568 C. A. GIBSON-HILL : THE CAPE GANNET IBIS, 90

DISPLAY AND POSTURING I N T H E CAPE GANNET MORUS CAPENSIS.

By C. A. GIBSON-HILL, M.A. Received on 21 October 1947.

Through the courtesy of the South African Government I was able to spend a week on Bird Island, at the east end of Algoa Bay, in November 1945 (Courtenay-Latimer & Gibson-Hill 1946 : 75-86). The island has an area of about 55 acres. The breeding birds include the Cape Gannet Morus cupensis (Licht.), which \vas nesting on a series of bare, flat tables, some six acres in extent, at the south-east corner of the island. In 1945 there were about 25,000 pairs, with a maximum density equivalent to 5,500 pairs to the acre.

No detailed account has so far been published of the display and flight posture of this species, which are of interest in view of the bird’s general similarity to the Northern Gannet Morus bassunus (Linn.).

SEXUAL DISPLAY.

At the time of my visit the majority of the birds had not laid their egg, or were incubating: a small proportion of the eggs, amounting :D less than 5 per cent. of the total, had hatched. Sexual display in the colony was therefore at its height.

The gannets spent the greater part of their time, when not fishing, standing or sitting, mostly in pairs, on the nest sites. The majority normally took up a position facing the wind, unless it was very light. At intervals one would stretch its neck and point its bill straight upwards. At the same time it lowered its shoulders slightly, deflected its tail and partially spread its wings, half extending the elbow and keeping the wrist well flexed. Holding its body in this position, it usually took a few steps, uttering a low, squeaky, rattling note (PI. 7, fig. 1). If it met with any response from its mate or its neighbours it passed on to the next stage. If not, it gradually sank back to a normal stance, like a balloon deflating slowly.

One female displayed to four near-by hirds in succession, retreating each time that her advances began to be reciprocated. It was not possible to distinguish the sexes with certainty, except by their ultimate behaviour, but from the latter it would seem that the birds often displayed to members of the same sex during this period.

When displaying to each other, the birds stood as nearly face to face as their instinctive desire to look into the wind would allow. Normally they began by moving forward until their breasts touched, raising their shoulders so that their backs made an angle of 40 to 50 degrees with the horizontal, and

In the early stages pairs are not rigidly fixed.

IBIS, 1948. PI. 7.

Fig. 1.-A Cape Gannet displaying alone.

Fig. 2.-The heginning of a dual displu) : thc trio birds standing with their necks strctchcd up\vards and thrir hills tnectinE at a common point.

Fig. 3.-Dual display : the two birds crossing bills.

IBIS, 1948. PI. 8.

Fig. 4.-Dual display : the birds beginning mutual preening.

Fig. 5.-The initial move in copulation, the male (on the left) catchinK the female by the throat.

Fig. 6.-The normal posture preparatory to taking flight. (All photographs taken on Bird Island, Algoa Bay, South Africa, in November 1945.)

1948 C. A. GIBSON-HILL : THE CAPE GANNET 569

thrusting their bills upwards, as far as possible, to a common point (fig. 2). Then, swaying slightly, they crossed their bills, first to one side and then to the other, so that they moved like two knives being sharpened against each other (figs. 3 and 4). At intervals they relaxed and crossed their necks to stroke each other’s backs. Throughout the performance they uttered a low, bubbling, croaking note. Occasionally one of the pair would break away and run for a few steps with its bill in the air, whinnying with delight, and then return to its partner.

Occasionally it continued until the caressing became dominant, and the pair devoted their full attention to scratching each other’s necks and shoulders with the points of their bills. Sometimes this passed on to copulation. The male would catch the female by the neck, with his upper mandible on her throat (fig. 5), and attempt to turn her round, moving himself to meet her body, until he held her by the nape. Then he would mount her, shaking her head violently throughout the subsequent proceedings. A mated couple appeared to indulge in courtship about once in 40 minutes, but the intervals were very irregular. Often they were incited to begin by the actions of their neighbours, and frequently a single bird would display alone under this stimulus.

Mutual display seemed to be continued, with decreasing frequency and fervour, for some time after the egg had been laid. Occasionally it might even reach the stage of copulation, but an incubating female appeared to be much less inclined to tolerate intercourse. The more normal procedure at this point was for the birds to do no more than cross their necks, rather perfunctorily, rubbing against each other instead of pecking or preening.

