Bergmann's rule and obligatory overseas migration, K Williamson

Tags: Greenland, Iceland, Fair Isle, BRITISH BIRDS, southern Greenland, LI, population, conditions, Black-tailed Godwit, Oenanthe oenanthe, Dansk Orn, Atlantic, Icelandic Merlin, Bergmann's Rule, environment, typical race, Greenland Wheatear, overseas migration, Europe, Iceland Redwing, Greenland Redpoll, Greenland Dansk Orn, climatic conditions, Greenland and Iceland, migratory birds, body-weight, southern Europe, weight records, western Europe, overseas flight, Thus Hemmingsen, Falco columbarius, Snow Buntings, passage migrants, British population, Continental races, MERLIN, tropical environment, gm, migratory species, weather conditions, Salomonsen, BRITISH BIRDS BERGMANN, natural selection, warm-blooded animals, selection pressures, selection pressure, British Trust for Ornithology
Content: VOL. LI No. 6
JUNE 1958
BRITISH BIRDS BERGMANN'S RULE AND OBLIGATORY OVERSEAS MIGRATION By KENNETH WILLIAMSON (Migration Research Officer, British Trust for Ornithology) INTRODUCTION IN THE STUDY of natural history there are well-known ecogeographical (sometimes called "climatic" or "ecological") rules relating certain characteristics of warm-blooded animals to the conditions of their environment. One of the best-known in its application to birds is Bergmann's Rule, which states that in a polytypic species the body-size of a subspecies tends to increase with the decreasing mean temperature of its habitat. Thus body-size, as indicated by an average lengthening of the wing, shows a tendency to increase with latitude; and the wings of populations which live at high altitudes tend to be longer than those of the same species residing in lowland areas. As Huxley (1942) pointed out, this is really a part of the more general principle that in warm-blooded animals body-surface relative to bulk tends to diminish with decreasing mean temperature of the environment; it is an adjustment compensating the organism, since the smaller the body the more heat it radiates to the colder outer air. This correlation has been criticised by Scholander (1955) on the grounds that there are more efficient adaptations for conserving body-heat, but their existence does not necessarily invalidate the hypothesis as a partial interpretation of Bergmann's Rule. There are some striking exceptions (Rensch estimates 16 per cent in Palaearctic birds) to the general rule, and the reasons for such exceptions are not always clear. There are many aspects to the problem, and the selection pressures which have combined to produce (or, in some cases, suppress) this "latitude effect" are manifold. Mayr (1956) insists that it must be regarded as " a p u r e l y empirical finding which 209
can be proven or disproven no matter to what physiological theoryone might ascribe this size trend", and he warns strongly against the search for an all-or-none solution to this complicated biological problem. The scope of the present discussion is a limited one, aiming to show that one of the contributory factors which must be taken into account in considering this "latitude effect" in certain species is the selection pressure exerted during the comparatively brief periods of the biennial migration. For, as Mayr has emphasized, the working of natural selection is particularly efficient during catastrophes and other periods of great environmental stress, so that even the short-term influence of a long migratory flight may be expected to play its part. Some consideration has already been devoted to the nature of this "latitude effect" in migratory species, though much of this has been concerned with the bird in its winter quarters and little of it with the actual journey which takes it there, and which returns it to the breeding-ground in spring. Rensch (1939) showed that a size-correlation could often be found with the minimum winter temperature of the environment, selection being exerted by the most rigorous life-conditions. This suggests that in migratory species the body-size of a northern population is likely to be a function of selection due to the off-season environment rather than the high latitude of the breeding-area. Thus Hemmingsen (1951) proposed that if we are to test the validity of Bergmann's Rule in migratory species, it is to the winter rather than the summer range that we must look for an answer. H e further pointed out that the timing of migration might also be involved since some subspecies, and also members of closely-allied species, return earlier in the season, under colder weather conditions, than others--as, for example, the Curlew (Numenius arquata) and Whimbrel (N. phaeopus), the Slenderbilled Knot (Calidris tenuirostris) and Common Knot (C. canutus). In these couples an association is apparent, on the one hand, between early spring movement, greater body-size and relatively northern winter quarters, and, on the other hand, a later migration in milder weather, smaller body-size and relatively warmer and more southerly wintering grounds. Salomonsen (1955) has also discussed at some length the effects due to selection pressure in the winter range, and has shown convincingly that there are cases in which a marked correlation exists between Bergmann's Rule and the rigour of conditions in the winter environment. This is especially so with those races of a species which practise allohiemy, or which (in other words) are segregated in different regions outside the breeding-season. Once such allohiemy has been established, differences in the selective factors operating in the two regions will tend to encourage subspecific differentiation. The Ringed Plover (Charadrius hiaticula) affords an excellent example, for here we find a reversal of the
normal "latitude effect" in that the smallest race tundrae nests farthest north in the total range; however, its migration "leapf r o g s " the range of the typical race, and its smaller size is really a response to the warmth of a tropical wintering-area. Salomonsen sees a similar state of affairs in the races of the Redshank (Tringa totanus), but there is a difficulty, which we shall refer to again below, in the case of the Icelandic form. Salomonsen's contribution is of great importance to the study of evolution and subspeciation in b i r d s ; but there remain certain populations, notably among the land-birds of Greenland and Iceland, whose divergence from their Continental relatives cannot be satisfactorily accounted for on his, Hemmingsen's, or any other hypothesis so far advanced. There are striking cases among these migratory land-birds in which it seems probable that selection operates most powerfully during the brief migratory periods; for, in comparison with their Continental relatives, they are called upon to perform twice yearly an obligatory overseas flight, with all its attendant hazards of migrational drift away from the most direct route. The rigours of these long flights are such that survival favours the more robust individuals possessing the greatest resources of energy (stored glycogen and fats); so that in the course of many generations the stock has developed a greater body-size by comparison with populations following coastal or overland migration-routes. This increased size is primarily apparent in a greater weight, and secondarily in an allometric increase in the length of wing and tail. It is worth while looking more closely into this situation in respect of certain breeding-birds of Iceland and Greenland, whose brief arctic summer is passed under conditions not as a rule more rigorous than those obtaining at similar latitudes in continental Europe. The fullest discussion will be given to the most striking example, the Wheatear, and a number of other species will be discussed at shorter length, with a view to determining to what extent they fall into line with the above hypothesis. WHEATEAR (Oenanthe oenanthe) The typical race of the W h e a t e a r breeds over the whole of Europe, extending north of the Arctic Circle, while a large form Oenanthe oe. leucorrhoa is a common breeder in the low Arctic region of Greenland, and also nests, though more locally, in the southern part of the high Arctic region. Salomonsen (1950-51) says " T h e Wheatear is much more common in the interior fjord country with a dryer and warmer climate and a rich insect-life than in the coastal areas, where it is rather scarce. It has its greatest population density in the south (Julianehaab District) and becomes less numerous towards the north". The interior fjord country of the Julianehaab District is actually sub-Arctic in climate, and at about 61 °N. is considerably south of the Arctic Circle and indeed at much the same latitude as Shetland. Wynne-
Edwards (1952) found this form abundant at the head of Clyde Inlet, Baffin Island, about 70°N., and it nests sparingly in other parts of the east Canadian Arctic. The Wheatear is common in Iceland and the Faeroe Islands; in the south of Iceland and Faeroe the bird's are intermediate between oenanthe and leucorrhoa and have been named Oenanthe oe. schJ0leri, Table I gives wingmeasurements of these populations and is taken from Salomonsen
Males No. Range (mm.) Average
Females No. Range (mm.) Average
Faeroe Is.
