AGGRESSIVE DISPLAY OF THE CORN-CRAKE.

Transcription

1 163 AGGRESSIVE DISPLAY OF THE CORN-CRAKE. BY A. G. MASON THE accompanying photographs of the aggressive display of the Corn-Crake (Crex crex) were obtained by calling a bird up to a mirror. The technique of calling up a Corn-Crake, using bones, has been previously described (i, 2). When the photographs were taken two hides were used. Mr. F. W. Fox was in one, with the mirror immediately in front of it, and the photographs were taken from the other. The experiments were made on the afternoons of May 27th and June 4th, 1950, on the same bird, at Saggart, Co. Dublin. May 27th was cold and wet; June 4th was hot and sunny ; but the bird's performance was the same on both occasions. When the bird came up to the " call" and saw its reflection in the mirror it checked and craked and then made repeated displays and attacks, interspersed with bouts of craking, at its reflection, On both days it was still active when all my plates were exposed, and we came out of the hides, after three-quarters of an hour or so. The Corn-Crake uses' three postures in its aggressive display. In the present experiments they did not follow in any definite order, but it is possible that in nature they would do so. The first posture was craking with the wings half open and with the head and neck inclined forward. This posture was also seen when a bird made an aggressive display at a dummy in 1943 (3). It is well shown in the photographs (Plates 17-19). It is a different posture from that adopted when craking away from an opponent or a mirror. When craking " normally " the head and neck are more nearly vertical and the wings closed. In both cases when craking the lower mandible remains almost stationary and the beak is opened by throwing the head back and raising the upper mandible. In the second posture (Plates 20 and 21) the bird stands or walks with the body held normally though the head and neck are raised more than in the usual position adopted when walking through grass. The wings are dropped and touch the ground and are pressed into the sides so that the tips of the primaries touch, behind and over the tail, making a shape like the inverted bow of a boat. The back feathers project out over the touching wings. This posture was seen on one occasion when an experiment was made in a short meadow which contained two birds on May 31st, Both birds came up to the " call" and made this display at each other while running round in seemingly aimless curves presumably jockeying for position. In my notes of previous experiments with a dummy (4) there are several references to birds commencing to display in long grass, when approaching the dummy, with the wings stretched back instead of sideways as in the courtship display. It is probable that some, at least, of these birds were making an aggressive display. In the present experiments this posture was not adopted at the first

2 164 BRITISH BIRDS. [VOL. xirv. approach to the mirror, but later on after attacks and other displays. It is probably essentially an approach display made when the bird is moving towards ah opponent in sight. In the mirror experiments it was probably induced by the movement of the reflection as the bird passed in front of the mirror. The " growling-mew " note (i) was uttered, with the beak closed, while the bird held this posture. Naming this display the second posture is begging the question. In nature it may well be the first of the sequence, if there is a sequence, though it was only used after the craking posture in these experiments. The third posture is the display previously described (i, 3) in which the bird lunges forward towards its opponent. As it lunges it brings its wings forward twisting them at the same time so that in the final position the top surface of the wing is facing to the front with the leading edge down ; the head and neck are drawn in and held low with the neck almost touching the ground; the breast feathers are erected. The effect is very dramatic. Viewed from the front the bird seems suddenly to increase in size. When previously describing this display I stated that the upper tail-coverts were erected as in the courtship display. This may have been an error in observation, as in previous experiments I was looking down on the bird from almost above it. Certainly the photographs show no sign of these feathers being erected but individual birds vary considerably in the completeness of their postures. The photographs (Plates 22 and 23) of this posture are disappointing. The movement was extremely quick and was literally over in a flash. It proved extremely difficult to expose the camera at the right moment, and, even then, the highest speed, i/20oth of a second, proved inadequate to stop movement. In Plate 22, however, the reflection in the mirror shows the wing in the forward position with the upper surface to the front. Plate 23 shows a slightly earlier moment in the display. The bird is bringing its wings up and is lunging forward. The position of the legs in both photographs illustrates the vigour and swiftness of the lunge. At intervals, on both days, the bird crouched on its tarsi with its wings half open and out sideways (Plate 24). Its beak was usually open and the bird was panting as if distressed by its exertions. This was seen more often on May 27th when the weather was cold and wet. On both days the bird wandered, several times, round the side of the hide, away from the mirror. When this happened I craked from the camera hide. The bird immediately returned to the mirror, not, as is usual, towards the sound of the crakes. On one occasion on June 4th the bird came under the side of the mirror hide and looked at Mr. Fox. It had traces of slug adhering to its lower mandible. When I craked from the other hide it immediately ducked back under the edge and ran round to the mirror. On the 27th Mr. Fox craked when the bird went round the corner of his hide away