A bird alighting near its mate usually emitted a honking, rather goose-like call of “ kaa-kaa ”, “ kaa-kaa ”. In the early stages, before the egg had been laid, it would often attempt the display normal to this period, or at least adopt the initial posture. Later it contented itself with bowing, first to one side and then to the other. If the other responded it might begin a mating display. If not, it frequently accentuated the bowing for a few movements, bringing its head well down so that the bill passed under its partly extended wings, and then stopped.

Frequently the display stopped at this stage.

POSTURE PREPARATORY TO TAKING FLIGHT.

Flight was usually associated with a special posture. The bird stretched its neck and bill straight upwards, and slightly backwards. The wings were extended almost completely, and held at an angle of about 30 degrees from the vertical (fig. 6). In this attitude it marched forwards for four or five yards, if there was sufficient space for it to do so. A bird hemmed in too closely by its neighbours merely turned round several times before leaping into the air. In either case it emitted a prolonged bubbling, snoring note for the greater part of the time.

570 C. A. GIBSON-HILL : THE CAPE GANNET IBIS, 90

A bird that had landed in the wrong place did not usually waste time on this etiquette. It was immediately pecked at by those round it, and under this abuse generally put its head down like a goose and rushed blindly for the edge of the colony, trampling on and being pecked by all the birds in its path. Once it had reached clear ground it allowed its momentum to take it into the air, without any preliminary wing-stretching.

COMPARISONS.

Apart from its much greater size, the Algoa Bay colony was in many respects similar to that of the Northern Gannet on Grassholm, off the coast of Pembrokeshire, which I visited early in July 1946. During the same summer I inspected the two colonies in the Shetlands in June, and spent five days on Ailsa Craig at the end of July. From September 1938 to December 1940 I lived on Christmas Island, in the Indian Ocean, where there were excellent opportunities for observing Sulu sula rubripes Gould and Sulu leucogaster plotus (Forst.) (Gibson-Hill 1947 : 87-165), and in 1941 twice visited the island of North Keeling, where there is a small colony of Sulu dactylatru bedouti Math.

In general I would say that the postures and form of courtship of the Cape Gannet are very similar to those of the northern bird as seen during the summer of 1946, and as it has been recorded by Lockley (in Salmon & Lockley 1933 : 142-152), Townsend (in Bent 1922 : 219) and Kirkman (in Gurney 1913 : 370-380), except that Townsend, possibly in error, refers to the tail being elevated, whereas in my experience it was deflected in both species. In most respects they were quite unlike that of the tropical boobies.

The differences in the behaviour of the two gannets would seem to be quantitative rather than qualitative. The ecstatic posture, with the bill pointing straight upwards, appears to occur more frequently in the South African bird, and bowing less often. At nest relief the incoming bird was never seen carrying a stick or piece of seaweed, but the Cape Gannet, on this island at least, does not collect material for its nest. On the other hand, birds were twice seen to pick up a piece of stick from the ground and offer it to their partners, as the booby Sulu ductylutru does, but the gift was not accepted.

A physical feature of considerable prominence during display is the thin line of hare, black skin running from the interramal area down the centre of the throat. This occurs in all three species of MOYUS, and is one of the diagnostic features used by Mathews (1921 : 76) in defining his genus Sulita. I have not had an opportunity of making a direct comparison of skins, but from drawings it would appear to be of about the same length in both M . bussunus and M . sewutor, and rather longer in M . cupensis. The last- named also carries the greatest amount of dark colouring on its tail. There

1948 C. A. GIBSON-HILL : THE CAPE GANNET 571

can be no doubt that the black line is clearly visible to the opposing bird during courtship, and it would seem reasonable to suppose that in all three it provides a visual stimulus to further excitement. The possible objection to this is that, according to Miss Courtenay-Latimer (in litt.), it is a constant feature in the Cape Gannet, with no appreciable seasonal variations.

The significance of black or off-black interramal skin among the white- plumaged members of the genus Sulu (these birds have no bare line on the throat) is less clear. In Sula sulu rubripes on Christmas Island the interramal skin is normally blue grey with a hint of mauve, but it darkens to a dull purple, almost black, at the beginning of the mating season (Gibson-Hill 1947 : 116). This would suggest a sexual significance, but the point has not been recorded in races of Sulu sulu on other islands. Murphy (1936 : 861-862) in fact suggests that as a general rule the skin is velvety black in the male, and grey or pinkish grey in the female. In Sula ubbotti it appears to remain always a pale bluish green, in contrast to the bare facial skin which is almost black (Gibson-Hill 1947: 120). In the adult Sulu ductylutru bedouti on the Cocos-Keeling Islands both facial and interramal skin appear to be black, or almost black, throughout the year. Moreau (1940: 51-53) has reason to believe that on some of the islands off the east coast of Africa this bird breeds while still in immature plumage, that is, presumably, while the interramal and facial skin are still only a dark lead grey. The colour of these areas would, in fact, seem to be variable, and of doubtful significance, among the tropical boobies.