102-110 99-107 97-103 92-99
105.00 102.55 99.61 96.38
"J3'37 98.81 97-54 93.22
A group of W h e a t e a r s , of medium wing-length and decidedly heavier than the local stock (and presumably the so-called schi0leri of south Iceland and Faeroe origin) migrates through Fair Isle regularly each autumn during the last ten days of August, Fair Isle and Shetland birds are smaller and are referable to oenanthe. The Greenland Wheatear (including the Canadian Arctic stock) migrates through Western Europe and winters mainly in tropical West Africa, along with birds of the typical race. The majority of the Greenland birds arrive on their breeding-grounds in middle and late May, but many do not reach the northern limits till early June (Salomonsen, 1950-51). This agrees well with the passagedates at Fair Isle, movement being evident in early and mid-May, and occasionally as early as the last week of April--though Ticehurst (1909) gives this as the normal time of arrival in southern England. This means that the bigger race is subject to " w a r m e r " climatic conditions during its spring journey than the typical form, which reaches Britain late in March and continues to pass throughout April; and thus the Wheatear provides an exception to Hemmingsen's rule that in related forms the smaller one moves later and under more congenial weather conditions. In autumn there is a difference of up to a fortnight, local and Shetland birds leaving in bulk from mid-August, the intermediate type passing during the last week or so of the month, and the big leucorrhoa from the beginning of September. It is interesting to consider the migration in relation to the greater body-weight of the Greenland race. Spring passage at Fair Isle is steady and protracted, with birds much less numerous than in autumn, when of course the majority are young of the year. (1956 showed a reversal of this situation, the spring migration being quite exceptionally heavy, and the fall migration unusually slight.) At both seasons, specimens of leucorrhoa show a wide
variation in weight (Table II), though in general the heaviest birds pass in the spring and the lightest occur (often in fairly homogeneous groups) in the fall (note the low weights in Table IV). It is possible that the light spring birds reach Fair Isle as the result of wind-drift when attempting to cross the Atlantic from an Irish or west-coast starting-point, while others apparently come in with Continental migrants displaced from the North Sea coast and Skagerrak by easterly winds.
Heavy Bird s
No. Range (gm.) Average
Light Birds No. Range (gm.) Average
·949 1950 I951 '9S2 1953 ·954 1955
4 29.20-37.40 5 29-U-33-33 3 30-I3-3S-6S 8 28.00-38.08 S 33.62-43.19 6 27.21-38.81 7 27-43-37-39
33-63 30.67 32-38 32-S2 37-75 32.01 32.82
5 21.45-24.71 1
3 21.48-26.07
23.07 26.13 24.94 24.50 24.18 23-95 24.88
For the most part, however, these spring birds are pursuing a northwards course under optimum conditions, for much of their migration takes place when there is anticyclonic weather over the British Isles. Since they reach Fair Isle from the north of Scotland and Orkney they have made only a negligible sea-crossing, and may be presumed to have expended very little of the reserves of energy built up by an adequate food-supply whilst travelling through Britain. The opposite situation occurs in autumn when col or anticyclonic weather provides calms or light airs to the north, and in some seasons a vast W h e a t e a r movement is released by such weather in the Faeroe-Shetland region in the last week of August. As is to be expected, the best weights are recorded in spring, with individuals occasionally exceeding 40 gm.--viz. cf cf, 30th April 1953, 43.2 g m . (wing 109 mm.), 5th May 1956, 41.2 g m . (wing 107 m m . ) ; 9 9 4 t n June 1953, 42.7 g m . (wing 98 mm), 5th May 1955, 41.1 g m . (wing 97 mm.). A detailed analysis of the weights of Wheatears captured in the spring at Fair Isle has been undertaken by Alec Butterfield.and this reveals a striking difference between leucorrhoa on passage and locally-resident oenanthe (Table III).
