BRITISH BIRDS NUMBER II, VOL. XLVI, NOVEMBER, 1953.

AN OBSERVATIONAL STUDY OF THE GULLS OF .

BY

JOHN' H. CROOK.

INTRODUCTION. THE work described in this short paper was carried out in the winters of 1950/51 and 1951/52 mostly in the Southampton area. I am most grateful to my friends from the Orni­ thologists' Club, Gordon Wooldridge, Alan Moody, Barry Goater, and Keith Edwards, all of whom gave invaluable assist­ ance; and I am also indebted to my correspondents, A. J. Bull, Dr. B. Campbell, K. G. Spencer, Miss I. Werth and R. Whitlock. Dr. David Lack has been most kind in giving advice during the preparation of this account. Studies of bird populations in winter often reveal a system of flocking-, feeding and roosting that seems best suited to the needs of the species. In Southampton Water the winter population of gulls (Lartts spp.) is organised into such a system. The birds roost at defined localities, spreading- out along well canalised routes during the day, feeding, and returning in large flocks in the evening. I propose to call this a "dispersal system." The population of the roost may be divided into two, those birds that fly inland every day and those that remain in the estuary. The following species roost in Southampton Water, Black-headed Gull [LOTUS ridibundus), Common Gull (L. canus), Herring Gull (1.. argentatus) and Great Black-backed Gull [L. marinns). In the morning the birds start moving away from the roosts as soon as there is an appreciable lightening of the sky. Counts on evening flights and qualitative observations show that although all species will fly inland the Great Black-backed Gull does so the least. Common and Black-headed Gulls are the most numerous inland and also occur in large numbers in the estuary. The Herring Gull is mainly "estuarine." The proportions of the species are noticeably different at the mouth of Southampton Water where there are usually fewer Black-headed Gulls and rela­ tively many more Herring and Great Black-backed Gulls. In winter migrants increase the number of birds in the valley sub­ stantially and this influences the proportions of the species pre­ sent. Fig. 1 shows the position of the roosts and the routes in the evening. 386 BRITISH BIRDS. [VOL. XLVI.

TOTTON COBDEN CAUSEWAY BRIDGE

SOUTHAMPTON

Fig. I. FLIGHT LINES AND ROOSTS IN SOUTHAMPTON WATER DIBDEN (diagramatic)

HYTHE

HAMBLE RIVER

FAWLEY

ASHLET

CALSHOT

STANSWOOD BAY

THE SOLENT

The activity of the "inland" birds differs very much from those which remain in the estuary and I have been able to study only the latter in detail. The feeding behaviour of the Black-headed VOL. XLVI.J OBSERVATIONAL STUDY OF GULLS. 387

Gull has been observed extensively throughout the two winters and most observations were made in the Itchen estuary between Cobden Bridge and the Floating Bridge. Others were made from the Hythe Ferry, Hythe Pier, in Dibden Bay and at Lymington.

DIURNAL FEEDING RHYTHMS IN "ESTUARINE" BLACK-HEADED GULLS. The Black-headed Gull exhibits a wide range of feeding activities in this area and adapts its behaviour to suit local con­ ditions, exploiting the resources of several very different types of feeding habitat. These may be listed as follows:

(i) Littoral. (a) A zone extending shorewards several yards from the reced­ ing tideline on an ebb tide. The prey consists of the sur­ face mud fauna which is left exposed to predation in a narrow zone above the tide line. This zone moves slowly down the shore with the tide. Most of the shore is thus exploited by gulls during an ebb tide. (b) A zone below the tide-line where the water is approxi­ mately 2-4 inches deep. The existence of this zone depends upon the slope of the shore. In certain localities a wide expanse of flat mud may be covered by water to this depth for a short period during the ebb and rising tides. (c) Certain favourable positions of calm water about 2 feet deep overlying mud. (d) The shore above the tide-line zones. Food here is usually waste or carrion left behind by the tide. The activities related to the first three habitats do not occur over ground which is unsuitable. Gravel is not exploited nor is gravelly mud. Apparently the requisite food can only be found in the very soft mud. The food taken from each habitat will belong to the same mud fauna, but the proportion of species is probably different depending on the state of the tide and the consequent feeding-activity.

