202 THE MIGRATION OF BRITISH BIRDS 



Robin {EritJiaciis supa'ba) is confined to Teneriffe and 

 Grand Canary, two of the most southerly and remote of 

 the islands, where the ordinary form of the Robin is 

 never seen ; and that the individuals of Parus itltravia- 

 riims (a sedentary species ranging from Algeria to the 

 Canaries) present some differences of size and colour. 

 I may also remark that the Stonechats resident in 

 Algeria may yet be found to belong to the Canarian 

 form, or to resemble it more nearly than the migratory 

 individuals of this species. Whilst dealing Avith the 

 birds of the Canaries, I may take the opportunity of 

 making a few remarks on the number of eggs laid by 

 various species in the islands. It is a most curious and 

 interesting fact that many birds breeding in the islands 

 lay fewer eggs for a sitting than they are known to do 

 elsewhere. Thus we have the Blackbird with clutches 

 of two or three, frequently one ; the Robin two or 

 three ; the Canarian Firccrest three to five ; the Tits 

 three to five ; the Teydean Chaffinch two ; the Barn 

 Owl two ; Bolle's Pigeon one ; and the Canarian Pigeon 

 one ! This small number to the brood, so general 

 among so many different species (especially in the case 

 of the Pigeons), is possibly caused by the scarcity of 

 food for the nestlings, or difficulty in obtaining it and 

 conveying it to the nest — the result of a long process of 

 natural selection. 



With Madeira and the Azores we come to a slightly 

 different class of phenomena. These islands are very 

 remote from continental land, and their avifaunas are 

 perhaps entirely derived from fortuitous emigration. 

 Although Madeira numbers some lOO species, and the 

 Azores 38, each of these island groups can claim but 