Males leucorrhoa oenanthe
Females leucorrhoa oenanthe
Numbers of specimens Mean Weight in gm. standard deviation Ratio of Mean Weight to Standard Deviation
30 30.98 5.62 S-5I
4S 24.1 r 2.38 t o . 13
24 30-25 4.41 6.86
35 23.80 2.08 11.44
W e should expect the weight of leucorrhoa, before setting forth in autumn, to be at least as much as that of spring migrants trapped under ideal anticyclonic conditions. In the absence of a series of weights collected on the breeding-grounds between the finish of moult and the birds' departure, our discussion must be limited to the samples available from passage migrants at Fair Isle. There is an even wider variation than in the spring, and the maximum records do not g o quite so high, the majority of those passing through in comparable anticyclonic weather registering around 35 gm. As these birds have had fairly long stretches of the sea to cross on the "inter-islands" route via Faeroe and Shetland, somewhat lower weights are to be expected. Sometimes, however, there -are incursions of long-winged birds at exceptionally low weights, around 20 g m . only; and the fact t h a t groups of these lightweight birds appear quite suddenly on the island suggests that an explanation for their condition should be sought in their recent migrational history. The events of early September 1953 present an excellent case for analysis. An interesting dual arrival of W h e a t e a r s took place between the 1st and 3rd, at a time when the picture was unlikely to be confused by the presence in the catch of local birds, since the majority of these had already departed. Trappings were about equally divided between long-winged, long-tailed leucorrhoa and small oenanthe with measurements less than the means for the local population. The meteorological setting showed an Atlantic low moving north-eastwards between Greenland and Britain, with a warm front extending across the North Sea from northern Scotland to the Skagerrak (Fig. 1). With strong anticyclonic developments over both Greenland and Norway conditions were ideal for departure from both countries, and the situation of this low resulted in a cyclonic westerly airstream over the Atlantic and an easterly one ahead of the warm front in the North Sea. W i t h the passing of a low to Norway on 4th September, and the advance of another towards southern Greenland, conditions inhibiting further migration reached both countries, and a long cigar-shaped ridge of a Central European high extending northwards beyond Shetland and Faeroe brought ideal migration weather, with calms and lightto-moderate northerly breezes, to the "inter-islands" route (Fig. 2). These changes effectively sealed off further influxes of Greenland and Continental Wheatears, and resulted instead in the passage through F a i r Isle on 5th September of intermediate schi0leri-type W h e a t e a r s from the islands to the north. This was, in effect, a resumption of the normal late August passage of such birds, which had been effecti%'ely blocked by the passing depression of ist-3rd. The birds of medium wing-length on 5th September showed the best weights, averaging close on 28 gm. (Table IV C). Although the leucorrhoa of ist-3rd must have been substantially heavier than this, and the small oenanthe lighter, at the start of their respective
Fig. I
Fig. 2
FIGS, I and 2--AUTUMN MIGRATION OF WIIEATEARS (Oenanthe oenanthe) Weather-charts for mid-day on 1st and 4th September 1953, to illustrate (Fig. 1) the cyclonic arrival at Fair Isle, on 2nd September, of oenanthe from the east and leucorrhoa from the west; and (Fig. 2) the resumption of passage at Fair Isle, on the 5th, of schi0leri intermediate birds in an anticyclonic ridge (see page 214).
journeys, both forms were extremely light and in fact about equal in weight on arrival at Fair Isle (Tables IV A and B). The small birds had made a fortuitous sea-crossing of 300-400 miles in the easterly airstream ahead of a warm front; the large ones, whose loss was proportionately very much greater, had made a transAtlantic crossing in the cyclonic wind-stream on the other side of the depression, covering a distance which seems likely to have been of the order of 1,600 miles. To complete this picture of the correlation between body-weight and the synoptic pattern of the migration, it might be added that some intermediate ("schi0leri") birds were trapped on 8th September in circumstances suggesting a similar cyclonic passage round the western half of a small low centred about 6o°N. 4 o 0 W . with a warm front and west wind reaching eastwards to Fair Isle. The drift-track for these birds, on a down-wind reckoning, would be of the order of 1,200 miles or about twice the airline distance via the "inter-islands" route, and it is interesting to compare their low arrival weights (Table IV D) both with the records of the better-favoured "schioleri" of the 5th, and the leucorrhoa of the ist-3rd. Wheatears of the Greenland race are frequently encountered out in the Atlantic in spring and autumn, and the large form is known as an accidental visitor or passage migrant in the Azores and Canary Islands, its occurrence in the latter archipelago showing a September peak (Bannerman, 1919). The spring and autumn distribution of records at sea has been plotted by Snow (1953), who concluded that crossings are made regularly in autumn from southeast Greenland direct to western Europe, while in spring " t h e shortest possible sea-crossing is normally made, the birds travelling northwards, largely overland, up through western Europe and the British I s l e s " . Such records as I have examined in juxtaposition
I A ) -- G r e e n l a n d birds (Oe. oe. leucorrkoa) Age/Sex Wing Tail Weight (mm.) (mm.) (gm.)
CB)--Continental birds (Oe. oe. Age/Sex Wing Tail (mm.) (mm.)
oenanihe\ Weight (gm.)
1 st w.
Ad. 9
Ad. rj
Ad. $
Ad. d
I St. W.
Ad. d
Ad. 9
Ad. <$
1 st w.
Average 103
1st w.


Ad. d
1st w.

Average 93

5*1 22.28
(C)--Iceland-Faeroe birds (Oe. oe. " s c h i 0 l e r i " ) Age/Sex Wing Tail (mm.) (mm.)
Weight (gm-)
Ad. S
1st w.

Average 96
55 55 57 56 55 ssh
30-50 25-44 26.70 29.83 27- iS 27.92
8. ix .. ,, 9 IX
(D)--Iceland-Faeroe birds (Oe. oe. "schi0leri") Age/Sex Wing Tail (mm.) (mm.)
We ght (gm-)
1st vv.
Ad. 9
2 2 25
2 3 19
22 22
23 20
Average 96
2 2 7»
to the appropriate weather charts substantially confirm Snow's conclusions. There is a crop of interesting records, from two sources, of spring birds in the Atlantic between 8th and n t h May 1952. A. P . Ryan, on board the weather-ship Weather Observer at 52°3o'N., 2 0 ° W . , saw 6 birds on 8th May, of which 4 came on board " i n poor condition--very t i r e d " , and 2 flew past, separately, heading north-east. Single Wheatears were also seen on the 7th and the 9th, and there had been a Greenland 9 > wing 100 mm., on the 5th. The wind was N.N.W, on the 5th-6th, veering northerly on the 7th and further to the N . E . 'on the 8th as a depression moved eastwards (Fig. 3): its force decreased from gale on the 6th-7th to force 4 on the 8th, a fair but cloudy day. A number of birds were also seen by E . F . Aikman, westwardbound in Empress of France on 9th and 10th May between S3°N. 3o°W. and 49°N.47°W. The weather was overcast and rather misty and the ship was just clearing the region influenced by this depression ( F i g . 4). W h e n Weather Observer was returning to base on n t h May 3 Wheatears came on board at 54°N.i4°W. (Fig. 5)-
Fig. 3
Fig. 4
Fig- 5 FIGS. 3, 4 and 5--OCCURRENCES OF WHEATEARS (Oenanthe oenanthe) AT SEA Weather-charts for mid-day on 8th, 9th and n t h May 1952, to illustrate the conditions that resulted in the arrival (Fig. 3) of six Wheatears at a weathership on 8th May; (Fig. 4) of several at R.M.S. "Empress of France" on the 9th; and (Fig. 5) of three at a weather-ship on the n t h (see page 216). In each case the position is marked " X " . It seems clear from these records that a large number of W h e a t ears, attempting a migratory flight from the Hebrides or Northern Ireland to either Greenland or Iceland, were deflected from their course by the approach of this depression, and were carried downwind in the easterly airstream of its northern perimeter, some being blown very far west. It is worth noting that wind and sky conditions were excellent all over Scotland, the Hebrides and Northern Ireland on the night of the 7th/8th, but that during the whole period fronts active in the Shetland-Faeroe region brought fog and drizzle to this section of the route. T h e 3 birds noted by A. P . Ryan on n t h May in the westerly airstream south of the now northwards-moving low may well have travelled all round this depression until brought within striking-distance of western Ireland. Of the large number of autumn migrants recorded at ships many appear to have been making a cyclonic approach to western Britain, and could be traced back to southern Greenland on a down-wind' track. In some instances Snow Buntings (Plectro-
phenax nivalis) were observed at the same time. W a t c h e r s in various weather-ships (F. R. Allison, M. A. Barras-Smith, A. Darlington and M. L. R. Romer) during September 1950 supplied records to British Birds (Anon., 1951), all of which fall into this category. There were 2 Wheatears at 59°N.i9°W. on 12th September and 9 at 52°N.2o°W. next day (Figs. 6 and 7). Birds moving down-wind through this depression could have reached the west and north of Scotland, and it is interesting to note that 8 long-winged birds trapped at Fair Isle between the 12th and t h e 14th showed a weight-range of 22.3 g m . to 26.7 g m . (average, 25.1 gm.), or about 10 g m . below the expected weight for leucorrhoa at this season.
