22

THE STATUS OF THE BLACK-HEADED GULL COLONY AT . BY S. MARCHANT. ALTHOUGH there is an interesting association of several species of sea and shore birds breeding on the 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 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 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/^

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. 12,150-6,000 respectively. Obviously the arithmetic mean of these figures could be reasonably accepted, if the extremes are agreed upon, and we therefore again reach a figure for the total population of about 10,000 pairs in 1941, In the writer's opinion a mean figure of 7-8 eggs per pair in itself seems more acceptable since it would better account for signs of exhaustion in egg laying than the low figures of 5-6, the higher figures (10-12) being altogether too high for an average. Applying this estimate to the 1951 season of four weeks approximately, when 24,568 eggs were collected by the Estate, we find that the total population would be between 7,500 and 3,750 or 3.33° : that is, if the total number of eggs collected was 30,000, and Mr. Farren estimates that about 6,000 eggs may be taken by people not working for the Muncaster Estate. The mean of these figures is 5,000-5,500, which agrees well enough with the count. 26 BRITISH BIRDS. [VOL. XLV

In conclusion, then, the evidence collected in 1951, together with local opinion and the record of the egg-collecting, suggests that not so very many more than 10,000 pairs are likely to have bred at this colony before the war. Consequently the 1938 estimate must be regarded as grossly inaccurate. Thus, the colony would probably have fallen into Hollom's fourth group (1,001-10,000) even if right at the top of it, and becomes comparable with some ten other colonies throughout the British Isles, rather than being so vastly superior in point of numbers, as formerly reported. By comparing the breeding area of this year with that of 1948 and pre­ war, it is quite clear that a decrease has occurred and is possibly still continuing. Further, it is seen that the deserted areas are those most quickly and easily reached when approaching from Drigg or Ravenglass : consequently they are probably the places most heavily cropped by the egg-collectors and at the same time least easily protected. It is reasonable enough, without going into details, to suggest that the decrease has been largely due to excessive egg-collecting by unauthorised persons continued too long into the summer, during the immediate post-war years : and that this has been connected with an inability to afford the same degree of pro­ tection and control as before the war. I should like to thank Mr. P. A. D. Hollom and Mr. E. M. Nicholson for helpful advice and criticism when preparing this article, and also the staff of the Muncaster Estate Office for giving me access to their records. To Mr. J. Farren of Ravenglass I am much obliged for providing many details of past occurrences and for the assistance of his experienced knowledge. J We accept Mr. Marchant's conclusion, in the light of his careful resurvey, that the level of the pre-war breeding population of Black-headed Gulls at Ravenglass should be taken as about 10,000 pairs, rather than as " some tens of thousands breeding pairs, proba­ bly less than 50,000 " previously stated (antea, vol. xxxiii, p. 220). The effect of this very large revision is to reduce the estimated breeding population for England in 1938 from about 70,000 pairs to about 35,000 pairs. This underlines the need for treating with the greatest reserve estimates of the size of large colonies unless they are made with the utmost thoroughness and are, if possible, re-checked by an independent observer. Reference to the original return shows that Mr. Marchant noted that owing to lack of time he had been unable to attempt an estimate himself, and that he made it clear that he was merely passing on and commenting on an estimate tentatively put forward by local watchers. Moreover the Schedule declared that it was " not intended to carry out a census, but any estimate of numbers will be welcome." At subsequent stages, however, the resulting figures tended to be taken more literally than the circumstances of their collection could justify. It is fortunate, therefore, that the estimate has now been critically examined while it is still possible to carry out a comparison between VOL. XLV.] BLACK-HEADED GULL COLONY. 27 the 1938 colony and the present one.—EDS.]