Great Crested Grebes at the Tring Reservoirs. by Chas
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(250) GREAT CRESTED GREBES AT THE TRING RESERVOIRS. BY CHAS. OLDHAM. THE Tring Reservoirs in Hertfordshire lie at about 350 feet, in open country, N.W. of the Chiltern escarpment. They comprise three contiguous pools—Marsworth, Startop's End and Little Tring, with an aggregate area of about 90 acres ; and, some half a mile away from the others, Wilstone Reser voir, 120 acres in extent. Here, in normal seasons, from twenty-five to thirty pairs of Great Crested Grebes (Podiceps c. cristatus) nest. The breeding birds arrive during the second half of February and the first half of March, but hard weather may retard them and frost may drive them away again for a time. The nests are built in the big beds of reed (Arundo phragmites), reed-grass (Glyceria), lesser reed-mace (Typha angustifolia) and, infrequently, among the floating leaves of Polygonum amphibium. The breeding birds, or most of them, leave in October and November. In 1934 none nested. At the beginning of the year the reservoirs were exceptionally low, owing to the drought of the previous twelve months, and even then a belt of dry mud, in some places more than a hundred, and nowhere less than twenty-five, yards in width, intervened between the water and the beds of reeds and rushes, whilst the areas on which the Polygonum grows were high and dry. The pumping of water for the use of the canal and the phenomenally low rainfall caused a constant shrinkage in the water area during the spring and summer, and by early autumn the bed of Little Tring Reservoir was dry altogether, whilst the area of the other pools was only about a third of the normal; the depth of the water was, of course, much reduced. The fertile mud produced an amazingly luxuriant crop of vegetation, but what in September was a breast-high jungle of willow saplings, goose-foot and other plants was, during the Grebes' breeding-season, an expanse of mud which effectually debarred them from their accustomed nesting-places. At the end of November, 1933, there were eight birds on the reservoirs, but hard frost in the second week of December banished them. On December 12th there was none, but on the 23rd a single bird was on a small patch of open water in the ice on Wilstone ; it was not there on the 31st nor on January 1st or 8th, but on January 13th and on ten different VOL. XXVIII] GREAT CRESTED GREBES. 251 occasions between then and February 16th there was a single bird on this reservoir. On February 20th there were four— on three different reservoirs—but on the 24th only one ; the birds seen on the 20th were presumably on passage. There were seven birds—including a party of six-—on Wilstone on February 27th, but none elsewhere. As there were only four—on three different reservoirs—on March 1st, the infer ence seems warranted that the birds seen on February 27th were on passage too. By March 6th there was a big increase ; twenty-six birds—some engaging in shaking-bouts—were on two reservoirs ; and on March 21st fifty-five, on three. Some of these were probably passage birds, for I did not see so many subsequently. My next visit to the reservoirs was not until April 22nd, but between that date and May 17th I was there frequently. Absence from home prevented my going again until June 14th. The following table suggests that, even when definitely paired, some of the birds moved about from one pool to another, and it opposes any idea of the annexation of definite territory in the beds of reeds and rushes, which indeed was, in the circumstances, impossible. It shows that there was a falling-off from the peak of abundance on May 10th and 12th, but whether this was due to some of the birds being on passage or to the premature departure of birds which would, in the ordinary course, have nested, is not clear. It is certain that .the total number at the reservoirs was less than usual, for in addition to the twenty-five to thirty pairs of nesting birds there is, in normal seasons on Startop's End, a loosely-spread flock of maybe thirty non-breeders. Startop't Little Wilstone. End. Marsworth. Tring. Total April 22nd 19 7 6 15 47 „ 24th 12 10 6 J 3 41 „ 25th 17 4 6 '4 41 May 5th 22 4 6 11 43 7th 24 6 6 8 44 ,, 10th 33 2 6 <) 5° ,, 12th 35 0 6 H 49 „ 15th 22 0 6 7 35 „ 17th 21 0 5 5 31 June 14th 23 5 2 2 32 At Wilstone, especially from mid-May onwards, there was a diminishing tendency for the birds to segregate in definite pairs. On May 12th twelve were gathered on a small, isolated pool, and most of the remaining twenty-three were associated in a loose flock, but there was some evidence of Abnormal nest of Coot on dry mud, Little Tring Keservoir. (Copyright photograph by O. G. Pike.) VOL. XXVIII] GREAT CRESTED GREBES. 