TRANSACTIONS OF SECTION D. 791 



strike either Iceland or the west coast of Norway, and in either case would 

 reach the east coast of Britain. But if, by storms and the prevailing winds of 

 the North Atlantic comino- from the west, they had heen driven out of their 

 usual course, they would strike the coast of Norway, and so find their way hither 

 in the company of their congeners. 



As to the elevation at which migratory flights are carried on, Herr Gatke, as 

 well as many American observers, holds that it is generally far above our ken, at 

 least in normal conditions of the atmosphere, and that the opportunities of observa- 

 tion, apart from seasons and unusual atmospheric disturbance, are confined chiefly 

 to unsuccessful and abortive attempts. It is maintained that the height of flight 

 is some 1,500 to 15,000 feet, and if this be so, as there seems every reason to 

 admit, the aid of land bridges and river valleys becomes of very shght importance. 

 A trivial instance will illustrate this. There are two species of blue-throat, 

 Ci/anecula suecica and C. leucocyana : the former with its red-breast patch is abun- 

 dant in Sweden in summer, but is never found in Germany, except most acciden- 

 tally, and the other is the common form of Central Europe. Yet both are abundant 

 in Egypt and Syria, where they winter, and I have, on several occasions, obtained 

 both species out of the same flock. Hence we infer that the Swedish bird makes 

 its journey from its winter quarters with scarcely a halt, while the other proceeds 

 leisurely to its nearer summer quarters. On the other hand, I have more than 

 once seen myriads of swallows, martins, sand-martins, and, later in the season, 

 swifts passing up the Jordan Valley and along the Bukaa of Central Syria at so 

 slight an elevation that I was able to distinguish at once whether the flight consisted 

 of swallows or of house-martins. This was in perfectly calm clear weather. One 

 stream of swallows, certainly not less than a quarter of a mile wide, occupied 

 more than half an hour in passing over one spot, and flights of house-martins, 

 and then of sand-martins, the next day were scarcely less numerous. These 

 flights must have been straight up from the Eed Sea, and may have been the 

 general assembly of all those which had wintered in East Africa. I cannot think 

 that these flights were more than 1,000 feet high. On the other hand, when 

 standing on the highest peak in the Island of Palma, 6,500 feet, with a dense mass 

 of clouds beneath us, leaving nothing of land or sea visible, save the distant Peak 

 of Tenerife, 13,000 feet, I have watched a flock of Cornish choughs soaring above 

 us, till at length they were absolutely undistinguishable by us except with field- 

 glasses. 



As to the speed with which the migration flights are accomplished, they 

 require much further observation. Herr Gatke maintains that godwits and 

 plovers can fly at the rate of 240 miles an hour (!), and the late Dr. Jerdon stated 

 that the spine-tailed swift (^Acanthylis caudacutus), roosting in Ceylon, would 

 reach the Himalayas (1,200 miles) before sunset. Certainly in their ordinary 

 flight the swift is the only bii'd I have ever noticed to outstrip an express train on 

 the Great Northern Railway. 



Observation has shown us that, while there is a regular and uniform migration 

 In the case of some species, yet that, beyond these, there comes a partial migration 

 of some species, immigrants and emigrants simultaneously, and this, besides the 

 familiar vertical emigration from higher to lower altitudes and vice versa, as in the 

 familiar instances of the lapwing and golden plover. There is still much scope for 

 the field naturalist in observation of these partial migrations. There are also 

 species in which some individuals migrate and some are sedentary. E.{/., in the few 

 primeval forests which still remain in the Canary Islands, and which are en- 

 shrouded in almost perpetual mist, the woodcock is sedentary, and not uncommon. 

 I have often put up the bird and seen the eggs ; but in winter the number is 

 vastly increased, and the visitors are easily to be distinguished from the residents 

 by their lighter colour and larger size. The resident never leaves the cover of 

 the dense forest, where the growth of ferns and shrubs is perpetual, and fosters a 

 moist, rich, semi-peaty soil, in which the woodcock finds abundant food all the 

 year, and has thus lost its migratory instincts. 



But why do birds migrate ? Observation has brought to light many facts 

 which seem to increase the difficulties of a satisfactory answer to the question.. The 



