NOTES ON THE BEHAVIOUR OF GREAT BLACK-BACKED GULLS By F. K .
COBB
Dßring the winter Great Black-backs are present in some numbers on the Orwell estuary and, in smaller numbers at Abberton Reservoir near Colchester. These two areas represent two very different types of h a b i t a t : the former with its rieh food supply on the exposed m u d and along the tide-line, the latter with no exposed mud, no tide-line and in the main enclosed by concrete walls. T h e Great Black-back is a notorious predator, but on this coast its attacks are usually directed at weak, sickly, or wounded birds— presumably it would in most cases be a waste of effort on the gull's part to attack a healthy bird which could escape by taking evasive action. This certainly seems to be the case on the Orwell, but at Abberton attacks on apparently healthy birds appear to be f r e q u e n t ; although it must be admitted that many more observations are desirable to confirm this. In the autumn Coots begin to arrive in both areas, flocks build up, and are present all through the winter ; at Abberton these flocks are frequently attacked by Great Black-backs, while on the Orwell they usually seem to be unmolested by the gulls. T h e defensive behaviour of the Coot flocks, which may run into several hundred, or even thousand birds, is to crowd closely together on the water when threatened or attacked. T h e y then present a formidable, closely packed, mass to an aerial aggressor, who usually does not press the attack home : a single Coot is a very pugnacious bird and in a closely packed phalanx seems to be more than even a Great Black-back dare attack. Frequently a Coot flock may be seen feeding at Abberton, its members spread loosely a b o u t ; a Great Black-back arrives overhead and the flock begins to contract inwards towards the centre, looking, at a distance, rather like a large dark flower closing its petals. If the threat does not appear to be urgent the contraction is slow, but if the gull is dangerously close and low over the water the movement is a fast one, with the outlying Coot half Aying, half scuttering on the water and sending up a shower of spray. Often a Great Black-back overhead is ignored by the flock, although there seems to be no visible difference in its behaviour, at least nothing that the human eye can discern, but the Coots appear to be fully aware whether or not there is danger. W h e n the Coots have packed closely together the gull flies round them for a time, then usually gives it u p as a bad job and leaves ; presumably its object is to catch an unwary Coot before
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it can get into the safety of the packed flock, and some indication of how this works out may be shown by the following incident. Abberton, Feb. 6th, 1955. A flock of between one and two hundred Coots were feeding at the top end of one of the concrete arms of the reservoir when an adult Great Black-back flew in. The gull came in very fast and low, apparently catching the Coots, at least to some extent, by surprise, for as they began to scutter along the surface the gull dropped amongst them on the fringe of the movement. The action was too fast to follow in detail, but the gull evidently failed to grab a Coot, for it rose from the water and again hurtled into the retreating fringe of the flock. This time it succeeded in separating one bird, and, as the remainder retreated towards the concrete edge, the gull was seen to be sitting on the water close to the solitary Coot and cutting it off from the safety of the pack. The Coot adopted the aggressive posture,headlowered and wings half raised like an angry swan, and the birds remained facing each other, about two feet apart, for perhaps two minutes. Suddenly the Coot charged at the gull, and the latter appeared to side-step the charge and yet managed to retain its position between single bird and pack, but not quite as squarely as before, and so they remained for about another two minutes. The Coot then made a second charge, and now in so determined a manner that the gull was forced to rise from the water to escape the impact, and in so doing lost its advantageous position. Coot and gull had now changed position, and the former, still posturing, began to edge away towards safety, but dare not turn its back to its foe, so making ground only very slowly and with the gull keeping almost within striking ränge. Suddenly the Coot got into the air and made a dash for the pack, now some thirty yards away and out of the water on the concrete sides, but the gull was quickly up after it, caught it, and the two birds plunged to the water together. But somehow the Coot had managed to escape injury in the aerial clash and, moreover, had retained its position on the " safe " side ; still more important, it had reduced the distance to safety by about half. Within seconds the Coot made its final dash through the air and, with the gull close on its tail, plunged into safety ; the gull swung away as soon as it reached the pack of Coots, but settled on the water a short distance away, where it sat eyeing the Coots and presenting a picture of patient malevolence. While this incident was only that of an unsuccessful attack itwas obvious on more than one occasion how near to disaster was the Coot: the slightest weakness, or inattention, which might easily have occurred in a sick bird, would have meant death. The courage and pugnacity of the solitary Coot also seems very worthy of comment: it is very difficult to think of any other species of approximately that size, less than half that of its massive and
