The increase and feeding habits of herring and lesser black-backed gulls at Orfordness, Suffolk

THE INCREASE AND FEEDING HABITS OF HERRING AND LESSER BLACK-BACKED GULLS AT ORFORDNESS, SUFFOLK G . J. THOMAS, J. PARTRIDGE, R . S. WOLSTENHOLME, P. RICHARDS, M . J. EVERETT a n d C . J. CADBURY

Herring Gulls Larus argentatus and Lesser Black-backed Gulls Larus fuscus have been increasing substantially in the twentieth Century to the extent that their numbers have had to be controlled in some countries, for instance on nature reserves and near airfields (Spaans, 1971; Thomas, 1972; Chabrzyk & Coulson, 1976). Studies in the NE USA indicate that more recently the Herring Gull growth rate has decreased and the population is stabilizing itself (Drury & Kadlec, 1974).

Both species often breed together in coastal areas, Lesser Black-backed Gulls preferring the flatter areas and Herring Gulls using rocky cliffs where available. In Suffolk the rnixed Herring/Lesser Black-backed Gull colony nests on the sparsely vegetated shingle at Orfordness. Nearest coastal colonies in the north are in Norfolk for the Herring Gull and Northumberland for the Lesser Black-backed Gull. To the south colonies of both species occur on the Kent coast.

The Orfordness colony is of particular interest because of its close proximity to the RSPB reserve of Havergate Island, where in recent years between 48 and 118 pairs of Avocets Recurvirostra avosetta have bred (Cadbury & Olney, 1978), in addition to other wader and tern species.

There is also a colony of Little Terns Sterna albifrons on Orfordness beach.

There were observations of predation by large gulls on Avocet chicks and other wader species so it was decided in 1973 to monitor the gull colony and investigate their foods. Systematic observations were made on Havergate as to the incidence of predation by large gulls and the species taken. The extent of any effect that the Herring/Lesser Black-backed Gull colony might have on the bird species nesting on Havergate could then be assessed.

Two Visits per annum were made to the colony in the years 1974-77, and 1979, one in June and the second in July, while in 1973, 1980 and 1981, only Single Visits were made. On the June Visits the colony was counted from an Observation tower (Fig. 1) using binoculars. It was possible to subdivide the colony into sections due to ground features and head counts of adult Herring and Lesser Black-backed Gulls were done for each. Each observer (usually 2 to 4) counted and an average value was taken. Separation of the two species was done visually as it was easy to distinguish the darker-backed Lesser Black-backed Gulls. The observers then systemtically covered two of the sections and recorded the clutch/brood size of each nest. From the total number of nests in each area, a value for nests present/birds counted was worked out. It was then possible to estimate the total nesting population of both species of gulls for the area.

To assess the food of the gulls, three methods were used on each visit, an analysis of food remains at the nest, pellets and the crop contents of any

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Orford Observation Tower

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Little Terns 1981 Little Terns up to 1975 Little T e r n s 1 9 7 6 - 8 0

Fig. 1 Location of Mixed Herring and Lesser Black-backed Colony at Orfordness.

young that regurgitated food. Except tor some pellets, most of the analysis and recording was done in thefield.Unfortunately it was not possible to distinguish between the foods of each species on the shortfieldtrips exce for one occasion in July 1977. Due to lack of time, no work was done o following theflightlines of gulls, either inland or out at sea.

Colony size The numbers of Herring and Lesser Black-backed Gulls breeding at Orford ness since 1963 are shown in Fig. 2. There was no information available 1964-67,1970-72 and 1978. Herring Gulls apparently colonised Orfordness about 1963 and Lesser Black-backed Gulls some time between 1963 and 1968. The Herring Gull population showed the greatest increase between 1969 and 1973 which must have been the result of a massive immigration Trans. Suffolk Nat. Soc. 18 part 4.

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breeding birds. Between 1973 and 1980 the population remained under 1400 pairs, with some minor fluctuations, but increased to about 2000 pairs in 1981. The Lesser Black-backed Gull population showed two big increases, in 1974 and 1976 when the numbers were approximately double those of the previous year. The mixed colony has increased from 140 pairs in 1968 to 3526 pairs by 1981. The areas colonised between 1973 and 1981 are shown in Fig. 1.

Foods There was little seasonal difference in the prey remains recorded, apart from an increase in the proportion of birds taken in July. The results in Fig. 3 are shown with both months combined.

Nest remains Fish remains were the most abundant items recorded, whilst the bones of young rabbit Oryctolagus cuniculus and rubbish (mainly chicken bones) also occurred at high frequencies. Marine invertebrates, which consisted largely of crab and marine molluscs were found in lesser amounts. Bird remains were few and included young Larus sp, young Lapwing Vanellus vanellus, young Shelduck Tadorna tadorna, Thrush Turdus sp, Redwing Turdus iliacus, finches, other passerines and an unidentified wader.