(2) Open water. (a) Open water with a calm surface. The food is restricted to a definite area and is usually evenly dispersed and of repetitive occurrence at the surface, e.g. shoals of small fish or shrimps. The food is living and part of the nektonic fauna. (b) Floating waste from ships, sewage outfalls, floating carrion, food from philanthropists on pleasure steamers and other miscellaneous sources. The food here is strictly localized. 388 BRITISH BIRDS. [VOL. XLVI.

(c) As above, but also including waste, etc., stirred up from the bottom by the wash of ships. (3) High concentrations of the prawn Palaemonetes varians in salt pans on reclaimed land at Dibden.

(4) Non-estuarine. (a) Agricultural land—especially behind the plough. (b) Flooded land. (c) Flying insects—e.g. ants. (d) Waste—rubbish dumps—sewage farms, etc. In other estuaries there are probably further feeding-habitats corresponding with local topography. The above classification is based upon the differing modes of exploitation. The feeding-activities related to these habitats are as follows:

(1) Tide-line feeding. [Littoral (a).] The birds follow the receding tide, maintaining a minimum individual distance of about a wing span. Infringement of indivi­ dual distance results in loud calling and threat posturing. Fights are rare. The food is evenly dispersed and so are the birds. Puddl­ ing with the feet may be used to disturb the prey from the mud.

(2) Scavenging. [Littoral (d). Open water (b). 4 (d).] The localised nature of the food seems responsible for the "all in" struggle that occurs in such places. The birds engage in a "free for all" in which the most active bird obtains the most food. There is no individual distance.

(3) Picking about. The birds walk about on the shore picking up food-particles occasionally. The birds appear to be in no need of food and the activity often occurs after intensive feeding such as "tide-line feeding." It may occur on the fringe of high water roosts on mud.

(4) Idling. . Similar to "picking about" in that it occurs often after inten­ sive feeding or on dispersal from an exhausted supply. It appears to be exploratory. If an "idling" bird discovers a localised source of food its descent is at once observed by others and aggregation at the site follows almost immediately.

(5) Paddle feeding. [Littoral (b).] The birds are evenly dispersed swimming actively with the head and neck inclined in a "searching attitude." The prey is obtained by a quick dipping of the bill under the water. The bird may VOL. XLVI.] OBSERVATIONAL STUDY OF GULLS. 389 paddle its feet rapidly in one spot so disturbing the mud and the organisms within it. This activity can occur on both a rising and a falling tide. Individual distance is maintained as in "tide-line feeding."

(6) Individual plunge. [Littoral (c).] The birds swim slowly around, usually in a small area and adopt the "searching attitude." The prey is obtained by a jump into the air, one or two balancing wing-beats and a plunge after the food.

(7) Plunge-flight feeding [Open water (a).] A compact flock of fifty birds or so hovers over a small area of water. As soon as the prey is located the nearest bird dives down and takes it. Only occasionally do two birds go after the same food particle. Even distribution over the feeding-area is main­ tained by an individual distance of several feet, or much more, depending on the concentration and frequency of the food. N.B. "Scavenging" over open water differs from this in that the food is localized and a "free for all" is the result.

(8) Liner following. [Open water (c).] The gulls (also many or even mostly Herring Gulls) will follow the liners in and out of Southampton Water. The number of birds involved seems to depend upon the strength of the up- currents created by the ship and also upon the direction of the wind. The actual taking of food is by the "scavenging" method.

(9) Scoop feeding. This activity enables the Black-headed Gulls to feed upon the dense shoals of the prawn Palaemonetes varians which occur in the 2-4 inch deep salt pans of Dibden Bay. The birds run or swim rapidly through the water with the beak and throat and much of the head submerged. The concentration of the prawns in these pools is very great indeed and when the birds feed they maintain a minimal individual distance of an inch or two and feed in a closed pack. There is no squabbling, but there is a continuous excited chatter from the flock. The birds run in nearly parallel lines and rarely obstruct one another while they feed. (See Barnes, 1949.) Individual distance during feeding is determined apparently by the concentration of food, the density of feeding birds on the ground and the intensity of the feeding activity. There is an interaction between these factors, but clearly much more observa­ tional study is required to elucidate the detailed behaviour of feeding flocks. 390 BRITISH BIRDS. [VOL. XLVI.

