(130) DISPLAY OF THE MUTE SWAN

Transcription

1 (130) DISPLAY OF THE MUTE SWAN BY J. S. HUXLEY ON reading A. W. Boyd's note on display of the Mute Swan (Cygnus olor), British Birds, Vol. xxxix, p. 182,1 turned up my own notes and from them I have been able to extract the following points, which do not seem to be covered adequately by the account in The Handbook* (1) Cuttmill Ponds, near Puttenham, Surrey, April nth, On the lowest (mill) pond a pair with nest containing three eggs. The hen was sitting, but left the nest for the open water as I approached. On seeing this the cock half flew, half flapped towards her, swimming the last portion with wings in the forwardly-arched position typical of the male. The female then assumed the same pose, facing her mate, though her wings were not so fully spread and arched as his, and the pair remained thus close to each other, until I left the vicinity. This (mutual) employment of a normally hostile attitude is mentioned by The Handbook as occurring "in certain circumstances," for instance, "after driving off intruders." In such cases, however, it is stated that "when breasts are almost touching, heads are suddenly stretched straight upwards and then returned to normal, while both utter prolonged snoring note." This latter pose and utterance was not exhibited by my birds which, however, had not driven off the intruder, in the shape of myself, but were apparently reinforcing each other's courage and emotional level. I have tested that mutual emotional reinforcement of this sort was habitually carried out by a pair of Whooper Swans (C. cygnus) at Whipsnade, but by means of a different display. On strangers threatening them or coming too close to the small pond where they were kept, the two would join each other on the water, in the same way as did the Mute Swans observed by me, and then both repeatedly raised and lowered their necks with a loud whooping note, and with wings partially extended outwards and downwards. This is almost exactly as described in The Handbook, except that there it is stated that this occurs either after an intruder has been driven off, or in greeting fresh arrivals at the winter gatherings of the species. At Whipsnade, however, as with my Mute Swans at Cuttmill, this action took place while a human intruder was still threatening, and was clearly concerned with "keeping up the spirits" and courage of the pair by mutual emotional reinforcement, instead of as an expression of relief at successful action. This was especially clear on one occasion when a Mute Swan had flown on to their little pond. Neither of the Whoopers at first attacked the intruder, but after they had swum together and indulged in this *Based on the account of O. Heinroth, Verh. d. V Internal. Omithol.- Kong., 1910, pp EDS.

2 VOL. XL] DISPLAY OF THE MUTE SWAN. 131 mutual ceremony, the cock, followed by the hen, swam at the Mute Swan and drove it away. Otherwise my observations tally with The Handbook, except that I should agree with E. Selous (Zoologist, 1913) whom The Handbook does not cite that the call is a "glad cry," not a "mournful-sounding note." (2) Same place and date, but on the next higher pond. Striking pre- and post-coitional performance, similar to the description in The Handbook except in the points indicated below (in italics). The pre-coitional display was already in progress when I first saw the birds, and continued for over ten minutes before coition occurred. Throughout the birds were side by side, facing in the same direction. The dipping of heads under water as described in The Handbook took place repeatedly, but was usually accompanied by a dabbling in the water, apparently from side to side, with the beak, which seemed to be repeatedly opened and shut, lasting from three to ten seconds. This seems to be a "habit-action," similar to the habit-preening described and discussed at length by me for the Great Crested Grebe (Huxley, Proc. Zool. Soc. Lond., 1914: ) and Red-throated Diver (1923, /. Linn. Soc, 35: ), where during the excitement of display an action subserving some quite other and non-sexual function is performed, but in a non-functional "ritual" way. These birds also occasionally showed a further development of this "habit-feeding," by first dipping the neck much further under water than in the normal head-dipping, and then dipping the whole fore-part of the body under water. Sometimes the neck appeared to be stretched vertically downwards, as in feeding on weeds on the bottom. In addition to " habit-feeding," the birds showed habit-preening as in my Grebes (a) ritual actions symbolic of preening, or (b) (more frequently) between head-dippings, rubbing the head along one flank or the other, as is often seen in true preening. The ritual habit-preening was quite different from the true preening seen after the post-coition ceremony. The head-rubbing and the fuller type of habit-feeding were apparently performed more often by the male. Coition lasted seconds (as against the 3-5 seconds of The Handbook) and was terminated by what I noted as " a hoarse attempt at a trumpeting cry " (instead of The Handbook's " snoring note"). The post-coition ceremony lasted only 10 to 15 seconds. It was as described in The Handbook except that I heard no note accompanying it. The vigorous preening, tail-wagging, etc., which then ensued was still going on when I had to leave about five minutes later ; but I saw none of the "prolonged bathing" described in The Handbook. (3) Same place, lowest (mill) pond, March 24th, A pair were occupying a nest, and an odd bird (a hen) was standing in a smallish heap of nest material 20 or 30 yards away. (It was rare for more than one pair to occupy this pond). When first seen, the

