(138) Publication of the British Trust for Ornithology
Total Page：16
File Type：pdf, Size：1020Kb
(138) PUBLICATION OF THE BRITISH TRUST FOR ORNITHOLOGY. REPORT ON GREAT CRESTED GREBE SAMPLE COUNT, 1935. BY P. A. D. HOLLOM. DURING the summer of 1935 a sample count of Great Crested Grebes (Podiceps c. cristatus) was taken in England, Scotland and Wales. The main object of this count was to discover the effect of the severe drought in some parts of the country during 1933 and 1934, and also to trace the general trend in numbers and distribution since the complete census of 1931 (see British Birds, Vol. XXVI., pp. 62/92, 102/131, 142/195). The areas chosen were well spread geographically and included waters in Wales and Scotland ; various other waters were also reported on, but the request in British Birds for casual observations met with very poor response. Most of the counts were made in June, and the numbers of non-breeders in particular refer to that month. THE 1935 SAMPLE COUNT. The following table gives the counts made in the selected areas together with the numbers for the same waters in 1931. For full particulars reference should be made to the detailed lists of occupied waters given at the end of the report. T935- i93i- Breeding Non-B. Total Breeding Non-B. Total. Pairs. Birds. A dults. Pairs. Birds. A dults. *Berks. ... 34/36 13/17 85 35/37 5 75/79 •Bucks. 17/18 15/17 51 11 2 24 •Cheshire 74/80 38/42 190/198 78 38 194 I I •Essex 7/ 8 30 64/66 33 31 97 •Leicester. 34 8 76 32 — 64 •Middlesex 10 27 47 15 5 35 •Oxford 14 1 29 15/16 — 30/32 •Somerset 17 15 49 17 5 39 Herts, (part) 23/28 18 64/74 31 35 97 Norfolk (some broads) no — 220 107 — 214 Surrey (part) 27 9 63 32 — 64 Yorks. (part) 48 8 104 47 — 94 South Wales (part) 16 — 32 15 — 3° Forth & Tay areas (part) 45 9 99 35/38 — 70/76 Other waters 33/35 52/55 121/122 57 19 133 Total 0.519/536243/2561,294/1,315! c.560/566 140 1,260/1,272 *A complete census was taken in these counties. VOL. xxx] REPORT ON GREAT CRESTED GREBE. 139 1935 1931 Breeding Non-B. Total Breeding Non-B. Total Pairs. Birds. A dults Pairs. Birds. Adults. Say 528 249 1.305 563 14° 1,266 Increase or Decrease on 1931 —35 + 109 + 39 Percentages on 1931 figures 94% 178% i°3% In 1931 about 1,240 pairs were found in England, Scotland and Wales, so that the 1935 count represents a sample of 45 per cent. The 1935 figures show a decrease of about 35 breeding pairs or 6 per cent. ; an increase of 109 non-breeding birds or 78 per cent.; and an overall increase in population of 39 birds or 3 per cent. In 1931 the question of non-breeding birds was one of several supplementary points on the reverse side of the enquiry schedule, whereas in 1935 there were adjacent columns in which to enter breeding and non-breeding birds. Moreover it was known that one of the objects of the present count was to find to what extent the breeding population had been upset by the drought. There was, therefore, a tendency to require more positive evidence in 1935 than in 1931 before a pair was entered as breeding, especially as no indication had been given to observers as to what should be counted as breeding birds and what as non-breeding birds. One observer who found two Grebes, evidently paired, on a water and did not see nest or young, but was unable to search the available cover, reported them as a breeding pair. Another observer, on similar evidence, reported non-breeding birds. But the majority did not give their reasons for including birds as breeding or otherwise, and so no attempt has been made to sort the records on a uniform basis ; they have been accepted as they stand. Consequently the increase in the proportion of non-breeders to breeders has been exaggerated and the 1935 figures for breeding pairs are not strictly comparable with those for 1931, when, generally speaking, pairs present on territories were included as breeding.* Therefore in the tables comparing the 1931 figures with those for 1935 both breeding and non- breeding birds have been included, and the total number of adult Grebes on each water is shown. •As an example, reference may be made to the last table on p. 80 of the 1931 report (Brit. Birds, Vol. XXVI.) which shows that on ten Northamptonshire waters twenty-one of the forty " breeding " pairs did not nest. 140 BRITISH BIRDS. [VOL. XXX. THE DROUGHT. An exceptionally long period of dry weather began in November, 1932, and lasted for two years. Over England and Wales the rainfall for 1933 was 6.6 inches less than the Rainfall, Seasonal year, as per cent, of average. average and in 1934 there was a further deficiency of 1.7' inches, in spite of an excess of about 3 inches in December which in many places was the wettest December on record. The year 1935 continued to make good the shortage, and the excess for the year was about 5 inches. In Scotland, although VOL. xxx] REPORT ON GREAT CRESTED GREBE. 