptured (though the hood was still
Some Effects Produced by the Hooding of Birds 325 iiyjosition) it showed resentment by biting and scratching every time it was touched ^Ppproached. In this case, therefore, the effect of the hood seems to have passed off after half-an-hour, a fact not noted in any other bird. The passive condition was well shown in the case of a hooded Kestrel. The keeper feared under normal circumstance to.interfere with this bird, owing to its biting and scratching habits; but after the hood had been placed in position the bird could be prodded with the point of a pencil and stroked with the hand without resenting it. If made to walk it progressed backwards and soon came to rest, sinking down on its legs with the head thrown backwards and remained thus. The moment the hood was removed its antagonism and temper were displayed to the full. The Small Owl attempted to dorsi-flex its head so much, that it lost its balance and fell over backwards. It lay as it fell with its feet in the air and made no attempt to assume the normal position. It allowed its feathers to be ruffled and stroked and did not appear to be conscious of it. The various types of Pigeons reacted in much the same way, thus the Jacobin, which possesses a broad collar of feathers in which it can hide its whole head, assumed almost at once the usual position, but made attempts at varying intervals to scratch the hood off, but not succeeding sank again into apathy after each attempt. With Tumblers the effect of the hood varied somewhat; their gait became so ataxic that they were practically incapable of walking; when pushed they were not absolutely passive, but attempted to walk of their own accord. A Bantam's head was gently placed under one wing in the sleeping position and the bird whirled round three times. It was then laid upon a table on its breast, in which position it remained absolutely passive until its head was again taken from under its wing, whereupon it got up immediately and ran away. This occurred in other birds also. This might suggest that the condition was brought about by disturbance of the fluid in the semicircular canals due to the whirling, but had that been so, the bird should have become ataxic and shown loss of the sense of equilibration instead of remaining passive. Again the head under the wing suggests sleep, but the bird does not usually sleep lying on its side; nor was it asleep. When Falcons were used for sport, the hoods they wore were made of metal and did not touch the top of the head, yet the birds remained quite passive so long as the hood was in position, taking flight at once when shown the quarry. Pressure of the hood therefore is not a cause of the passive condition of hooded birds. So far no pathological changes have been observed in the brains or cerebella of the pigeons experimented with. From these experiments it may be concluded that hooding induces a hypnotic condition in birds and that the position assumed by them resembles rather that of death than that of sleep. A pigeon, in this country at any rate, usually sleeps with its head on its breast, with shortened neck; whereas in the experiments detailed ahove, the head is dorsi-flexed and the neck extended. ^Pi.ccording to Flack and Hill, hypnosis may be induced in many animals merely by holding them in a strange position, as in the well-known experimentum mirabile. This is quite sufficient to explain the effect produced on the bantam, but the other
326 DOROTHY M. PATRICK birds were held in a natural position while being hooded. Possibly fear shamming death reflex may be factors in some cases but no definite exj here offered as to how or why hooding induces the hypnotic condition. My best thanks are due to Prof. Carlier for both help and advice throughout the prosecution of this investigation. PLATE XIII. Fig. 1. Photograph of Hood. The beak protruded through one hole, the right eye through the other (see p. 322). Fig. 2. Homer pigeon wearing hood. Note position of the head. Fig. 3. Brooding hen wearing hood.
JOURNAL OF EXPERIMENTAL BIOLOGY VOL. IV, PLATE XIII SOME EFFECTS PRODUCED BY THE HOODING OF BIRDS (pp. 322-326).