Where the shore slopes too steeply into the water for "paddle feeding'" to occur only "tide-line feeding" has been observed. The same is true apparently where the water is too disturbed for "paddle feeding" to be effective.

Correlation with tidal and diurnal periodicity. Most of the above activities are related to certain periods of the tide and a cycle of activity can thus be constructed. At the high tide period most birds collect into high water roosts*. These occur in Dibden Bay and on log piles in the Rivers Itchen and Test—also on sheds and buildings in the dock area. There is thus very little activity at high water—perhaps some "idling" birds and' a number of "scavenging" parties. After the low tide most birds seem replete as a result of "tide- line feeding" in which the great majority of Black-headed Gulls take part; they accumulate at the Dibden Bay roost soon after the start of flood tide. Other birds go "idling" or join "scaveng­ ing" or "plunge-flight feeding" parties. The time of arrival at the high tide roost may in fact depend upon how long it takes to satisfy hunger. The cycle showing majority activities may be represented as follows: HIGH TIDE. (i) Majority in high water roost, (ii) Some " scavenging," " plunge-flight feeding." (iii) " Liner following."

RISING TIDE. FALLING TIDE. (i) " Paddle feeding." (i) " Tide-line feeding." (ii) " Individual plunge." (ii) " Paddle feeding " in suitable (iii) " Scavenging," " plunge-flight areas. ^ feeding " or " idling." (iv) Build up of high water roost.

Low TIDE. (i) " Picking about." - (ii) " Idling." (iii) Some " scavenging." The daily activity -of the Black-headed Gulls may be deter­ mined by the superimposition of the day/night cycle upon the tide cycle. The state of the tide at dawn will to a great extent determine the first activity of the birds dispersing from the roost and from this the diurnal sequence will follow. * The use of this word here and in other places in this paper is perhaps rather confusing as it refers not to a roost in the accepted sense but to a resting-place occupied during the day. While we have felt disinclined to alter the word (because it is the author's choice and it is impossible to consult him on the subject due to his absence abroad) we feel that this must be made quite clear. —EDS. VOL. XLVI] OBSERVATIONAL STUDY OF GULLS. 391

It is suggested very tentatively that the number of birds flying inland every morning may be related to the availability of food in the estuary at dawn. There would be relatively little food available on a high dawn tide and much more on a falling tide. On high dawn tides it is possible that a greater proportion of the birds will move up river from the estuary.

ROOSTING LOCALITIES AND BEHAVIOUR. There is some evidence that each "dispersal system" is con­ fined to a definite area, often coincident with a river valley, but there is no information as to how much interchange occurs be­ tween the population of one area and another. Observations in Southampton Water indicate some degree of regional differentia­ tion between the populations along different sections of the coast and this appears to be related to the dispersal areas of the roosts. The "dispersal system" may be considered to be a definite unit of population, but fluctuating both in distribution and numbers. Locally there are roosts with dependent systems in Southamp­ ton Water, , the and the Needles Cliffs. It seems very probable that such systems occur at regularly spaced intervals around the coasts. The distance between each roost is probably determined by the area coverable in a day's feeding dispersal and by the topography of the country. The systems occur usually in river valleys and the major roosts are situated on the coast or in an estuary. Inland roosts also occur, but are apparently less constant than those near the coast. In the evening the gull population of Southampton Water is concentrated into three roosting places, at Dibden Bay, Ashlet Creek and Calshot Creek. The "up-river" birds are distributed between the roosts in the following manner: Dibden Bay receives gulls from , River Itchen, and "estuarine" gulls from upper Southampton Water. Ashlet Creek receives gulls from River Itchen, River Hamble, and "estuarine" gulls from mid Southampton Water. Calshot Creek receives gulls from River Hamble, and "estuarine" gulls from Meon-Lee Shore, The Solent, Stanswood Bay. In addition, on two evenings, large numbers of gulls have been found apparently roosting on the water off Fawley and near Cad- land Creek. Scattered individuals or small groups may often be found around the dock area at night. No inland roosts have been discovered in either the valley of the Itchen or the Test and it seems likely that the whole population descends into Southamp­ ton Water. 392 BRITISH BIRDS. [VOL. XLVI.

