THE MIGRATORY STATUS OF THE HERON IN GREAT BRITAIN.

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1 (242) THE MIGRATORY STATUS OF THE HERON IN GREAT BRITAIN. BY N. F. TICEHURST, O.B.K., M.A., F.R.C.S. THE Common Heron (Ardea c. cinerea) is a species whose movements within the British Isles are easy to observe, but extraordinarily difficult to interpret. This probably accounts for the great paucity of material available for any attempt to reduce them to order. Our native birds habitually make nights of relatively long distances for ordinary food-gathering purposes*, which not infrequently take them to the neighbourhood of the coast-line, while single birds, or two or three together, are far more frequently met with than parties of larger dimensions, though apparently when actually migrating parties of up to thirty in number may sometimes be encountered (see Eagle Clarke, Studies in Bird Mign. II., p. 163). Even on the coastline, unless one happens to see birds actually arriving, departing, or obviously coasting, it is almost impossible to be sure whether any given individuals are actually migrating or not. This is well illustrated by the evident difficulties of the Misses E. V. Baxter and J. L. Rintoul in their Vertebrate Fauna of Forth (p. 246) in interpreting the observations made at the Isle of May. This has been for years a well-watched island station, where we can be sure from evidence from both north and south of it, that migrant Herons must occur sometimes, if not regularly, yet these authors are only able to say that Herons are met with in every month of the year, but'that no definite evidence of migration appears to have been recognized. Besides many such stray observations made on different parts of our coastline and at island and rock stations, we have the positive statement by Nelson (B. of Yorks, p. 389) that he had annually in autumn seen Herons coming in from the sea from the east, flying at a great height and passing on inland. He seems to have been the only observer fortunate enough to have seen this event sufficiently often to be able to come to a positive conclusion. Having collected and analysed all such scattered observations and records I think that they form substantial grounds for concluding that there is an annual autumn and spring passage of continental migrants to and from the British Isles, more marked and of larger numbers in autumn than in spring. In autumn these migrants arrive *E. M. NicljolSon, Report on the " British Birds " Census of Heronries, British Birds, Vol. XXlI, p. 359.

2 VOL.XXXII] MIGRATORY STATUS OF HERON. 243 at irregular intervals and in varying, probably mostly small, numbers between the end of July and the beginning of November, chiefly during September, on the east coast from Shetland to at any rate as far south as Suffolk. As will be seen later on, from a consideration of the recoveries of ringed birds, this southern limit will have to be extended to the Straits of Dover at least. The spring passage, probably by the same route, takes place from the last half of March to June. From a consideration of these latter dates it must follow that the majority of these passage migrants must be immature non-breeding birds and this is borne out by Gatke's statement about Heligoland (B. of H., p. 455) " one or two young birds... are met with on some of the days of the general autumn migration, but old birds are seen comparatively rarely". The bearing of this point will be considered later on. After arrival there is evidence that a proportion of these migrants pass south through the western isles of Scotland to Ireland and down the west coast of England and Wales. Eagle Clarke's two observations at the Eddystone (i.e., I., p. 319) of birds passing south there in October suggest that winter quarters south of our islands are at any rate occasionally sought by these passage migrants. There are also a few similar records from Hanois (Channel Islands) and Eagle Clarke records the arrival of Herons at Ushant, though neither of these may necessarily relate to birds that have passed through the British Isles. With regard to the destinations of birds arriving on the east coast of Scotland and England there is no evidence from direct observation, but fortunately it is available as the result of recoveries of ringed birds.* From these it is clear that some of these migrants penetrate inland and scatter over a considerable area. One hesitates to assume that they cover the whole country, as most of the recoveries have been made in. eastern and southern counties, though individuals have reached Dumfries, Glamorgan, Devon, Cornwall and central and southern Ireland. These recoveries also, of course, give us information as to whence these migrants come, viz., S.E. and S.W. Sweden, S.W. and S. Norway, the North Sea coast of Germany, E. Denmark, Schleswig, W. Flanders and N. France, to which list no doubt Holland should be added, *Movements of Ringed Birds from Abroad to the British Isles and from the British Isles Abroad, by H. F. Witherby and E. P. Leach. British Birds, Vol. XXV, p. 175 ; Vol. XXVI, p. 356 ; Vol. XXVIII, p. 133 ; Vol. XXIX, p. 137 ; Vol. XXXI, p. 19.