The posture naturally adopted immediately before taking to the air appeared to be identical in the Cape Gannet with that occurring in, and already recorded for, the Northern Gannet (4. Armstrong 1942 : pl. 1). The only difference seemed to be that it was seen more frequently, and was almost the invariable procedure under normal circumstances. I never saw it taken up,by any of the three tropical boobies. If the significance of behaviour patterns be admitted in considering relationships, one would suggest that the two gannets are much closer to each other than any of the boobies are, and that from this aspect they might well be treated as races of a single species (cf. Hartert 1912-21 : 1407).

REFERENCES.

ARMSTRONG, E. A. (1942). ' Bird display.' Cambridge, England. BENT, A. C. (1912).

allies.' CouRmNAY-LATiMEH, M. and GIBSON-HILL, C. A. (1946).

the Bird Island group in Algoa Bay.' GIBSON-HILL, C. A. (1947).

Ocean).' GURNEY, J. H. (1913). ' The Gannet.' London. ~ R T E R T , E. (1912-21). ' Die Vogel der paliarktischen Fauna.' Berlin.

' I.ife histories of North American Petrels and Pelicans and their

' A preliminary note on

' Field notes on the birds of Christmas Island (Indian

U.S. Nat. Mus. Bull. 121.

Ostrich, 17 : 75-84.

Bull. Rd. Mus. 18 : 87-165.

572 PERCY R. LOWE : WHAT ARE THE CORACIIFORMES ? IBIS, 90

MATHEWS, G. M. and IREDALE, T. (1921). ‘ A manual of the birds of Australii.’ London.

MOREAU, R. E. (1940). ‘ Contributions to the ornithology of the East African islands.’

MURPHY, R. C. (1936). ‘ Oceanic birds of South America.’ SALMON, H. M. and LOCKLEY, R. M. (1933). ‘The Grassholm Gannets.’ Brit.

Ibis, (14) 4 : 48-91. New York.

Birds, 27 : 142-152.

WHAT ARE T H E CORACIIFORMES ? By PERCY R. LOWE, O.B.E., B.A., M.B. (Cantab.).

Received on 22 October 1947.

SOME PRELIMINARY REMARKS.

After careful dissection of the anatomy of the various groups or families of birds which are included under one order, the Coraciiformes, in the ‘ Sysrema Avium Ethiopicarum ’ (1924), I find on anatomical examination that there are a good many families and isolated forms therein which require reconsideration. It seems to me doubtful if some of them ought to be included in the order, and they are, in fact, quite odt of their proper place.

But the Coraciiformes have for many years been loaded with a hetero- geneous collection of forms which custom has blindly accepted. For instance, in a list prepared for the Zoological Society of London (1929), the Swifts and the Humming Birds, to say nothing of the Colies and the Hoopoes, are included in the Coraciiformes.

In ‘ Trans. 2001. Soc.’ 24 (4), 1939, I was able to show that the Swifts were nearer to the Passeres-if, in fact, they were not actually Passerine- than to any other order, and, in my opinion, have no right to be placed anywhere near the Coraciiformes or, at any rate, anywhere near non- passeriform birds.

Again, in his ‘ Systema Avium Ethiopicarum ’ Mr. Sclater makes the Hoopoes a family of the order Coraciiformes, which seems quite unjustifiable, and the same remark applies to the Colies. Moreover, it seems impossible to believe that the Swiftsor the Humming Birds, or the Colies or the Hoopoes, can be related in any way with the Rollers. This may be seen by studying their anatomy or their habits, and seems to furnish some excuse for an attempt at a reconsideration of the problem.

THE CHARACTERS OF THE CORACIIFORMES.

Linnaeus, in the 10th edition of his ‘ Systema Naturae ’, cited Co~ucius garrulus as the type of that order. Thus, in order to gain a knowledge of what the Coraciiformes are, we must make ourselves familiar with the characters of that genus as typified by that species. They are as follows :-