The "dispersal system" breaks up in April when the roost in Dibden Bay becomes very small and finally deserted. The site is in use again by the end of July and the numbers increase throughout the autumn. Bull informs me that a few Black-headed Gulls continue to fly inland from Poole Harbour during the summer months. These are possibly non-breeding birds. At the roosting sites the birds alight on mud or water depend­ ing on the state of the tide and usually preen or wash before resting. Most observations on their behaviour have been made from Dibden Bay where a consistent pattern of events can be observed during the winter. The arriving birds are at first widely dispersed around the shores of the bay and on the water, but they are nearly always more densely packed over the definitive roosting site. As the light fails the most outlying birds begin to fly over the heads of the others in small groups and to concentrate nearer the centre of the roost. This continues, reducing the area covered by the birds very considerably, until the failing light makes further observa­ tion impossible. I have called this gradual concentration "shuffle flighting" which is descriptive of the appearance of the move­ ment. Water-movements influence the distribution of the birds to some extent, but the creek is essentially a backwater and drift­ ing is not extensive. At low water the birds sit or stand on the muddy shingle. There is some division amongst the birds since the Great Black-backed Gulls occupy a long, raised ridge of shingle while the mass of Black-headed and Common Gulls stand on the flats around them. Great Black-backed Gulls have been observed chasing the other gulls piratically and their presence in the roost is not conducive to social stability. Herring Gulls may stand with both groups of birds. At high tide the species are mixed together more evenly as a result of water movements. The roost receives both "up-river" and "estuarine" birds, but the two groups tend to remain separate until late in the afternoon. This is due, very largely to the activities adopted by the "estuarine" birds. If the tide is falling in the late afternoon the latter feed along the tide line. The "up-river" birds meanwhile are streaming down river and alighting at the roost. Very little attraction has been observed between the two groups and it is not until much later that the "estuarine" birds join the others. Observations indicate that feeding stops with nightfall, but this is not certain. If the afternoon tide is high the majority of the "estuarine" birds are gathered into high water roosts. The largest is situated on some reclaimed land a little upstream from the night roost in Dibden Bay. The birds collect here in one or more groups of VOL. XLVI.] OBSERVATIONAL STUDY OF GULLS. 393 up to a thousand individuals. The "up-river" birds on their way downstream pass very close to this site, and very often portions of the flocks have been observed to fly down and join the birds on the ground. The majority of the birds however continue down­ stream and settle at the night roost. With weakening light the high water roosts become increasingly restless and in small or large groups all the gulls eventually fly down and join the main body. In spring and early autumn when the number of birds is low the behaviour at the roost becomes variable and the site of the roost less certain. During a falling tide in April the small number of "up-river" birds at the roost were observed to move away in groups until all of them had joined the numerically superior mass of "estuarine" birds feeding along the tide line. They were not observed to feed with them however, and the roost site that even­ ing was probably well out in the bay near the low tide line. One August evening during high tide the "up-river" birds fly­ ing downstream alighted at the high water roosting sites and only a small number of birds flew on to join the minority in the night roost. Eventually all the birds in the latter flew upstream and settled into the high water roost which thus became the func­ tional night roost. Spring and early autumn are the times of break-up and recon- stitution of the "dispersal system" and the site of the roost is least certain at these times. The nightly use of this roosting site seems to depend on the size of flocks using" it, the intensity of flock integration and the amount of attraction exerted on the birds by other flocks in the neighbourhood.