Fig. 6
Fig. 7
FIGS. 6 and 7--OCCURRENCES OF WHEATEARS (Oenanthe oenanthe) AT SEA Weather-charts for mid-day on 12th and 13th September 1950, to illustrate the conditions that resulted in the arrival (Fig. 6) of two Wheatears at a weather-ship on 12th September; and (Fig. 7) of nine at a weather-ship on the 13th (see this page). In each case the position is marked " X " .
Two Wheatears spent 20 minutes aboard at the same station on 22nd September, and of 9 Snow Buntings seen that day 6 passed the ship flying south singly or in pairs. These appear to have travelled south through a col after leaving Greenland and entered a westerly airstream in the complementary wind-system between a trough in the Iceland-Faeroe seas and an Azores high farther south (8 Snow Buntings were also seen on 29th under similar cyclonic conditions). Two very late Wheatears seen by A. P . Ryan at 52°3o'N. i7°2o'W. on 21st October, with rain falling and a fresh S . W . wind (Fig. 8), and 3 recorded at 49°22'N. 3 4 ° i 5 ' W . by Goodwin (1954) on 16th September 1949 (Fig. 9), are other cases of cyclonic migration. W h e n H . M . S . Vidal w a s returning from the annexation of Rockall on 20th/2ist September 1955, and was about half-way between south-west Ireland and Cornwall, a flock of about 24 W h e a t ears came aboard soon after midnight, very tired (Fisher, 1956). Those handled were leucorrhoa, and Fisher suggests they were cyclonic migrants from Greenland. Before 0800 hours the previous
Fig. 8
Fig 9
FIGS. 8 and 9--OCCURRENCES OF WHEATEARS (Oenanthe oenanthe) AT SEA Weather-charts for mid-day on 21st October 1950 and 16th September 1949, to illustrate the conditions that resulted in the arrival (Fig. 8) of two Wheatears at a weather-ship on 21st October 1950; and (Fig. 9) of three at a ship on 16th September 1949 (Goodwin, 1954) see page 218). In each case the position is marked " X " .
day 3 leucorrhoa at very low weights had been trapped at Fair Isle--2 adult 9 9 a t 21.1 g m . and 20.4 g m . (wings 101 and 103 mm.), and a ist-winter cf at 23.2 g m . (wing 100 mm.). It is apparent from the synoptic situation that these birds were finishing a long approach to Britain round the southern periphery of an eastwards-moving low situated a good way west of the Hebrides, a n d t h e distance flown m a y well have been of t h e order of 1,800 to 2.000 miles (Fig. 10).
FIG. 10--OCCURRENCE OF WHEATEARS (Oenanthe oenanthe) AT SEA Weather-chart for mid-day on 20th September 1955, to illustrate the conditions on the day before the arrival of two dozen Wheatears on board H.M.S. "Vidal", south of Ireland, on 21st September 1955 (Fisher, 1956) (see page 218). There is thus abundant evidence in the meteorological correlation of W h e a t e a r records at sea in the North Atlantic, and of movements passing through Fair Isle, of extended cyclonic
journeys, and of the need for a greatly increased capacity in the Greenland and Iceland stocks for the storage of glycogen and fats to withstand the severe drain on their resources caused by long overseas flights. These flights may well be in excess of 1,000 miles in most cases--and much nearer 2,000 miles in some--and may reduce the Greenland Wheatear to a weight which is even less than the normal for the much smaller oenanthe. It will be seen from Table III that the range of possible weights for leucorrhoa is almost twice as great in comparison with the mean as that of the typical race--implying that the northern population is able to sacrifice a greater proportion of its total body-weight during this hazardous migration without falling below the threshold (the "lebensminimum") beyond which recovery is impossible. REDWING (Turdus musicus) The Iceland Redwing (Turdus m. coburni) breeds at approximately the same latitudes as the bulk of the Scandinavian population of typical musicus. The Icelandic race is bigger, the difference being most pronounced in body-weight, and reflected in length of wing and tail: viz., mean wing-length 121.3 mm. and tail-length 85.2 mm. in coburni, against 116.6 mm. and 79.9 mm. in musicus (Williamson, in press). A substantial difference in body-weight can be demonstrated from the laboratory records of samples trapped when on migration through Fair Isle. In autumn 1956 over 300 birds were weighed and the average difference was 5^ g m . in favour of coburni. In previous seasons, with fewer coburni trapped, a difference of up to 10 g m . (1954) was noted. The mean weight of a sample taken on any one day varies according to the type of weather and the length of the journey involved, as with the Wheatear, and in 1956 we were dealing largely with musicus which had made a short sea-crossing in ridge or col weather from western Norway, and coburni which may well have travelled three times that distance as cyclonic migrants. There are only a few weight records for spring migrants, and these suggest a norm for musicus of around 70 g m . and for coburni of up to 90 gm.--viz., 10th March 1950, 89.2 gm. ; 25th April 1950, 91.7 g m . ; 6th April 1952, 86.6 gm. Scandinavian Redwings winter over most of western and southern Europe, including the whole of Britain. The migration of Scandinavian birds is rather "fanned o u t " , as Holgerson (1953) puts it, so that many reach Italy and the central Mediterranean countries. Recoveries of birds ringed as young in Iceland' have been made in winter in Ireland and the Outer Hebrides, and a coburni ringed on passage at Fair Isle in October 1955 was found in November 1956 in Co. Kerry. Another autumn migrant, wing 122 mm., ringed during an influx of Iceland Redwings and Merlins on 14th October 1953, was found dead' in November near Antwerp, Belgium; and although Molyneux (1930) includes Holland and France in the wintering-range these Continental occurrences may
be merely a slight overspill from the main headquarters in the British Isles. It could be claimed that since the influence of a warm Mediterranean climate is enjoyed by a large number of Continental Redwings, there is partial allohiemy with the Icelandic race; but it seems hardly likely that selection-pressure due to this spatial separation can be very great, since equally a vast number of musicus share with coburni a mild Atlantic climate. Furthermore, the Continental race is either peculiarly prone to the vagaries of drift or has an unusually nomadic instinct, since many individuals (perhaps the majority) seem to change their wintering-area from year to year. Thus, birds marked in winter in Gloucestershire (1934), Worcestershire (1937) and Cornwall (1939) were all recovered in the following winters in northern Italy (Leach, 1941); and a bird ringed at Gibraltar Point Bird Observatory, Lincolnshire, on 1st November 1952 was recovered at Kyrenia in Cyprus on 13th February 1955 (Spencer, 1955). I can find only one record of a bird returning to the same winter-quarters in Britain in successive years, at Shrewsbury on 10th February 1947, and 22nd January 1948. If such wide dispersal over the winter range, with frequent alteration between the Atlantic