These results can only be regarded as providing an approximation of the foods taken. There is a definite bias towards components with hard parts, such as fish and rabbit bones, which may last from the previous season. Also' the June data are the result of a build-up since March when the gulls first arnved at the colony. It is likely that rabbit formed an important part of the gulls' diet early in the season, then gradually assumed less importance.

In July 1977 when nest remains from the two species were recorded separately, little difference was found between them.

Pellets The results of this analysis (Fig. 3) emphasize the importance of terrestrial foods which was not seen in nest remains. In 1977, cereal grain and stems were found in many pellets, in addition to those with grass roots and stems found in other years. Although gulls have been recorded feeding on corn in autumn (Davis, 1956), the same behaviour in summer is not easy to explain The terrestrial invertebrates included coleoptera and annelids. The other food items show similar relative proportions to the nest remains.

Pellets are likely to be a more accurate indicator of the relative proportions of the food items in the diet as they are more recent, but still show a bias towards hard parts. However, older ones are likely to break up during w e a t h e n n g and then may be recorded as nest remains. This analvsis will include few pellets of gull chicks, as although they will form pellets at 3 - 4 days old, they digest fish bones and only form pellets containing fish when 3 - 4 months old (Spaans, 1971).

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The results (Fig. 3) show clearly that fish was the main food of the young.

O t h e r food identified was rubbish which included chicken. other meat, bread, potatoes, bacon and fried eggs, while marine invertebrates included shrimp, lugworm Arenicola, crab and squid. There was no detectable difference in the diets of the chicks of Herring and Lesser Black-backed Gulls.

The fish recorded in the study were always small ( < 10 cm) and examination of otoliths collected in 1977 identified several specimens of Whiting Merlangios merlangus and one of the Poor Cod Trisopterus minutus, both members of the Gadidae. A study in the Netherlands (Spaans. 1971) showed that just over 90% of the fish foods recorded were from Gadidae and over three-quarters of these were Whiting. As most of these were longer than 15 cm, and because the adult Whiting are bottom dwellers, it was assumed that they were obtained by the gulls from commercial fisheries (Spaans, 1971). In the present study, as the fish were small and likely to be near the surface, it was possible that the gulls could have caught them by plunge-diving.

Discussion The colony as a whole has increased considerably in the 18 years it has been in existence. It can be seen from Fig. 2 that the Herring Gulls have increased in several phäses.^There was a rapid phase of growth from 1968 to 1973, and then a slower Increase, up to 1980. It will be interesting to see how the numbers change in the next few years, in view of the large numbers recorded in 1981. Between 1968 and 1981, Herring Gulls at Orfordness increased at an average annual rate of 25.9% p.a. This is twice as much as the national average of 1 2 - 1 3 % (Chabrzyk & Coulson, 1976). Other colonies have increased at comparable ratesâ&#x20AC;&#x201D;Herring Gulls in the Isles of Scilly increased at an average annual rate of 25% between 1969 and 1974 (Allen, 1974). The mixed Herring and Lesser Black-backed Gull colony on Walney Island increased from 700 to 18-19,000 pairs between 1950 and 1965 (Brown, 1967).

A large net immigration of breeding birds must have taken place to account for these high rates of increase.

The increase of Lesser Black-backed Gulls at Orfordness has been fairly steady from 1968 to 1981. Although the pattern of increase is different from that of the Herring Gulls, the average annual rate of increase of 24.7% is very similar. Harris (1970) estimates that Lesser Black-backed Gulls can increase at 1 7 - 2 0 % p . a . , so some immigration must be taking place at Orfordness.

Comparable rates of increase for Lesser Black-faced Gulls have been found at the Isles of Scilly where they increased at 23% per annum (Allen, 1974) and Skokholm and Skomer where Harris (1970) reports that the rates of increase of Herring Gulls was 10% and that of the Lesser Black-backed Gulls was almost double.

The increase of Herring Gulls at Orfordness in the period 1963-68 could not have been caused by recruitment of h o m e produced young birds into the population. In the study of the Herring Gull colony on the Isle of May (Chabrzyk & Coulson, 1976) the mean age of first breeding was found to be

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5.25 years, with 55% of the birds breeding for the first time when 5 years old.

Thus this increase, and that of the Lesser Black-backed Gulls from 1968 to 1973 must largely be due to immigration.