FLIGHTING AND DISPERSAL. The behaviour of the flocks coming down the Rivers Itchen and Test was watched from Cobden Bridge, Totton Causeway and from various positions in Southampton. The birds travel in com­ pact flocks and early in the year it is possible to see that these are usually monospecific. Black-headed Gulls and Common Gulls flock together frequently, but Herring and Great Black-backed Gulls are rarely associated with another species. In the middle of winter, however, the number of birds passing is so great that the sky seems full of a continuous stream. Great care is needed in identifying flocks in poor light and reliance is often placed on flight behaviour, which is usually distinctive. Wind and weather influence the flighting to a considerable degree, in heavy rain or squally weather the birds fly low over the houses in Southampton, on calm evenings they fly steadily at much greater heights and usually in V-formations (see also Bryanston School N.H.S. Report, 1950). Flight integration within the flocks seems to be at a high level and in direct contrast with 394 BRITISH BIRDS. [VOL. XLVI. the straggly hosts of Rooks (Corvus frugilegus) which pass up• stream at the same time. In a strong wind the V-formations are no longer maintained and the birds make great use of the wind for gaining height and gliding. When the wind is directly opposed to the line of flight the gulls fly steadily into it making use of up currents over houses and ships to gain height. They then enter a glide, in which they slowly lose height before returning to steady' flight again.

FIG. 2. INFLUENCE OF WIND ON FLIGHTING

A) - A2 Flying into wind gaining height on up currents, etc.

B, - B2 Long glide with loss of height. Direction of wind

Direction of wind

Spiralling into a hind wind with gain of height (see text) VOL. XLVI] OBSERVATIONAL STUDY OF GULLS. 395

When the wind is at an angle to the line of flight the sequence is as follows : (i) Long glide at an angle inclined to the direction of the wind. (2) Turn into the wind. (3) Rising, with a few wing beats, into the wind and gaining height. (4) Turn out of the wind and enter long glide. The resultant is a straight line to the roost or down the river valley. The birds steer in this way and maintain a definite line of advance. The greater the angle between the glide line and the direction of the wind the more the bird has to turn in order to gain height. When the wind is blowing from behind the bird has to turn through an angle of approximately 180 degrees in order to face the wind. It may then continue, completing the circle or entering a spiral before returning to the long glide again. I have often watched a flock enter a spiral and become trapped in it, apparently as the result of flock integration. The flock then drifts with the wind rising higher all the time. Eventually the birds glide out of the spiral and return to the original line of flight. These flight patterns are illustrated in Fig 2. The flocks inland usually fly in well canalised routes along the river valleys. This is not always so however, for Whitlock (per­ sonal communication) informs me that near Salisbury, evening and morning flights cut across hills regardless of contour.

Inland feeding. Whitlock remarks that these "inland" gulls feed wherever there is food to be had, newly ploughed fields, pig pens and so on. Bull (personal communication) writes from Bryanston: "There is un­ doubtedly a tendency for the Black-headed Gull to follow the plough or to feed on ploughed land where the soil is being dis­ turbed, but the Common Gull on the whole prefers grassland . . . in times of flood the numbers of birds feeding on waterlogged land in the valleys is much increased." Campbell (personal communi­ cation) records particularly the importance of flooded land near Oxford where Black-headed and Common Gulls feed almost en­ tirely upon limp or lifeless lumbricids taken from the water or around the edges of the flood pools. The Southampton Sewage Farm and Council Tip are major scavenging grounds and the use of such resorts seems usual over most of the country. Spencer writing from Lancashire informs me that while Black-headed Gulls spend much time on ploughed land the "larger gulls" are the more frequent at offal tips near the towns. These observations reflect Sparck's findings (1950), based on stomach analyses, that Common and Black-headed Gulls are 396 BRITISH BIRDS. [VOL. XLVI. mainly insectivorous, while the main constituent in the diet of the Herring Gull is offal. In the upper estuary of Southampton Water a great many Black-headed and Common Gulls feed on the mudflats. The diet of these birds must differ considerably from those which feed inland. It is not known how much interchange occurs from day to day between the "inland" and "estuarine" feeding flocks. Bull (personal communication) has observed in Dorset that in times of frost the movement inland is reduced to a trickle. This was noted on one extremely snowy day at Southampton. The "dispersal system" on the Itchen and the Test has no known inland roosting sites such as Spencer (personal communi­ cation) has observed on the reservoirs of the River Ribble valley. Inland roosts appear to be secondary roosting sites and liable to be abandoned in severe weather. The "estuarine" roosts are the primary and most constant roosting sites. The whole "dispersal system" appears to be based upon these in nearly every case.

