396 NATURE-STUDY REVIEW [16:9— Dec, 1920' 



As to the little, soft-billed birds, he seems to have settled the 

 question of migration beyond a doubt, for he says, "I was much 

 pleased to see, among the collection of birds from Gibraltar, some 

 of those short-winged Eng. summer birds of passage, concerning 

 whose departure we have made so much inquiry. Now if these 

 birds are found in Andalusia to migrate to and from Barbary, it 

 may easily be supposed that those that come to us may migrate 

 back to the continent, and spend their winters in some of the 

 warmer parts of Europe. This is certain, that many soft-billed 

 birds that come to Gibralter, appear there only in spring and 

 autumn, seeming to advance in pairs towards the northward, for 

 the sake of breeding during the summer months; and retiring in 

 parties and broods towards the south at the decline of the year; 

 so that the rock of Gibralter is the great rendezvous, and place of 

 observation, from whence they take their departure each way 

 towards Europe or Africa. It is therefore, no mean discovery, I 

 think, to find that our small short winged summer birds of passage 

 are to be seen spring and autumn on the very skirts of Europe; 

 it is a presumptive proof of their emigrations." Though the 

 swallow kind mystified him sorely, he came to this conclusion, 

 "We must not deny migration in general; because migration 

 certainly does subsist in some places as my brother in Andalusia 

 has fully informed me. Of the motions of these birds he has- 

 ocular demonstration for many weeks together, both spring and 

 fall: during which periods myriads of the swallow kind traverse 

 the Straits from north to south, and from south to north according 

 to the season." 



As to the reasons for migration, he of course realized the influence 

 of food and weather, but was also aware of other factors which he 

 did not altogether understand. He says, "Now it is no wonder 

 that birds residing in Africa should retreat before the sun as it 

 advances, and retire to milder regions, and especially birds of 

 prey, whose blood being heated with hot animal food, are more 

 impatinet of a sultry climate; but then I cannot help wondering: 

 why kites and hawks, and such hardy birds as are known to defy 

 all the severity of England, and even of Sweden and all northern 

 Europe, should want to migrate from the south of Europe, and be 

 dissatisfied with the winters of Andalusia." He accounts for the 

 spring migration in a particularly quaint way. 'The two great 

 motives which regulate the proceedings of the brute creation are 