and Mediterranean types of climate, is in any way usual, then the "winter-influence" in the adaptive variation must be reduced to a minimum and should tend to diminish any gap in size between musicus and coburni. A study of the meteorological environment of the autumn influxes of Icelandic birds (Williamson, 1953a and in press), shows that the situation in this subspecies is very similar to that in the Greenland Wheatear. "Island-hopping" migration takes place when col and anticyclonic weather affect the direct route via Faeroe and Shetland, but there are frequently longer cyclonic journeys, and since the Redwing is a late autumn migrant the majority of the arrivals belong to this kind because of the rapidly deteriorating weather in the north-east Atlantic in October. There is good cause for believing that at least some movements of coburni take place in two stages, culminating in an overseas flight from southern Greenland. This would appear quite clearly to be another case of the north-western population having developed greater body-size as the result of strong selection-pressure due to a mandatory overseas migration. REDPOLL (Carduelis flammed) The redpolls have a holarctic distribution, forming an emergent interspecies which most taxonomists treat as two distinct species and split up into several recognizable forms. They comprise an Arctic group (hornemanni and exilipes) a sub-Arctic group (rostrata and islandica, and perhaps also pallescens), and a boreal group (flatnmea, and the Lesser Redpolls disruptis and cabaret). The two Lesser Redpolls are isolated in the wooded country of the
British Isles and the mountain system of south-central Europe respectively, and present no problem. The pale, white-rumped Coues's Redpoll, exilipes, inhabits the Arctic scrub of Europe, Asia and North America, and Hornemann's is a larger edition of it in the high Arctic regions of Greenland and Baffin Island. The typical race, the Mealy Redpoll, extends across Europe, Asia and North America as a forest bird south of the range of exilipes; the American population, pushing eastwards, has colonized parts of Baffin Island' and low Arctic Greenland and become the larger, darker, thick-billed rostrata. In years of late spring many exilipes breed south of their normal limit and within the range of the Mealy Redpoll, and the white-rumped pallescens recorded from Arctic Norway is probably a hybrid form. The Iceland bird appears to be an intermediate population in which extreme types are hardly separable from the very pale hornemanni on one hand and the dark rostrata of southern Greenland on the other. Salomonsen (1951) has shown that, with the amelioration of the climate in southern Greenland, rostrata is pushing its range farther north into the breeding zone of Hornemann's Redpoll, and interbreeding occurs where they meet, so that the trend which produces pallescens in Northern Europe is here reversed. Iceland, being within the migratory orbit of both hornemanni and rostrata, seems to have captured both and welded them into a hybrid population, islandica; and with an intermediate form occupying so great an area the recognition of two " s p e c i e s " of redpolls, very similar morphologically and in habits, does violence to taxonomic principles, as Salomonsen has shown (1928, 1951). T h e largest race, hornemanni, and the isolated forms inhabiting Britain and the alpine region of southern Europe are only partially migratory and do not move far: they are not considered further in this discussion, except to say that Hornemann's Redpoll returns to the breeding-area from its low Arctic winteringgrounds some 10 days t o a fortnight ahead of the slightly smaller rostrata (Salomonsen, 1951; W y n n e - E d w a r d s , 1952)--so that in this instance Hemmingsen's rule applies. The extent to which the Icelandic bird migrates is not known: certainly a number stay in Iceland throughout the year, but it is possible that others are numbered among the rostrata which reach Britain from southern Greenland in the fall. This bird and the Mealy Redpoll of northern Europe are the only forms which are in any sense strongly migratory, and they breed at much the same latitudes. The Greenland Redpoll is the bigger bird, Salomonsen (1951) giving these wing-measurements: rostrata, cfcf 77-83 mm., 9 9 75-82 m m ; flammea, cfd* 7 1 78 mm., 9 9 69-75 m m - The g r e a t majority of rostrata depart from West Greenland during September to winter in the eastern provinces of Canada, whilst the east-coast population winters for the greater part in Iceland, Scotland and western Ireland. The Mealy Redpoll has only a short overland migration, wintering for
the most part in middle Europe and in some years in fair numbers in the British Isles. For practical purposes the two forms can be regarded as allohiemic; there is little difference in the latitude of the wintering range but it should be emphasized that the larger form enjoys the milder, Atlantic climate, and the smaller Mealy Redpoll is exposed to the colder, Continental one--so that in this case there is an apparent reversal of the expected size-trend. W i t h 4 exceptions, the 41 weight-records of redpolls obtained at Fair Isle between 1948 and 1955 are of birds identified with rostrata on plumage characteristics and their strong, bulging bills. T h e 4 Mealy Redpolls give from 13.8 to 15 g m . , average 14.4 g m . ; and Weigold (1926) gives a longer series, 19 examples ranging from 12 to 17.5 g m . , average 14.4 g m . He adds a "lebensm i n i m u m " of 9.5 g m . for an exhausted bird. These were migrants at Heligoland, so had not travelled far. The 37 records {including recaptures) of rostrata a t F a i r Isle give a range of 10.7 to 21.2 gm., average 16.3 gm. Birds retrapped whilst "off passage" show gains of between 20 and 40 per cent, of their initial arrival weight over periods of from 9 to 17 days, and suggest that the normal weight of this subspecies must be close on 20 gm. (see Table in Williamson, 1956). Birds have been seen occasionally at s e a : 3 rostrata flew aboard a ship at 6o°N. 23°3o'W. on the afternoon of 29th August 1955 in cyclonic weather (R. Meinertzhagen, pers. comm.), and small flocks were noted about 100 miles west of Faeroe in June 1929 (Saemundsson, 1934). The synoptic picture in the north-east Atlantic at the time of their immigration at Fair Isle is frequently indicative of down-wind drift from south-east Greenland in cyclonic airstreams, and several movements of this type were a feature of an unusually large and protracted irruption which took place in 1955 (Williamson, 1933b, 1956). In these respects the migration into Europe affords a close parallel with the other cases considered in this paper, and this is apparently a further instance of selection working through the medium of the migration-flight to produce a greater body-size and longer wing. That the effect is less strongly marked than in the case of the W h e a t e a r and Redwing may be due to the fact that a part of the Greenland community winters in the New World and has only the comparatively narrow Davis Straits to cross. OTHER PASSERINES The Lapland Bunting (Calcarius lapponicus) is predominantly a low Arctic breeder in Greenland. The Snow Bunting is a common summer visitor to the high Arctic as well as low Arctic regions and is the only one to nest in Iceland, where it is differentiated by the name insulae on plumage characters. In Greenland, according to Salomonsen (1950-51), both species seek the warmer and drier anticyclonic climate of the interior fjord country and are relatively scarce in the less benign coastal regions.