Behaviour in the upper estuary. Usually the greatest numbers of Black-headed Gulls and Com­ mon Gulls pass Totton Causeway half an hour or so before sun­ set, but the evening flight begins a little earlier on dull days. Usually the main peak of Herring Gulls is later, the greatest numbers passing immediately after sunset on a clear day. Werth (personal communication) also remarks that the arrival times of birds at Eccup Reservoir depend largely upon the light intensity —earlier in the poorer light. When the flocks arrive over the estuary there is a rapid change in their behaviour. This occurs where the Rivers Test and Itchen broaden quite suddenly into navigable waters at the Totton Causeway and Cobden Bridge respectively. The flight behaviour alters as the birds approach, the steady wing beats (in calm weather—see above) giving place to a long glide down on to the water. The method of descent is of necessity more precipitous in a high wind and is characteristic for each species. The flocks thus aggregate in the upper estuary of both rivers. There may be one large aggregation or a number of smaller groups. The presence of these on the water seems to act as an attraction to later flocks, most of which descend to join them. Later in the evening when the number of birds has been greatly reduced there is an increas­ ing tendency for the flighting birds to continue downsteam. These may even act as an attraction to the sitting birds. Late in the evening there are usually no birds below the bridges and new arrivals fly straight on down the estuary. The birds alight on water or mud depending on the state of the tide and washing or preening usually follows flighting. Below Cobden Bridge there are usually large numbers of gulls that have re- VOL. XLVI] OBSERVATIONAL STUDY OF GULLS. 397 mained in the estuary all day. It is often possible to distinguish the "inland" birds from the "estuarine" birds since the former tend to remain in separate groups, either washing or preening while the latter are usually "idling" or engaged in one of the several feeding activities. Later as the "estuarine" birds move towards the roost this distinction becomes less apparent. With decreasing light the aggregations break up progressively. At first small parties detach themselves from the main group and fly downstream. The passing of a train, plane or small boat may scare a large number of birds and after wheeling around many of these fly down stream also. As the light weakens the excitability of the group increases and sudden spontaneous departures will occur, without extrinsic causes being apparent. In addition there is usually a continuous "dribble" of individuals and small groups from the edges of the main aggregations. Local excitation whether intrinsic or extrinsic, flock integration and an increasing excit­ ability probably related to the failing light seem to be the main factors determining the size of departing flocks. At the head of the estuary there is thus a reshuffling of birds into different flocks from those that fed on feeding sites inland. Flock continuity from day to day would appear very unlikely.

SUMMARY. An outline study has been made of the "dispersal system" of the gulls in Southampton Water. "Estuarine" Black-headed Gulls have nine different feeding activities which are related to the types of feeding habitat available in this estuary. A cycle of activity may be constructed by correlating feeding behaviour with tidal and diurnal periodicity. The population of the Itchen, Test and Hamble valleys roosts with the "estuarine" birds at three main sites in Southampton Water. There is a dispersal every, morning and an evening con­ centration. The roosts are in constant use throughout the winter, but in spring and autumn when the numbers are low the activi­ ties are less constant and the roost sites variable. The flocks flying to or from the roosts are shown to steer in high winds in such a way as to maintain a straight line advance. Deflection from the usual routes is unusual at Southampton. The behaviour of the flighting birds changes as soon as the estuary is reached. Flock continuity from day to day is considered unlikely. The "system" is broken up in the summer for the breeding season.

REFERENCES. BARNES, J. A. G. (1949). "Feeding behaviour of Black-headed Gulls," Brit. Birds, xlii: 334. SPAKCK, R. (1951). "The food of north European gulls." Proc. X. Int. Orn. Congr. Uppsala, 1950: 588-591.