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rapacious antagonist, which would have had the courage, whether or not of desperation, to charge head-on on two separate occasions. A further contrast between reservoir and estuary Great Blackbacks seems to be in their relation to the various species of duck. It was a fellow bird-watcher, Mr. R. V. A. Marshall, who first brought this home to me while we were watching at Abberton. His remark " Here comes a Great Black-back, it will put the duck up ", Struck me as curious, but sure enough the duck did get up, and not only on that occasion. Mr. Marshall informs me that such behaviour is almost commonplace at Abberton, but that the various species of duck react in different ways : Teal, Pochard, Tufted Duck and Goldeneye are very frightened of the Great Black-backs, Wigeon less so, Mallard do not bother much, Sheldduck are unaffected. I have not seen this sort of behaviour on the Orwell, where Great Black-backs and various species of duck are numerous, and where the duck generally seem quite unconcerned at the presence of the large gulls. However, the Orwell is not a Teal river, Pochard and Tufted Duck are few in number, and it would be the smaller species which would suffer most from predatory Great Black-backs. Goldeneye appear to be unmolested, and Wigeon, the most numerous duck in winter on the Orwell, are also unaffected, but I have a note of a party of Red-breasted Mergansers crowding together and pointing their bills upwards, obviously alarmed at the presence of a Great Black-back overhead. The Abberton pattern of behaviour is more commonly seen in Suffolk on the meres, but here it is the passage overhead of a Marsh Harrier which almost invariably puts up the duck : Great Blackbacks are regulär visitors, Teal very numerous, but it is the harrier, not the gull, which causes the alarm. As previously stated it seems desirable to obtain more evidence before concluding that Great Black-backs show a greater degree of predatory behaviour on reservoir than estuary, but perhaps enough has been said to suggest the probability. The reason for this, it is suggested, may be the lack of a rieh tide-line at the reservoir. One thing that the reservoir and estuary Great Black-backs certainly have in common is their propensity for robbing other species of their food, and this applies in particular to Cormorants. The Cormorant usually surfaces with its catch, and the fish or eel has to be turned about in the bill until it is in the correct position for swallowing. Should the prey be large this action may take quite an appreciable time, long enough at any rate for a Great Black-back to see what is going on. The gull will usually settle on the water beside the Cormorant, or, if the Cormorant has brought its fish to land, will sidle up beside it. There is a quick snatch by the gull, and the Cormorant is left making a futile display of anger.
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I have never seen a robbed Cormorant make any attempt to retaliate on the gull, and it would, of course, be very unwise for it to do so, but I have a note of a rather curious display of " sympathy " by other Cormorants in the neighbourhood. River Orwell, March 6th, 1955. Several Cormorants were scattered about the reach and one surfaced with quite a large eel. Immediately a Great Black-back dropped on to it from above and it looked for a moment as if the gull would drown the Cormorant, instead of which a tug of war developed, with the eel as a rope. The gull won the contest and commenced to fly away with the eel, when, to my surprise, the Cormorant began to chase the gull, pattering along the surface with wildly flapping wings and stiffly outstretched neck. As usual the gull had difficulty in holding on to the eel and another five Great Black-backs joined in the fray, the eel changing bills several times, both in the air and on the water. Then, and more surprising still, four of the Cormorants which had been fishing singly in the vicinity joined in the scrum. T h e Cormorants, now five in number, appeared to behave with a group animosity towards the quarrelling gulls, but made no effort to press their attacks home, looking, in fact, rather ludicrous as they made clumsy dashes along the surface of the water with half-opened wings, stiffly outstretched necks, and open bills, presumably hissing with anger, although the distance was too great for the hisses to be audible. Finally one of the gulls flew away with the eel, whether the original robber or not, it was impossible to say ; the Cormorants ceased making their futile dashes, dispersed to fish, and the river was peaceful again. It would be interesting to know what brought the other Cormorants into the action: sympathy for the robbed member of their species seems most improbable : perhaps it was the combined effect of the excitement of the scene, the sight of a large eel being bandied about, and their understandable antipathy towards a species which frequently robs them of their food. Scientific names of birds mentioned : Cormorant Phalacrocorax carbo L. Mallard Anas platyrhyncos L. Teal Anas crecca L. Wigeon Anas penelope L. Tufted Duck Aythya fuligula L. Pochard ' Aythya ferina L. Goldeneye Bucephala clangula L. Marsh-Harrier Circus aeruginosus L. Coot Fulicia atra L. Great Black-backed Gull Larus marinus L.