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