J. E. LITTLEBOV THE MIGRATION OF BIRDS. 29 



well-known s]i(H'ios is so wide that every variety of climate is in- 

 cluded withiu their breeding-area. Let us take a few illustrations. 

 The sand-raartin breeds occasionally within the arctic circle ; * it is 

 also abundant in Egypt, and breeds in colonies on the banks of the 

 River Nile.f The hooded crow breeds profusely in Scandinavia 

 and Western Siberia ; in England and central Europe it is only 

 a winter visitant, but it breeds also in Algeria, Egypt, Palestine, 

 and Greece. The blue-throated warbler breeds in Europe only 

 within the arctic circle or at high elevations above the limit of 

 forest growth, J but it was recently found by Mr. Scully breeding as 

 far south as Yarkand, in Turkestan. § (b) Xot only do many of the 

 birds which breed in the north of Europe breed also in Egypt and 

 Algeria, but there are immense numbers which breed exclusively or 

 nearly exclusively in the tropics. Wallace || asserts that there are 

 about 140 species of parrots resident in tropical America, and 150 

 in tropical Australia and the tropical Australian islands ; and that of 

 346 species of pigeons only 1 6 frequent temperate regions, the whole 

 of the remainder being tropical, if it be correct that the necessities 

 of incubation compel a certain number of birds belonging to a par- 

 ticular species always to migrate northwards in the spring, how 

 comes it that others of the same species are not governed by 

 a similar necessity, but breed regularly and successfully in tropical 

 climates ? This is a query, to which it will, I think, be difficult 

 to find a satisfactory answer. Again, can it be shown that the 

 eggs of the four or five pigeons which frequent the British Isles 

 differ in any way from the eggs of their tropical congeners, or that 

 the 339 parrots and 400 humming birds, which breed within the 

 tropics, possess special powers enabling them to do so ? Unless this 

 can be done, it is difficult to understand how the physiological 

 necessities of incubation can satisfactorily account for the phenomena 

 connected with northern migration. That special facility for incu- 

 bation is one among the chief desidei'ata that help to make up the 

 ideal of a "Natural Home," I most willingly admit, but there are 

 certain other conditions that appear equally essential. 



The question as to what constitutes the "Natural Home" of 

 a migrant is a very important one, and exercises, as I believe, no 

 inconsiderable influence in determining the causes of migration. 

 I shall detain you a few moments in discussing it. 



There are three primary requirements which the term " Natural 

 Home," when applied to the resort of birds, seems to imply : — {a) 

 locality of birth ; (h) abundant supply of food ; (c) facility for in- 

 cubation ; and it will be necessary to consider whether the wide 

 regions in the north of Europe, that are selected by migrants as 

 their summer breeding-grounds, or their winter retreats in Africa 

 and elsewhere, best fulfil these requirements. Dr. Weissmann's 



* Seebohm, ' Siberia in Europe,' p. 156. 



t Shelley, ' Birds of Egypt,' p. 122. 



J Seebohm, 'Siberia,' p. 127. 



§ 'Ibis,' 1881, p. 447. 



II ' Tropical JVature,' p. 102. 