Greenland Snow Buntings cannot be separated from the populations of Canada and E u r o p e : birds from north-west Greenland are slightly longer in the wing (cf cf, 107-118 mm.) than European (cfcf, 105-115 mm.) but are no bigger than pallidior from Siberia. Greenland " L a p " Buntings have been distinguished from the typical race as subcalcaratus on account of slight plumage differences coupled with a more robust bill. The wing is a little longer on average, Horring (1937) giving for 52 Greenland c? cf 91-101 m m . , and for 20 European o* cf 90-96 m m . At best, however, subcalcaratus is a poorly defined form. In these two buntings, therefore, there is but little tendency towards a greater size as compared with the corresponding Continental populations, despite the fact that a minor part of the Greenland population (probably from the east coast only) crosses the Atlantic to Europe under weather conditions similar to those already described for the Wheatear, Redwing and Greenland Redpoll. The synoptic background of a phenomenal Lapland Bunting invasion of northern and western Britain in September 1953 was studied by Williamson and Davis (1956), and it was shown that this must have originated in southern Greenland and that several of the movements in that and previous years took place under cyclonic conditions. Ten Snow Bunting recoveries show movement from west Greenland to the eastern provinces of Canada (one to Minnesota, U.S.A.), and a recent record concerns a spring migrant ringed in north-east Greenland in May 1955 and recovered in Archangelsk District of Russia in April 1956. The most recent list also notes the only recorded movement of a ringed Lapland Bunting, from Egedesminde District in the west to the interior of southern Canada. It is highly probable that the bulk of both populations winter in the New World and show little increase in robustness since they have only the short sea-crossing of the Davis Strait to make. Two other species which are common breeding-birds in Iceland and northern Europe, and which are strongly migratory, are the Meadow Pipit (Anthus pratensis) and W h i t e W a g t a i l (Motacilla a. alba). The former winters largely in the Mediterranean basin, the latter crossing it to winter in tropical Africa. It is not known if the Icelandic populations are synhiemic with the Continental, but this seems very probable in the case of the wagtail at any rate. These Icelandic populations must make an obligatory overseas flight, and passage at Fair Isle under the same weather conditions as reported for the previous species is probably derived from this source. In neither species does the Icelandic bird differ from the Continental in size, so far as is known. It is worth while noting that the W h i t e W a g t a i l does not fit into Salomonsen's theory, since it has a "leap-frog" migration over the winter range of the Pied W a g t a i l (Motacilla a. yarrellii)-- the British race, some of which cross the Channel to winter in France--to Africa, and ought therefore to be smaller if the
"winter influence" of a tropical environment is strong. It may be that the tropical "winter-influence" is countered in this subspecies by the potential for greater size due to the need to survive a long overseas hop from Iceland to Britain, so that one effect balances the other. However, it should also be borne in mind that the recent spread of both Meadow Pipit and W h i t e W a g t a i l to eastern Greenland suggests that their presence in the north-west may be the result of a comparatively new, and continuing, expansion of range, and that the time-lapse since their arrival has been too brief for subspecific differences to develop. MERLIN (Falco columbarius) The Icelandic Merlin (Falco c. subaesalon) breeds between the same latitudes as many Falco c. aesalon of Scandinavia and northwest Russia: it is considerably more robust than the Continental race, the mean wing-length of cfcf being 209 mm., and of $ $ 227 mm., against 197 mm. and 215 mm. respectively for the European bird. The joint non-overlap between the two races is 96 per cent, in cf d1 and 94 per cent, in $ 9 (see Table and discussion in Butterfield, 1954)- A series of measurements is also given by Salomonsen (1935). There are some limitations in the use of weight-records for a comparison of the Icelandic and Continental races, since the weight of any individual bird-of-prey may be expected to vary within a fairly wide range. Obviously, a newly-arrived migrant at Fair Isle would register much less than an "off passage" bird which had made a recent kill, especially as the prey is sometimes as big as a Redwing. When all the available weights are averaged, sex for sex, including records of the same individuals if recaptured, it is probable that these inequalities are to some extent reduced. If the figures are comparable, then it would appear that subaesalon is considerably heavier than aesalon-- viz., 21 cfcf, 182 g m . and 21 $ $ , 255 g m . , against 9 cfd% 175 g m . and 11 $ $ ? , 210 g m . Kluz {1943) gives for an unspecified number of aesalon a range of: c?cf> 150-180 g m . , $ $ 188-210 gm., the upper limit in each case being less than the average for subaesalon at F a i r Isle. The sedentary British population shows a c? wing-length close to the mean for Continental birds, but a $ wing-length nearer that of Icelandic ? $ . The Continental race winters over much of Europe south to the Mediterranean, while the Icelandic population spends the off-season mostly in western Britain with (in all likelihood) some slight overspill on the Continent between southern Norway and France. Four recoveries of birds ringed as young in Iceland are known from Eire (Offaly), central and south-west Scotland (Stirlingshire, Dumbartonshire) and north-west England (Lancashire), and there are specimens of this race in museum collections from most Scottish and many English and Irish counties. Icelandic birds ringed on passage at Fair Isle have been recovered
