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Publication of the British Trust for Ornithology (304) PUBLICATION OF THE BRITISH TRUST FOR ORNITHOLOGY. THE INDEX OF HERON POPULATION, 1939. BY W. B. ALEXANDER. THE number of heronries in which the nests were counted in 1939, and the figures reported to the Edward Grey Institute, was again greater than in any previous year. Particulars were received for one or more heronries in every county in England and Wales except Monmouth and Flint (and London, Middlesex, and Rutland, in which none are known to exist). Figures were also received from 11 counties in Scotland and 15 in Ireland, a large proportion of the latter being due to the assistance of the Rev. P. G. Kennedy. In England and Wales the number of sites reported on was 139, in Scotland 16, and in Ireland 33. Of these 188 heronries 166 were counted also in 1938, when they contained 2,820 nests as against 2,720 in 1939. The totals thus indicate a decrease of 3 per cent, in the breeding population compared with the previous year. Mr. E. M. Nicholson, in his report on the 1937 figures (Vol. XXXI, p. 341), has explained the reasons that have led us to- adopt two indices, one based on 1928 and the other on 1936. The statistical problems arising in connexion with the 1928 standard grow more troublesome every year. At the outset it was decided that the annual sample should contain heronries of various sizes in proportion to their relative abundance in the total in the 1928 census. This meant that only one heronry in the class with over 100 nests could be included. It happens that the one included has in the last 6 years varied between 54 and 68 nests. It has thus tended to keep the index down. On the other hand two heronries which had. respectively 51 and 65 nests in 1928 both had well over 100 nests in 1939. Ought they now to be excluded from the sample ? Of two other heronries in the class with over 100 nests in 1928, on which we have regularly received reports, but which have hitherto been excluded from the sample used for calculating the index, one still contains over 100 nests but the other has this year dropped to 61. Thus, of 5 heronries in the class with over 100 nests either in 1928 or 1939, two have fallen out of it since 1928, one has remained in it hroughout and two have risen into it since 1928, It seems to the writer that it is more satisfactory to include them all in the sample especially now that its size warrants VOL. XXXIII.] HERON POPULATION, 1939. 305 the inclusion of at least two heronries of this class. Adopting this procedure we get an index figure of 109 for England and Wales in 1939, compared with 100 in 1928. This figure is based on 2,288 nests, well over 50 per cent, of the total population in 1928. It is the same as the index for 1938 (Vol. XXXII, p. 139) and indicates that the severe weather experienced at Christmas, 1938, when many rivers and streams were frozen and deep snow covered most of the country, did not affect the Heron population as had been anticipated. It will be of interest to see whether the more prolonged cold spell of January and February will reduce the numbers of Herons breeding in 1940. NEW INDEX, BASED ON 1936. Number of nests Year. Index. on which index is England and Wales. based. 1936 100 1,982 1937 100 1.974 1938 101 2,017 1939 98 i,936 Great Britain. 1936 100 2,129 1937 99 2,111 1938 IOI 2,069 1939 98 1,997 British Isles. 1936 100 2,208 1937 99 2,183 1938 IOI 2,226 1939 99 2,149 It will be seen from the above table that the fluctuations during the past 4 years have been very slight. The index has in no case varied by more than 2 per cent., an amount which is not statistically significant. The main reason why the index based on 1936 has varied so much less than that based on 1928 is no doubt that the figures for each year are for almost exactly the same list of heronries counted by the same observers at approximately the same season. On the other hand the figures used for comparison with 1928 have each year included those for a number of heronries from which regular reports have not been received, but which can be utilized since we have a complete set of figures for 1928. Some of the 1928 figures, however, are for dates much earlier in the season than those adopted in recent counts, these latter having almost all been made between April 15th and May 10th. As in previous years the stability of the Heron population 306 BRITISH BIRDS. [VOL. XXXIII. as a whole is the result of opposite tendencies in different parts of the country. In comparison with 1936 the samples from south-west England, south-east England, and Wales and the border counties show decreases of more than 10 per cent., whilst the samples from the Midlands and Ireland show increases of over 10 per cent. As a check on the results obtained by the counting of sample heronries distributed over the country we have again been able to obtain figures for all the known heronries in four considerable areas of England, namely, the Thames drainage area; Sussex, where the survey was undertaken by Mr. E. M. Cawkell; Lancashire and Cheshire, where much help was received from Mr. A. W. Boyd ; and Yorkshire and Durham, where the figures for the West Riding were collected by Mr. H. B. Booth. In these four areas all or almost all the heronries have been counted for several years past. By interpolating figures for a few heronries not counted in certain years it has been possible to compile the following table. Figures in brackets indicate that in those years a number of figures have had to be interpolated so that the index is only approximate. INDEX FIGURES FOR SAMPLE AREAS. No. of Area of nests 1928 1934 I935 x936 1937 1938 1939 in 1928 Thames Basin ... 325 100 99 94 94 105 105 108 Sussex 294 100 (85} 95 (106) 103 HI 93 Lanes, Cheshire and part of Westmorland... 329 100 — — (124) 125 127 127 Yorks and Durham ... 129 100 (85) (IOI) 106 92 100 100 It will be noted that the Heron population breeding in Yorkshire and Durham in the last two years has been the same as in the census year 1928, whilst in Lancashire and Cheshire it has been approximately 25 per cent, greater than in 1928 for the last four years. It can perhaps hardly be expected that so many returns will be received in the present year, but it is hoped that as many helpers as possible will make a special effort to count nests in heronries between April 15th and May 10th and send particulars to the Edward Grey Institute of Field Ornithology, 39, Museum Road, Oxford. The data accumulated in past years provide a basis which has hitherto been lacking for determining the effect of an unusually severe winter on the Heron population of the British Isles, and such an opportunity may not occur again for many years. .
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