141 1933 was an unusually dry year, the average rainfall was exceeded in both 1934 and 1935. On page 140 will be found a map reproduced by kind permission of the Controller of H.M. Stationery Office from British Rainfall, 1934, showing the rainfall for the year October 1933 to September, 1934. During these twelve months the deficiency of rain steadily accumulated, and almost reached its peak, and it is therefore perhaps the most significant period of the drought so fat- as Grebes are concerned. ACCUMULATED DEFICIENCY OF RAINFALL. (Taken from British Rainfall, 1934). Period. England Scotland. and Wales. inches. inches. November 1932 to March, 1933 1.7 0.2 July, 1933 3-5 0.9 September, 1933 • 5-8 4-3 December, 1933 9-7 9-7 ,, March, 1934 . 11.4 11.2 July, 1934 13.1 7.8 ,, September, 1934 . 13-1 5-8 November, 1934 14.9 5-7 ,, December, 1934 . 11.5 4.6 The average annual rainfall in England and Wales is 35.2 inches, and in Scotland 50.3 inches. Although sufficient information is not forthcoming to obtain an exact idea of the extent to which the Great Crested Grebes were affected at the height of the drought, it is clear that there were considerable disturbances and readjustments, particularly in 1934. This is well illustrated by the following examples of some of the more startling changes in that year compared with 1931. At Tatton Mere, Cheshire, there were 7 pairs in 1931 and 2 pairs in 1934 ; at Tring, Hertfordshire, 25 pairs in 1931, and none in 1934 ; at Blagdon, Somerset, 12 pairs in 1931 and 3 pairs in 1934 ; at Hornsea, Yorkshire, 10 pairs in 1931 and about 19 pairs in 1934 ; at Llangorse, Brecon, 13 pairs in 1931 and 20 pairs in 1934. Only a small proportion of the waters was still affected by the drought in 1935, and the waters concerned were spread throughout the country, very few areas escaping altogether. The Grebes suffered on only half the waters that were affected in this way, while in three localities (in Essex, Leicestershire and Middlesex) the lower level resulted in a better growth of nesting cover. The reports on the state of the waters in 1935 are tabulated below. 142 BRITISH BIRDS. [VOL. XXX. DROUGHT EFFECTS. Material No. of Reduction A Averse Waters. in water Effect on level .1935- Grebes, 1935 Berks. 22 — .—. Bucks. II 1 — Cheshire 36 6 4 Essex 10 1 1 Leics. 12 3 •—. Middlesex ... 9 3 2 Norfolk 15 —. Oxford 8 1 — Somerset ... 8 — .—. Surrey 18 4 2 Yorks. 15 4 2 Wales ... •. 2 Scotland 26 3 2 Other waters 28 9 5 220 35 18 DECREASE OF BREEDING PAIRS. In considering the decrease of breeding pairs it is noteworthy that it is more than covered by the decreases in pairs at reservoirs ; in fact, the decreases in almost every area are due to losses at waters of this type. The net reduction of about 60 pairs at reservoirs is made up as follows:—• Increase. Decrease. Bucks. Weston Turville .. — Cheshire Baddiley .. — 3 Bosley .. 0/1 -— Hurleston 1 — Sutton .. — — Walton 1 — Essex Lea Valley .. — 17 Herts. Cheshunt .. — Elstree ... .. — 3 Tring .. — 5/1° Leics. Blackbrook .. — 2 Cropston .. — 1 Knipton ... .. — •— Moira — 1 Saddington .. — —• Swithland 4 — Thornton 2 — Middlesex Ruislip ... .. 4 Stoke Newington .. 2 Northants. Stanford .. 12 Westhorpe ,. 6 Oxford Clattercote .. — Carried forward: 8/9 56/61 VOL xxx] REPORT ON GREAT CRESTED GREBE. 143 Increase Decrease Brought forward: 8/9 56/61 Somerset Blagdon ... ... — 2 Chard ... 1 •— Litton ... ... ... — Staffs. Bellfields — 6 Dimmingsdale ... ... — 1 Warwicks. Wormleighton ... ... — I Worcs. Bittell — 3 Yorks. Chelker ... ... — — Gouthwaite ... ... 1 — Ryhill ..' 2 — Fife Kinghom 1 •— Peebles, Portmore ... ... 1 — Midlothian Bavelaw ... ... ... •— 1 Cobbinshaw ... ... •— 2 Gladhouse ... ... — — 14/15 72/77 Less increases 14/15 58/62 18 Reservoirs showed decreases totalling 72/77 pairs. 9/10 ,, ,, increases ,, 14/15 ,. 9/10 ,, „ no change — 37 Reservoirs. Net Decrease 58/62 pairs. It follows that the natural waters show a net increase of about 25 pairs, and it would appear that many local decreases and increases are merely the redistribution of birds between natural waters and reservoirs where the effect of the drought was accentuated. But much more significant than the failure of the reservoirs to maintain their population of breeding Grebes, is the fact that if the areas counted in 1935 are grouped according to their increase or decrease of nesting pairs, they fall into two main geographical divisions.