In Caithness and Perthshire, and a (f subaesalon ringed on 18th August 1953 was found at Hannut, Liege, Belgium, two months later. An interesting capture of an Icelandic Merlin (c?, wing 213 mm.) was made in the North Sea 26 miles east of Spurn Point, Yorkshire, on 12th October 1910 (specimen in the British Museum). Recently Holgersen (1954) has recorded autumn and winter specimens from south-west Norway, and has since added a further record (pers. comm.). The Icelandic Merlin is therefore synhiemic with the British population, and although it is clear that there is partial allohiemy with the Continental race both forms winter largely in the mild maritime climate of western Europe. It seems doubtful if selection due to factors of either the breeding or the wintering environment can be so markedly different as to have produced the remarkable size discrepancy which we find between these two races, and this would appear to be a clear case in which selection pressure is strongest during the actual migration period; for whereas the southwards movement of aesalon is largely overland, that of subaesalon must necessarily encompass a sea-crossing of several hundred miles, as a recent synoptic study of the autumn migration has shown (Williamson, 1954b; Williamson and Butterfield, 1955). Little is known of the spring migration of subaesalon: the Merlin is a very scarce bird at Fair Isle in April and May, and perhaps the return is made direct from northern Scotland and the western isles (the species was of daily occurrence at St. Kilda in April and early May of 1957). There are in the Universitetets Zoologiske Museum, Copenhagen, 3 spring specimens taken on board ships in the Atlantic--a VOL. LI] BERGMANN'S RULE AND MIGRATION
stouter tarsi (Harrison, 1944), indicating a greater body-weight. Salomonsen (1935) gives a maximum wing-length of: 163 mm., for Danish birds, and cfcf 171mm,, 9 $ 172 m m . for Icelandic. Witherby et al. (1938-41) give similar maxima for robusta but mention British-taken specimens with wings up to 175 m m . : for britannicus they g i v e : d?d1, 1 6 3 m m . , 9 9 i 1 6 5 m m . Harrison (1944) gives for robusta, 7 cfcf, 165-172 mm., 3 9 9 164-177 mm., and for Norwegian birds. 10 r/cTj 148-158 mm. There is little data on w e i g h t s : presumed Norwegian birds with a wing-length of 154-156 mm., trapped at Fair Isle in early autumn, weighed 102 g m . , 120 g m . and 134 g m . ; and birds referred to robusta on a wing-length of 163173 mm. weighed' 127 g m . , 128 g m . , 130 g m . and 148 g m . (an average of 133 g m . against 119 g m . in the first group). Redshanks ringed in Iceland have been recovered in winter in localities as far apart as Orkney in the north, Flintshire in the west and the W a s h in the south-east, so we may take it that the whole of Britain lies within the winter area. According to Witherby et al. (1938-41) some robusta are recorded in winter along the west coast of Europe, and there is one recovery as far south as Morocco. The same authority says that only a small part of the British stock crosses the Channel, but that some birds from northern localities g o to Ireland. T h e winter ranges of the European populations have been worked out by Salomonsen (1954), who gives maps based on ringing returns. Danish birds make an overland migration to the central Mediterranean countries, whilst those from the Low Countries and* West Germany winter in the Iberian Peninsula and North Africa. British and West European Redshanks, though wintering in widely separated areas in the Mediterranean and Atlantic maritime regions, do not differ materially in size, whereas the synhiemic British and Icelandic populations show a difference of 10 mm. in the mean of the wingmeasurement. The lowest mean wing-length belongs to Norwegian and Swedish birds, which migrate coastwise through Europe to tropical Africa, and thus enjoy the warmest winter environment. Although the pattern of allohiemy which dominates the winter distribution of the Continental populations firmly supports Salomonsen's conclusion that there is a correlation between Bergmann's Rule and selection in the winter quarters, it would appear that the more robust proportions of the Icelandic bird are better explained as an adaptation to a mandatory overseas migration. OTHER WADERS There is a small average difference in wing-length between Continental and Icelandic Whimbrels (Numenius phaeopus), and the Iceland bird has been separated despite the existence of a large overlap in this character. Salomonsen (1935) gives for 39 Scandinavian specimens, cfef, 231-250 mm., 9 9> 241-254 mm.,
and for 77 islandicus, cj*cf> 240-260111111,, 9 9 i 251-272111111. Faeroe birds are intermediate, giving for (f cf, 238-252 mm., and for $ 9 1 245-264 (23 measured). (See also the Table in Salomonsen, 1947.) Both forms breed at roughly the same latitudes and, so far as is known, winter in tropical Africa. There would appear to be little influence towards differentiation in either the breeding or winter climates, and the somewhat longer wing of the Iceland birds probably reflects the need for greater stayingpowers during the obligatory overseas journey to and from the breeding-grounds. Oystercatchers (Haematopus ostralegus) of Iceland and the Faeroe Islands are also somewhat longer in the wing than Scandinavian birds, but there is a wide overlap and many taxonomists cannot accept the north-western birds as a distinct race, malacophaga. Measurements given by Salomonsen (1935) indicate a maximum wing-length of : d1 d1, 2 7 2 m m . , 9 9> 276 mm., for Icelandic birds ; c? 9 > 270 mm. for Faeroe breeders ; and cfcf, 263mm., 9 9» 265mm. for Continental. British examples are intermediate. Many Continental Oystercatchers breed at much the same latitudes as the Faeroese and Icelandic stocks, and winter over most of south-west Europe and the Mediterranean region, some on the coast of west Africa; while all recoveries of Iceland and Faeroe (and also Fair Isle) birds are from Scotland, the west of England, Ireland and (a few only) France. A different "winter-influence" may well be involved, derived from Mediterranean and Atlantic climates, and the case for attributing the greater size of the Iceland and Faeroe populations to the overseas migratory flights is less strong in this species. The wing-length data given by Salomonsen (1950-51) for the Purple Sandpiper (Calidris maritima), and reproduced in Table V, show a peculiar distribution, Icelandic birds being distinctly larger than either Greenland or Scandinavian. Lovenski0ld (1950) has also made a detailed examination of this species and presents the measurements of the various populations in histograms. In all probability Icelandic birds are also heavier: Weigold (1926) gives 60-84.5 Sm- f ° r 3 migrants at Heligoland, and quotes Hantzsch for Icelandic birds (presumably breeders), 4 cfcf, 74-80 gm., 5 9 9> 80-95 Sm- This northern species has a Holarctic range and practically nothing is known about the winter distribution of the various populations. The only recovery of a bird marked in Iceland is a curious one--an adult ringed in May 1942, recovered at Cape Dorset, Baffin Island, in late April 1943. I t may be that Iceland and Faeroe Islands have been colonized from the Palaearctic region, the increase in size being a subsequent adaptation to a mandatory overseas migration to Old World wintering quarters; and that the Greenland stock is derived from the North American continent, to which it has access by the short seacrossing of Davis Strait.