3 VOL. XLIV.] AGGRESSIVE DISPLAY OF CORN-CRAKE. 165 from the mirror. On this occasion, too, the bird went straight back to the mirror and not towards the sound of the call. On May 27th Mr. Fox held the dummy out under the hide beside the mirror. The bird paid no attention to either his hand or the dummy though it must have seen them. When photographing the sexual display using a dummy (1) some years ago the bird was driven away and the dummy taken into the hide while the camera was packed up. The bird came back and Mr. G. F. Mitchell held out the dummy under the hide. The bird attempted coition and Mr. Mitchell brought the dummy, with the bird on it, into the hide and picked the bird off the dummy with his other hand. As well as displaying to its reflection the bird made numerous attacks. Sometimes it pecked vigorously at the image ; sometimes it half jumped, half flew up and struck with its feet. When the dummy was attacked in previous experiments the attacking bird always struck with its beak. When a mirror was used on a previous occasion the bird struck with its beak only (4). The mirror used in this case was, however, a different shape and only nine inches high ; had the bird jumped up its reflection would have disappeared. I have only once seen a fight between two Corn-Crakes (4). The birds were half hidden in long grass. Their wings were seen stretched almost straight up and beating vigorously. At the time I thought they lunged at each other with their beaks but they may well have been springing up and striking with their feet. Once when a fight occurred in long grass, out of sight but close enough to hear, there was much beating of wings (5). In describing the " second " posture I mentioned that in nature it might well be the first of a sequence, if there is a sequence. There probably is no sequence in the sense that the different postures form a connected whole with one following another automatically. It is much more probable that the different postures are induced by different behaviour on the part of the opponent and that the other bird's movement or behaviour supplies the stimulus which evokes the appropriate posture. Thus the craking-with-wings-half-open posture is probably normally used at the first visual contact with a craking intruder. In experiments with a dummy an approaching bird's normal reaction to sight of the dummy, after the middle of May (2), is a burst of crakes. The wings in this case are not opened, probably because the dummy is passive and motionless and thus does not supply the stimulus for aggressive behaviour. It takes two to make a fight and this should never be forgotten when studying displays made in unnatural experimental conditions. I have already suggested that the second posture is induced by the sight of a moving opponent. The signal for the third posture may well be an opponent who stands his ground but who does not actually

4 166 BRITISH BIRDS. [VOL. XLIV (I) MASON, A. G. (1945)- " The Display of the Corn-Crake." British Birds, vol. xxxviii, p (2) MASON, A. G. (1950). "The Behaviour of Corn-Crakes." ibid, vol. xliii, p. 70. (3) MASON, A. G. (1944). "Combat Display of the Corn-Crake." Irish Naturalists' Journal, vol. viii, p (4) MASON, A. G. (1941). " Further Experiments with Corn-Crakes." ibid, vol. vii, pp (5) MASON, A. G. (1940). "On Some Experiments with-corn-crakes." ibid, vol. vii, pp

5 British Birds, Vol. xliv, PI. 17. AGGRFSSIVE DISPLAY OF CORN-CRAKE (Crex crex). FIRST POSTURE. COMMENCEMENT or " CRAKE " WITH WINGS HALF OPEN. (Photographed by A. G. MASON).

6 British Birds, Vol. xliv, PI. 18. AGGRESSIVE DISPLAY OF CORN-CRAKE (Crex crex). FIRST POSTURE. " CRAKING " WITH WINGS HALF OPEN. NOTE THAT THE LOWER MANDIBLE IS IN THE SAME POSITION AS IN PLATE 17 ; THE BEAK IS OPENF.D BY RAISING THE UPPER MANDIBLE. (Photographed by A. G MASON).

7 British Birds, Vol. xliv, PI!!). AGGRESSIVE DISPLAY OF CORN-CRAKE (Crex crex). FIRST TOSTURE. REAR VIEW OF " CRANING " WITH WINGS HALF OPEN. NOTE UPPER MANDIBLE. (Photographed by A. G. MASON).

8 British Birds, Vol. xliv, PI. 20. AGGRESSIVE DISPLAY OF CORN-CRAKE (Crex rrr.r). SECOND POSTURE. (Photographed by A. G. MASON).

9 British Birds. Vol. xliv, PI. 21. AGGRESSIVE DISPLAY OF CORN-CRAKE (Crex crex). SECOND POSTURE. JS'OTK PRIMARIES TOUCHING BEYOND TAIL. (Photographed bv A. G. MASON).

10 British Birds, Vol. xliv, Pi. 22. AGGRESSIVE DISPLAY OF CORN-CRAKE (Crex crex). THIRD POSTURE. THE LUNGE FORWARD. (Photographed by A. C. MASON).

11 British Kirds, Vol xliv PI 23. AGGRESSIVE DTSPT.AY OF CORN-CRAKE (Cre.x rrrx). THIRD POSTURE. THE LUNGE FORWARD. (Photographed by A. G,.MASON).

12 British Birds, Vol. xliv, PI. 24. AGGRESSIVE DISPLAY OF CORN-CRAKE (Crex crex). RESTING ON TARSI WITH HALF-OPEN WINGS. (Photographed by A. G. MASON).