3 132 BRITISH BIRDS. [VOL. XL. hen of the pair was brooding and the cock sitting on the rim of the nest. After 12 minutes the cock, after three snorts, began to pull material from the outer rim of the nest or just beyond, higher up on to the nest. After three minutes of this, with three further snorts on the cock leaving the nest, the hen continued what he had been doing. The cock went off a few feet and proceeded to pick up reed-stems and floating debris. However, he never brought them to the nest, but merely dropped them behind him over his shoulder. This continued for 13 minutes, after which he swam off. Curiously enough, I had just seen this ritual non-functional gathering of nest-material for the first time, by another bird (probably the cock, but the sexes were hard to distinguish) of a pair nesting on the pond next above. In this case the bird was "about z\ yards from the nest, picking up loose debris from among the bulrushes, and turning to deposit them not on the nest but on the water nearer to the nest." On my first approaching, some time earlier, the hen had slipped off the nest and the cock had "swam about nearby, pulling at growing bulrushes and debris, often in a curious attitude with the neck stretched right out along the surface of the water." This may have been an expression of fear or frustrated anger. Neither of these habits is mentioned in The Handbook. To return to the pair on the mill-pond, the cock, after chasing a tame white duck into the reeds, then returned slowly in the characteristic aggressive "arched-wing" attitude, but found the hen dozing unresponsive on the nest. He then swam off in the direction of the unmated hen. When he was close to her, the two birds came out together on to the open water and started a prolonged display, culminating in attempted coition by the hen, the whole lasting just over 20 minutes. Neither the "bigamous" behaviour nor the reversed pairing (as in Great Crested Grebes Huxley, Proc. Zool. Soc, 1914 ; or Moorhens Howard, Bird Behaviour, Cambridge, 1929), is noted for this species in The Handbook. My notes on this lengthy display provide a rather fuller description than that given in The Handbook, and I summarize them here. The display was built up of five distinct actions: (1) Dipping of the whole head and neck below water, with a slight forward tilt of the body so that the entire base of the neck, including its dorsal side, was submerged. The head was then brought quickly up and shaken as it emerged, all in a single movement, making a splashing sound. (2) Less frequently, the head alone was dipped and held below the surface for a few seconds. Here again, at the finish it was usually shaken, either while still below the water or immediately after emergence. The only action of this type noted by The Handbook is a repeated dipping of the head under water, followed by quick withdrawal. This apparently refers to my No. 2.

