The Status of the Black-Headed Gull Colony at Ravenglass
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22 THE STATUS OF THE BLACK-HEADED GULL COLONY AT RAVENGLASS. BY S. MARCHANT. ALTHOUGH there is an interesting association of several species of sea and shore birds breeding on the Drigg sandhills opposite Raven- glass in large numbers, the site has received little serious study and is generally known only for the size of the colony of Black-headed Gulls (Lams ridibundus) and for the yearly presence of Sandwich Terns (Sterna sandvicensis) in somewhat fluctuating numbers. When in England during the summer of 1948, the writer noticed an obvious decrease in numbers of Black-headed Gulls and immedi ately realised that it was possible to make a more accurate estimate of the size of the colony than was done in 1938. The interpretation of the results of observations in 1948 was open to some doubt, and in consequence a more detailed check was made in 1951. The present writer was responsible for the figures for the Raven- glass colony supplied for the Survey of Black-headed Gull colonies carried out by the British Trust for Ornithology in 1938. In his report on that survey (antea, vol. xxxiii, pp. 202-221, 230-244) Hollom described the colony as by far the greatest in the Kingdom, over five times the size of its nearest rival, and amounting to two- thirds of the total breeding population of Black-headed Gulls in England. The more important details given were that the birds bred over an area of about f square mile and that " in 1938 (there were) some tens of thousands breeding pairs, probably less than 50,000," figures which were based on the present writer's submission. It will be shown that at the present time the size of the colony is far smaller, the difference being so great that, together with what evidence can now be collected for pre-war numbers, it must indicate that the 1938 estimate was much exaggerated. At the same time there is clear evidence for some natural decrease. It is not necessary to discuss the results of observations made in 1948, though they will be mentioned later, since they show that a decrease has occurred since the war. The accompanying map shows the breeding area of the gulls as measured in 1951, when the writer visited the colony on June 24th and 25th. These areas were mapped as carefully as possible by rather primitive pacing and compass methods : though they are not thought to be absolutely precise, they are probably quite reasonably accurate. As far as possible all nests were counted directly, and, though naturally the final figures are not exact, they are probably on the generous side since they include all nests which showed signs of occupation this year, whether they contained eggs or young or were empty : finally the figures were rounded off upwards. The numbers on the map against each more or less easily separable area show the nests counted and their total is 5,640. If then we say that no more than VOL. XLV.] BLACK-HEADED GULL COLONY 23 6,000 pairs bred at Ravenglass in 1951, we cannot, it is thought, be making an underestimate. In 1948 the birds bred over a slightly larger area than this year. The extension is shown on the map. It is not likely that there were more than an extra 500 pairs in that year. The map also indicates roughly those areas where the gulls used to nest before the war : it is now impossible to be exact on this point, partly because of lack of records and partly because the topography of the sandhills changes somewhat in the course of time, obliterates landmarks and makes it difficult to be precise as to localities. Nevertheless the areas indicated are not wildly inaccurate. Measured as generously as possible they comprise approximately 400,000 square yards. This is a very different figure from the § square mile implied in 1938, though in the broadest sense it is not far out to say that the birds did nest " over " that area. Now, this year (1951) it is possible to estimate the breeding density fairly accurately. There were five to six thousand breeding pairs in an area of approximately 200,000 square yards, i.e., one pair per 35-40 sq. yds. Incidentally in 1948 a sample count in an area of 100 x 150 yards showed 150 nests or one pair per 100 sq. yds., but that was a selected area which included some bare sand, a factor which has been excluded as far as possible in the 1951 figures. In both years the density in the breeding patches seemed to be much the same as before the war. On this basis, then, say one pair to 40 sq. yds., we could expect a population of just about 10,000 pairs in pre-war years. Certainly it would be wrong to suggest a figure anyway nearly approaching 50,000, and, even by putting the most liberal interpretation on the estimates of area and density made above, it would be hard to reach a figure of 20,000. The writer's opinion is that 10,000 pairs is a very fair estimate of the pre-war population and there is support for this from two independent sources. The semi-permanent watcher at Ravenglass who has been associated with the gullery for 30 years or more, considers that, though undoubtedly some decrease has occurred, as is obvious from the decrease of the breeding area, this is nothing like of the order of ten times ; but that on the contrary it is remarkable that the gulls have been able to maintain their numbers so well, in spite of bombing and shelling during the war, when the sandhills were used to some extent as a practice area, and also in spite of a general falling-off in protection leading to more intensive and excessive egg-collecting. Moreover the records of eggs collected by the Muncaster Estate over the last 20 years are instructive. These show very great fluctuations, but in an average year before the war almost exactly the same number was collected as in the past two or three years. The pre-war fluctuations can best be attributed to outside factors such as bad weather or inability to collect systematically. The average figure is between 20-25,000 eggs, and as this year that amount was collected from about 6,000 24 BRITISH BIRDS. [VOL. XLV. /If'. * ^Ss>^ ' • • /"NT^i, ' - I-•-••• •-••/- •{ \\mhu, \ \\. ^V*^' 4l III I life \ T"» N.- * Ljyirfe^ * * I § »-" yftou±jir /^ id's ^* -/^gR^v^ "s x s /• -'' "l/^ <o ui < Z /* pairs, it is hardly likely that before the war it came from a much larger number, since the collecting methods have not been appre ciably intensified. I am obliged to Mr. E. M. Nicholson for pointing out that a further check on numbers both before and after the war is possible. In particular by considering the year 1941 when an especial effort VOL. XLV.] BLACK-HEADED GULL COLONY 25 was made by the Muncaster Estate to collect as many eggs as possible, a more reliable figure for early war-year, and so, perhaps, pre-war, numbers might be found. If we knew, or could reasonably estimate, the number of eggs collected from each pair on an average, by divid ing that figure into the total number of eggs collected, we would get the total number of breeding pairs. In 1941, it is recorded that 72,498 eggs were collected for the Estate in about six weeks, i.e. probably from about April 20th to about June 1st. Hardly any other eggs can have been collected by people not working for the Estate during that season, as the sandhills had been taken over as a military area. Mr. Joe Farren, the watcher at Ravenglass for more than 25 years, considers that a pair usually lays again on about the second day after the nest has been robbed of a single egg, until the normal clutch has been laid, and that then there are a few days before the birds lay again. He also says that he has taken up to 13 eggs from one nest. It is certainly a matter of fact that in pre war years dwarf, deformed and unnaturally marked eggs with poor chalky shells could often be found towards the end of the collecting period—probably a sign of exhaustion in some of the birds. As it does not seem that we have much exact knowledge of the laying behaviour of the gulls when their nests are robbed, i.e. whether they really do go on laying till the normal number of eggs in a clutch has been laid, or whether they are discouraged for longer than a couple of days, or indeed whether a pair actually uses the same nest the whole time, some uncertainty still surrounds the number of eggs likely to be provided by one pair during a given period. However, without going into details we might suggest that an average pair would provide either two or three clutches of 2 or 3 eggs each in a four week collecting period, or an average of 1 to 2 eggs per week. In the 1941 season of six weeks the limits of eggs per pair would be, taking the lower figure, probably 5 to 10, taking into account the longer period : taking the higher figure it would be 6 to 12. Taking a round figure of 73,000 eggs collected, we would thus get limits for the total population of c. 14,600-7,300 or c.