3 244 BRITISH BIRDS. [VOL. XXXII. though there is no evidence at present. Of six birds from Norway, three reached Shetland by a course almost due west. One that reached Ross may also have arrived first of all in Shetland, while the other two subsequently recovered in Lewis and Cork probably did so as well and followed the west coast route already defined. All the other immigrants except the Belgian and French ones, which must have travelled west or to some point north of west, came by an almost pure south-westerly course. Of the twenty-nine recoveries twenty-three were of young birds less than a year old and the distances they covered from point of ringing to point of recovery varied between 250 and 800 miles, except the Belgian and French birds, whose flight direction as already mentioned differed from all the others. The distances covered by them were only from 120 to 200 miles. Coming now to the case of British Herons there is, perhaps naturally, an entire lack of direct observational evidence of migration. Fortunately a large number of nestlings have been ringed in this country and their percentage of recoveries (10.9) is amongst the highest of any birds.* There is, therefore, a considerable amount of fact to work on. In the first place recoveries include birds that were ringed at heronries covering a considerable proportion of the British Isles (except Wales) so that it may be assumed that in these recoveries we have a representative sample of our Heron population. The first point that emerges from a tabulation and analysis of the 185 cases considered is that 93, or almost exactly half of them, were recovered at a distance of 50 miles or less from their original homes, at periods ranging from a few months to over 15 years. A few of these, recovered in the early spring at an interval of a few years, should of course be left out of account, as they might quite possibly have been birds that had returned to nest in their home heronry, as actually happened in a single instance, after an interval of five years.t They do not, however, affect the conclusion that a high proportion of our native Herons are entirely sedentary. Of the remaining 92 recoveries 21 covered between 100 and 200 miles during the first nine months from ringing and eleven others distances of between 200 and 384 miles in the same period. One bird alone went farther, accomplishing about 780 miles *Recovery of Marked Birds, by H. F. Witherby and E. P. Leach, British Birds, Vols. Ill to XXXII. fow some Results of Ringing certain species of Birds, by H. F. Witherby, British Birds, Vol. XIII, p. 295.

4 VOL. xxxn.] MIGRATORY STATUS OF HERON. 245 in 7 months. Only twelve of the total, therefore, accomplished a journey at all comparable with that of the majority of our continental immigrants. It might, therefore, be concluded that some small proportion of our native birds are migratory. In confirmation or otherwise of this let us consider the directions that they took, for if there be any true migratory, movement, it is almost an axiom that this must be in some constant direction, with but little variation to one side or the other. This is clearly displayed in an analysis of the journeys of our continental immigrants. Taking the whole of the recoveries together we find that 49 (34-5%) travelled towards some point between N. and WNW., 40 (28.1% between W. and SSW., 32 (22.5%) between S. and ESE., 21 (14.8%) between E. and NNE., a result which, if it means anything more than casual wandering, does certainly not point to migration. As a more strict test let us consider the cases of birds recovered after three and up to eight months after ringing, a period during which, if they had migrated, they would be expected to be at or on their way to winter quarters. There are 68 of these and they give the following results: x 8 (30.5%) went between N. and WNW., 19 (32.2%) between W. and SSW., 13 (22%) between S. and ESE., 9 (15.2%) between E. and NNE. The percentages do not differ markedly from those of the first group. Of the twelve long-distance birds one went NE., two NW., one ESE., one SSE,, one SE., three SW. and three SSW. With these therefore there is obviously a slightly different result and one might be'tempted to hazard a suggestion that there is a tendency in some birds to a southward movement. The numbers, however, are probably much too small upon which to form such a conclusion, which also would certainly be invalidated by the absurdity that whereas birds from Sussex and Kent travelled between SE. and SW., others from Buckinghamshire went NW. The only view that one can take on the premises is that British Herons are residents and to a considerable degree sedentary, but with also a considerable tendency, especially amongst birds of the year, to disperse and wander, in the doing of which they may, though perhaps only in leisurely

5 246 BRITISH BIRDS. [VOL. xxxn. stages, accomplish a distance from their original homes comparable in length to a true migration. Perhaps it is not too far fetched to hazard a speculation that this tendency is the remains of the migratory habit that is still in process of dying out in our British race of Herons. From a consideration of the distribution of the species on the continent it is obvious that, from climatic and food procuring reasons alone, seasonal migration to some extent from some parts of its range is a necessity. In the British Isles on the other hand it is only irregularly and in quite restricted areas that these reasons would act and so the urge to migrate, which in the case of such a bird as the Heron may easily be less strongly developed than in that of many other species, might become in course of time altogether suppressed, or only manifest itself in a sporadic and irregular manner. A similar suppression of the migration habit in the British stock of species that are migratory on most parts of the continent appears to have taken place to a complete extent in the cases of our Jay, Magpie and * Chaffinch and to a greater or less degree in those of the Rook, Jackdaw, Starling, Mistle-Thrush, Blackbird, Robin, Mallard and some others. The probability that Herons do not breed until they are at least three years old possibly has some bearing, in that the absence of the breeding urge for that period may give scope for this wandering tendency to manifest itself, as apparently it does to its greatest degree, amongst juvenile birds. There are two other points that seem to have some bearing on this. The first is Gatke's statement mentioned above, that he practically only saw young birds passing through Heligoland. The second is the case of the small group of immigrants recovered in England from Belgium and N. France. The data of these six birds are very comparable with those of our British Herons in that the distances covered are comparatively short and the direction taken is widely different from that of other continental migrants. Necessarily they had to fly towards some point between west and north to reach England at all, but the fact that they did so suggests wandering rather than migration and it may be that the same process of suppression of the migratory urge is taking place there as has been suggested is happening here. Before coming to such a conclusion, however, it would, of course, be necessary to make a complete analysis of all recoveries of birds ringed in Belgium and N. France.