THE SPRING MIGRATION OF THE OVER EUROPE.

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1 (34) THE SPRING MIGRATION OF THE OVER EUROPE. BY H. N. SOUTHERN. REDSTART THIS study forms the third of a series of five whose object is to show the characteristic migrations of various widespread passerine species northward over Europe every spring. Maps showing the rate of spread of the Swallow (Hirundo r. rustled) and of the Willow-Warbler (Phyttoscopus irochilus) have already been published [i and 2], and the present map and article deal with the Redstart (Phanicurus ph. phasnicurus). This is an abundant and vigorous species, though only in some parts of its range attaining the commonness of the Willow-Warbler. Like the latter it feeds in the vegetation. This means that it probably will suffer no delay in its migration, once started, as might be occasioned in species which live mostly on flying prey (for a discussion of this point see the article on the Willow-Warbler [2]). The Redstart winters rather farther north than the bulk of the Willow-Warblers, being concentrated in North Africa, while a large proportion of the Asiatic population winters at comparatively high latitudes. Some are said to winter in Asia Minor, and, if this is true, it is curious to notice from the map that the spread northwards starts latest at this corner of Europe. It is interesting to note that we are dealing for the first time in this series with a species whose numbers in the British Isles are small, at any rate in comparison with the two summer visitors previously considered. This may be the reason why the quickest rate of spread is seen over the Continent, and there seems to be a slight lag in the bird's appearance on our shores. This contrasts with the Swallow especially and also with the Willow-Warbler, and such contrast can best be seen in the conformation of the April 15th line, which in the case of the two earlier species tilts up towards the west. In the Redstart the westward tilt starts over continental Europe, but declines again when the North Sea is reached. Thus, the main thrust in the spreading of this migrant northwards is through France, and then directly from Holland and north-west Germany across to western Scandinavia. It would appear from this that the British Isles receive only the wash of the main movement, thrown off along its flank, and therefore the arrival is comparatively later. If migratory behaviour is one result of changes in the

2 VOL. XXXIII.] MIGRATION OF REDSTART. 35 balance of internal secretions, and an expression of a more general raising of the level of activity by reason of these changes, then it is reasonable to suppose that in a crowd of birds this effect will be most sharply marked. Heightened activity on the part of one individual, whether in feeding or in any other competitive way, must react upon neighbours, and, where neighbours are thickest, the total resultant activity will be much more than the sum of individual exertions, if the birds were more spaced out. Thus the main drive of migration in the Redstart is earlier than the progress of sparser breeding populations on the edge of its range. It may be remarked here that the Redstart shows particularly well how careful one must be to distinguish between the general arrival date and isolated birds appearing much earlier. The analysis of the data showed such isolated records much more commonly than in the other species, and it seems as if this may be another manifestation of the same phenomenon. In a sparse population not only will the general level of activity be lower and later but it will be less well regulated and more ill-defined. This whole point of view accords well with the theories advanced recently by Fraser Darling [3], that breeding activity is synchronized and forwarded by the proximity of a number of individuals. The fact that these passerine migrants subsequently take up territory and subject themselves to intense isolation in no way invalidates the idea suggested. That is a subsequent phase of behaviour and belongs to a totally different environment. In any case Venables and Lack [4] have shown in the case of the Great Crested Grebe how closely related to each other are territorial and colonial behaviour. Such splitting of breeding behaviour into two radically opposed principles is phylogenetically speaking of late occurrence. That heightened activity may produce changes not only in behaviour but in morphology is known from the case of the migratory locusts, where the activity caused by the collection of great numbers induces the appearance of a form different in colour and shape as well as in behaviour (i.e., migratory as opposed to sedentary). CHARACTERISTICS OF THE REDSTART MIGRATION. The map has been compiled in exactly the same way as the previous two. Data were more numerous than for the Willow-Warbler, and the picture may be regarded as fairly complete. Records from the Mediterranean were difficult

3 36 BRITISH BIRDS. [VOL. xxxm. to trace, and many of them too dubious to be included, while there is a signal lack of information for Sweden, Norway and Finland are, however, so well supplied with observers that this creates little disturbance, The thick black lines represent the appearance and settling in of a Redstart population at fortnightly intervals, the dates being given on the right-hand side of the map. The dotted lines represent the northward movement of the 48 0 F. isotherm, as shown in Bartholomew's Physical Atlas [5]. The black squares give the points for which average arrival dates were worked out in most cases from a ten-year period. The migration of the Redstart over Europe lasts from March 15th (Pyrenees, south Italy and the north-east African coast) to May 15th (Inari), a period of 61 days, in which a distance of 2,000 miles in the west is covered and 2,500 miles in the east. This means that the average rate of spread varies from 33 miles per day in the west to 41 in the east. Thus it may be seen that the Redstart spreads northwards at a greater rate than either the Willow-Warbler or the Swallow, which only average 29 and 25 miles per day respectively. This progress is not quite regular, for the last part of the spread takes place at a much faster rate. Thus, from the Crimea to the White Sea (1,500 miles) a month suffices, and the average rate of progress is here at its greatest, 50 miles per day. Both the previous migrations dealt with have shown a similar tendency to accelerate as they go on, but neither of of them to the same degree. Even then the behaviour of the Redstart does not nearly approach that of some American birds mentioned in the first article of this series [1]. If the isochronal lines are compared with the isotherms, the lag along the western seaboard becomes even more pronounced, as also does the drive forward through France and western Germany. This is shown well in the lines for April 1st and April 15th. Apart from these anomalies there is a rough tendency for the isochronal lines to keep pace with the spring from April 1st to April 15th, but after that the isotherms are left behind, and arrival in the high north is six or seven weeks ahead of the 48 0 isotherm. In the east, however, the effect is marked in the wide gap between the isochronal lines for April 15th and May 1st, and between the isothermal lines for the same dates. It is evident that all migrants, as well as the spring, spread fast over the steppes of southern Russia, and that a similar synchronous effect is shown in the delay, which occurs in southern Sweden and Denmark.

4 VOL. XXXIII.] MIGRATION OF REDSTART. 37 Map showing Rate of Spread of the Redstart (Phmnicutus ph. phcenicurus) over Europe during the spring. The continuous black lines, dated at the right-hand ends, are isochronal lines, showing the stage achieved every fortnight. The dotted lines, dated at the left-hand ends, show the movement of the 48 F. isotherm for comparison. Each square represents a place for which an average arrival date was worked out from local data. {Projection ; Bonne's.]

5 38 BRITISH BIRDS. [VOL.XXXIII. The spread of the Redstart on its spring migration therefore is marked by : (i) a faster rate than either of the two previous species; (2) a lag along the western seaboard of Europe, possibly correlated with a sparser population in these districts, and a thrust forward through France and western Germany to western Scandinavia ; and (3) a marked tendency to outstrip the spring from April 15th onwards. REFERENCES. [1] SOUTHERN, H. N. (1938). " The Spring Migration of the Swallowover Europe." British Birds, Vol. XXXII, pp [2] SOUTHERN, H. N. (1938). " The Spring Migration of the Willow- Warbler over Europe." British Birds, Vol. XXXII, pp [3] DARLING, F. F. (1938). " Bird Flocks and the Breeding Cycle." Cambridge. [4] VENABLES, L. S. V. and LACK, D. (1936). " Further Notes on Territory in the Great Crested Grebe." British Birds, Vol. XXX, pp [5] Bartholomew's Physical Atlas (1899). Vol. 3. London.