4 vol. xt] DISPLAY OF THE MUTE SWAN. 133 (3) One of the two kinds of habit-preening observed in the 1921 pair (see above), namely the rubbing of the back of the head against the bird's flanks and back. (4) A frequent attitude : close together, with both necks erect but heads arched somewhat forward. While in this pose the birds might be side by side, either facing in the same or in opposite directions, or facing each other. (5) No cry was heard, but there was a good deal of what I note as "heavy breathing" possibly the "snoring" of The Handbook, but there not noted for the pre-coition display. All these actions were performed by both birds, though the cock did much more neck-dipping. There was no regular sequence of the different actions, though usually the ritual head-rubbing (No. 3) alternated between two neck-dippings (No. 1). It will be noted that this display contained some elements lacking in that previously described, and vice versa. In the first five minutes, the cock performed the neck-dipping 44 times. Then the hen nipped hold of the cock's tail and attempted to mount. This was followed by confused napping in which the cock pursued the hen towards the reeds. Shortly afterwards the display was resumed for a further five minutes, during which the cock gave 35 more neck-dippings, making 79 in all. Then the hen once more attempted coition, but the cock shook her off and then "apparently angrily" flapped at her, upon which she retired into the reeds. On this, the male approached her, went through the neck-dipping performance four more times (solo neck-dipping is not recorded in The Handbook), and then swam off to near his own nest. 14 minutes after the second attempt at coition the odd female bird was sitting quietly just outside "her" area of reeds, while the male had gone off out of sight up a small stream. The male's rightful mate never evinced any sign of jealously ; for the most part she brooded quietly or dozed, once repeating the performance of picking material from the outside of the nest, putting it in the inner rim. My notes conclude : "the whole incident was very Jupiter-and-Leda-ish." I want to stress (a) that the sexes of the mated pair were noted by me as easy to distinguish, (b) that the odd bird seemed definitely to have been a female (my notes describe it as having "a small knob on a pale-yellowish bill), (c) that in both cases I note of the bird which attempted coition that "I am quite sure," and "again absolutely sure," that it was the unmated bird, and therefore almost certainly a female, and in any case not the male of the mated pair. While on the subject of swans, I may conclude with an unpublished observation of Edmund Selous, which he told me verbally. When he visited Tring Reservoirs in early March of one year shortly before World War I, he noted a pair of Black Swans repeatedly driving away a pair of the much larger and heavier Mute Swan. The keeper informed him that a little later, when the Mute Swans began nesting, the tables were turned, for they then became

5 134 BRITISH BIRDS. [VOL. XL. bellicose and easily drove away the Blacks in fact in a later year they killed one of them. This would appear to depend on the difference in season of ripening of the gonad in the southern hemisphere species, and in any case is a pretty example of the dependence of behaviour upon internal secretion. SUMMARY. (I) Notes are given on the display and reproductive behaviour of the Mute Swan, which supplement the account in The Handbook. (2) The normal aggressive male arched-wing attitude is used as a mutual ceremony by the mated pair, not only after driving off an intruder but during the presence of an intruder who cannot be of has not yet been driven away. It is interpreted as a method for mutual reinforcement of emotion and courage, as is a rather different mutual ceremony of the Whooper Swan. (3) In the prolonged pre-coitional display there are more elements (elementary actions) than the repeated dipping and quick withdrawal of the head which is the only action noted in The Handbook, These additional elements (not all seen on every occasion) are (a) dipping of the whole head and neck, accompanied by a forward tilt of the body, (b) mutual neck-erection both necks held nearly vertical, but with head somewhat arched forward, (c) occasional "habit-preening" (ritualized preening actions), (d) more frequent ritualization of the head-rubbing seen during normal preening, when the back of the head is rubbed against the flank or back, (e) "habit-surface-feeding" gobbling and dabbling with the beak, accompanying the head-dipping, (f) "habit-bottom-feeding" stretching the neck vertically downwards, accompanying the neck-dipping described in (a) above, (g) a good deal of "heavy breathing" (perhaps the "snoring" of The Handbook though this is there noted only during and after coition), but no definite note. (4) Pre-coitional display was noted between an already mated male (with sitting mate) and an unmated female. (5) In this case, two attempts at coition were made, both by the female. The Mute Swan is therefore to be added to the list of birds in which "reversed coition" may occur or at least be attempted. (6) Ritual gathering of nest-material may occur. Here the male (in all cases observed by me) picks up nest material off the surface of the water, but merely drops it again over Ms shoulder. (7) Before their gonads have fully ripened Mute Swans may submit to being chased away by Black Swans, but later in the season will expel and even kill them.