253 pairing in frequent shaking-bouts. On May 15th and 17th eighteen were in a loose flock, as were all the birds on June 14th ; but on each occasion shaking-bouts were engaged in. At Startop's End the birds seen between April 25th and Mav 10th were obviously paired ; the five on June 14th comprised two pairs and an odd bird. At Marsworth the six birds that were always present between April 22nd and May 15th comprised three obvious pairs. At Little Tring the fourteen birds on April 25th comprised seven obvious pairs, but, despite occasional shaking-bouts, the five on May 17th did not seem definitely to be paired ; the two on June 14th were an obvious pair. I was not able to go again to Tring until July 6th ; the only Grebes then on the reservoirs were two—on Wilstone—and on July nth there was only one, which was possibly under some disability for I saw it frequently, always in the same place, approximately, until November 12th, but not thereafter; and whether it died or left the place for its winter quarters in the ordinary course I do not know. When the other thirty birds that had been present on June 14th left, and whether they went gradually or all at once, I could not learn, but it is certain that by July 6th virtually all the Grebes had evacuated the reservoirs. I never saw any attempt at nest building, but was told by the keepers that in early June a pair gathered a little nesting material and deposited it in shallow water on the muddy margin of Little Tring, but that as the water shrank they abandoned it. This scanty foundation was taken over by a pair of Coots (Fulica a. atra) which, not finding their usual material at hand, collected a few small sticks, a piece of coarse string and a number of shells of the swan mussel (Anodonta cygnea), with which the dried mud was strewn. No eggs were laid* and this effort of the Coots, abortive indeed, was like that of the Grebes, so far as I could learn, unique. At no time during the spring or summer did I hear of any eggs being dropped on the mud or in the shallow water, although if any such there were they would probably be picked up and carried away by the Crows (Corvus corone). There is evidence that the Great Crested Grebe has some slight power of accommodation to unusual circumstances f but its nesting is a highly-specialized affair and corres- *The eggs shown in the photograph were " property " eggs, put in the nest to help the picture. fHarrisson, T. H., and Hollom, P. A. D., B.B., XXVI., pp. 185-6; and Venables, L. S. V., and Lack, D., ibid, XXVIII., p. 195. 254 BRITISH BIRDS. [VOL. XXVIII. pondingly inelastic ; the intervention of a belt of dry land between the water and the reed-beds was an insuperable obstacle, a violent interruption to their routine and something quite beyond the birds' powers to adjust. Unfortunately, we have no knowledge of the state of the birds' gonads when they left the reservoirs, nor do we know whether the urge to nest still persisted, nor whether all or any of them did actually nest, and, if so, where. Food shortage was not the reason of the Grebes' exodus. The shrinking waters implied a con gestion rather than a scarcity of fish, and made their capture more easy. At Little Tring many fish were removed by netting as the water receded, and it was only in the last phase of drying up that the survivors were stranded and died ; their abund ance was then revealed ; stranded on the mud I counted between a hundred and a hundred and twenty large roach (Rutilus rutilus) and smaller fish innumerable. Inability to achieve nesting-sites seems the most likely, and, at the first blush, the obvious reason for the Great Crested Grebes' failure, and a like explanation may meet the case of the Dabchick (Podiceps r. ruficollis) and even those of the Pochard (Nyroca f. ferina) and Coot, which incline to build their nests in the water, but this reason and shortage of food seem alike insufficient to explain the abstention of the Tufted Duck (Nyroca juligula). The problem is not simplified by the fact that whilst none of these species nested at the reservoirs, the Mallard (Anas p.