Males No. Range (mm.) Average
Females No. Range (mm.) Average
Faeroe Islands 10
123-137 124-132 118-131 117-131
129.1 127.2 124.6 123.7
127-141 134.6
125-133 130-1
125-135 129.8
120-137 123-7
The Black-tailed Godwit (Limosa limosa) of Iceland is separable from the typical race of Europe on account of its redder plumage in the breeding season and its shorter bill, sex for sex {Salomonsen, 1935). The wing-length range appears to be much the same in the two races, except that 9 9 islandica are perhaps a trifle bigger than Continental. Hachisuka (1927) gives 217-230 mm., and Salomonsen 210-228 mm. for $ $ islandica, the latter adding 209225 mm. for Danish limosa. Except for a small group in S. Sweden (and isolated instances in Norway), the Black-tailed Godwit does not nest on the Continent north of the Kattegat and Skagerrak, so there is a considerable latitude difference in the breeding-ranges of the t w o forms. It is now known that a large part of the winter population of Black-tailed Godwits in Ireland consists of islandica (Williamson and Ruttledge, 1957), so that, since many limosa winter southwards to tropical Africa, islandica ought to reflect Bergmann's Rule on both counts. Since it obeys Allen's Rule of decreasing bill-size with increasing latitude, it has presumably been established in Iceland for a very long time, and its lack of greater robustness despite more northerly breeding and wintering areas, and an obligatory overseas migration, makes it a puzzling case. As with the Merlin, little is known about the spring migration: its rarity in the Outer Hebrides and northern Scotland suggests that the main movement avoids these areas, though the sporadic breeding of pairs possibly belonging to this form in Caithness (Pilkington, 1947), Shetland (Venables and Venables, 1955) and Faeroe (Williamson, 1954a) should be noted. Holgersen and Willgohs (1956) have recorded the interesting and perhaps significant fact that Black-tailed Godwits are more frequent in Norway in spring than in autumn, and that all Norwegian specimens so far examined have the short bill-length of islandica. It is possible therefore that a proportion may return to Iceland, by way of the Norwegian coast. Finally, the White-fronted Goose (Anser albifrons) may be mentioned. The Greenland form flavirostris winters exclusively in Ireland and south-west Scotland, as the numerous recoveries of ringed birds show. The flocks reach these areas in middle and late October largely by a cyclonic migration across the eastern Atlantic (Ruttledge and Williamson, 1952). Salomonsen (1950-51) gives wing-measurements of 368-440 mm. for typical albifrons of
Europe, which has a largely overland migration to winter quarters at much the same latitude in western Europe, and 420-455 mm. for flavirostris. This follows the pattern demonstrated for the majority of these trans-oceanic migrants from Greenland and Iceland to winter quarters in western Europe and beyond. ACKNOWLEDGEMENTS I am grateful to Dr. D. W. Snow and Messrs. E. F. Aikman and A. P. Ryan for making available information concerning Wheatears at sea; and to Mr. Alec Butterfield for undertaking a statistical examination of Wheatear weights. Recoveries of birds ringed in Iceland have been taken from the reports issued by Gudmundsson (1933 et seq.), and those of Greenland birds from the reports of Salomonsen (1949 et seq.). Weather-maps are reproduced from the Daily Weather Report of the Meteorological Office (Air Ministry) by kind permission of H.M. Stationery Office. SUMMARY 1. The selective influences which, contribute to the effect known as Bergmann's Rule are briefly discussed in so far as they concern migratory birds. A correlation between wing-length and the minimum temperature of the winter environment has been demonstrated for some migratory species with allohiemic populations, but there are cases of agreement with this ecogeographical rule which neither this nor any other reason so far advanced can satisfactorily explain. 2. The proposition is made that among certain land-birds whose migration demands an obligatory overseas flight, selection operates most powerfully during the actual period of migration, since survival favours those individuals with the greatest reserves of strength. In the case of a number of migratory birds nesting in Greenland and Iceland the resulting adaptation has been one of greater body-weight, coupled with an allometric increase in length of wing and tail, as compared with the appropriate Continental populations, whose migration is largely coastwise or overland. 3. Among Greenland birds the evolutionary consequence is most strikingly manifested in the Wheatear, which migrates through W. Europe to Africa. The migration-pattern at Fair Isle, and occurrences in the North Atlantic, show that trans-oceanic flights of 1,500-2,000 miles may be undertaken in cyclonic weather, resulting in a weight-loss considerably greater than the typical race could endure. The Greenland Redpoll shows this "migrationinfluence" less strongly, the Lapland Bunting slightly, and the Snow Bunting hardly at all. In these cases the bulk of the population crosses the comparatively narrow Davis Strait to wintering-grounds in North America, and the need to evolve greater body-size has been less urgent. 4. Among Icelandic birds the effect is most strongly marked in the Redwing, Merlin and Redshank, whose migratory flights
into the British area may exceed 1,000 miles under cyclonic conditions. The Meadow Pipit and White Wagtail give a negative result, but may be comparatively recent and still expanding colonists in the north-west. The Whimbrel and Purple Sandpiper show some response to the selective influence of a mandatory overseas migration, and the Oystercatcher perhaps does so, but the Black-tailed Godwit does not and indeed provides a puzzling exception to Bergmann's Rule.
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