T o see in which years net immigration must have occurred, calculations were done using Chabrzyk & Coulson's figures for first year and adult survival rates, and age of first breeding. A fledgling rate of one young per pair was used for Herring Gull and 1.5 young per pair for Lesser Blackbacked Gull (Harris, 1970). It was found that net immigration of Herring Gulls must have taken place at Orfordness between 1969 and 1973 and in 1981, and probably in 1976. Net immigration of Lesser Black-backed Gulls must have occurred every year excepting 1980 and 1981.

T h e r e is still ample space around the existing colony at Orfordness for the expansion of both species of gulls. It would seem too that there is no reason for the immigration rate to decline substantially while other established colonies continue to increase. Thus, the colony must be expected to continue to increase until other factors such as food and density or human disturbance become limiting.

The establishment of the colony and the subsequent increases cannot easily be ex'plained in terms of an increase in the availability of food from man's fishing activities. The Overall amount of white fish caught at Lowestoft, the only large fishing port nearby, has not changed substantially since 1963, although the amount of certain fish caught eg. Herring Clupea harengus and Mackerei Scomber scombrus has decreased considerably ( M A F F , Unpub.)

Harris (1970) tentatively suggests a time-lag effect between an increase in food and the increase in gulls due to the gulls having to adjust their behaviour to exploit a new food supply. Thus the present increase at Orfordness may be related to an earlier increase in food availability.

Although the gulls may be feeding on rubbish to a fair extent, there is no evidence for any increase in the amount or availability of man-produced rubbish in the area, nor any increase in the number of rubbish dumps.

A n o t h e r possibility is that the gulls have emigrated from other areas that were becoming overcrowded, or where food supplies were not enough for the growing population. Thus Orfordness may have become a more attractive breeding site than it used to be, especially as there is now little human activity and disturbance. Ground predators such as foxes seem to be absent.

T h e r e was little predation by large gulls on Avocet chicks at Havergate, in spite of large numbers of gulls passing over the island. Six were observed taken in 1973, 3 in 1975, 10 in 1979, 1 in 1980 and none was taken in 1974, 1977, or 1978. Most of this predation was by immature gulls. Moreover, no remains of Avocet chicks were found in the gull pellets, nest remains or crop contents of their young on Orfordness over 9 years 1973-1981.

The main predator of young Avocets at Havergate was found to be Kestrels Falco tinnunculus, which took about 30 in 1970, about 50 in 1972, one in 1975, possibly a high proportion of the 144 hatched in 1977 when 16 were observed taken, but only one in 1978. Remains of an Avocet chick were found at a Kestrel roost L-ite in a disused building on Orfordness in June 1977.

In 1979 the gull colony on Orfordness beach did not seem to have affected the breeding success of the adjacent Little Tern colony, although the

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breeding terns had been displaced. Twelve pairs nested on Orfordness opposite Havergate Island (Fig. 1) up to 1975. After this area was colonised by large gulls the terns have subsequently nested at two other sites.

However, the gulls may still pose a potential threat to the Avocet colony if they change their feeding habits. In the Camargue, France, Herring Gulls were found to have contributed to a decrease in an Avocet population (Johnson & Isenmann, 1971).

References Allen, R. (1974). Gulls and Other Seabirds on the Isles of Scilly. NCC Report.

Brown, R. G. B. (1967). Breeding Success and Population Growth in a Colony of Herring and Lesser Black-backed Gulls Larus argentatus and L. fuscus. Ibis 109,502.

Cadbury, C. J. & Olney, P. J. S. (1978). The Avocet population in England and its dynamics. British Birds 71,102.

Chabrzyk, G. & Coulson, J. C. (1976). Survival and recruitment in Herring Gull Larus argentatus. J. Anim. Ecol. 45, 187.

Davis, T. A . W. (1956). Gulls feeding on grain. British Birds 49, 400.

Drury, W. H . & Kadlec, J. A . (1974). The current status of the Herring Gull population in the North-eastern United States. Bird Banding 45, 4.

Harris, M. P. (1970). Rates and causes of increase of some British gull populations. Bird Study 17, 325.

Johnson, A. R. & Isenmann, P. 1971. L'Evolution Recente Des Effectifs Nicheurs D e Laro-Limicoles En Z o n e Saumätre D e Carmargue Alauda 39, 29.

Spaans, A . L. (1971). On the feeding ecology of the Herring Gull Larus argentatus pont. in the Northern part of the Netherlands. Ardea 59, 73.

Thomas, G. J. (1972). A Review o f G u l l D a m a g e and Management Methods at Nature Reserves. Biol. Cons. 4,117.

The Royal Society for the Protection of Birds, The Lodge, Sandy, Bedfordshire, SG19 2DL.

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