MIGRATION 553 



aftbrding tlie sole or the most convenient site for the nest in the 

 neighbourhood, but in so many instances such is not the case that 

 we are led to believe in the existence of a real partiality, while 

 there are quite enough exceptions to shew that a choice is frequently 

 exercised. The same may equally be said of the most migrant of 

 Birds, and perhaps the strongest instance that has ever come to the 

 knowledge of the writer refers to one of the latter. A pair of 

 Stone-CuRLEWS {GEdicnemus crejntans) — a very migratory species, 

 affecting almost exclusively the most open country — were in the 

 habit of breeding for many years on the same spot^ though its 

 character had undergone a complete change. It had been part of 

 an extensive and barren I'abbit-warren, and was become the centre 

 of a large and floiuishing plantation. 



With these two sets of facts before us we may begin to try and 

 account for the cause or causes of Migration. In some cases want 

 of food would seem to be enough, as it is undoubtedly the most 

 obvious cause that presents itself to our mind.^ The need which 

 all animals have of finding for themselves proper and sufficient 

 sustenance is all-powerful, and the diificulties they have to encounter 

 in obtaining it are so great that none can Avonder that those which 

 possess the power of removing themselves from a place of scarcity 

 should avail themselves of it, while it is unquestionable that no 

 Class of animals has this facility in a greater degree than Birds. ^ 

 Even among many of those species which we commonly speak of 

 as sedentary, it is only the adults which maintain their ground 



as one of the most southern points whence the midnight snn may be seen — is 

 mentioned by the French astronomer Maupertiiis as having been observed by 

 him in the year 1736. In 1799 the nest was rediscovered by Skjoldebrand and 

 Acerbi. In 1853 "VVoUey found it tenanted, and from enquiries he made of the 

 neighbours it was evident that such had yearly been the case so far as any one 

 could remember, and so it was in 1855 as I myself can testify. In 1779 accord- 

 ing to one account, in 1785 according to another, a pair of the Blue Titmouse 

 {Parus coeruleus) built their nest in a large earthenware bottle placed in the 

 branches of a tree in a garden at Oxbridge near Stockton-on-Tees. With two 

 exceptions only, this bottle, or a second which had been placed close to it, was 

 tenanted by a pair of birds of this species from the year in which it was first 

 occupied until 1873, when I saw it (see Yarrell's British Birds, ed. 4, i. pp. • ' t 



58, 486) ; but I regret to add that I learnt through Canon Tristram in 1892 that CJ- ((AKu^^ao- 

 the occupancy had ceased for four years. (/ 



1 At Elveden in Suffolk. 



- Far more so than variation of the temperature, though in popular belief 

 that probably holds the first place. But Birds generally, as compared with other 

 Vertebrates, are but slightly affected by extremes of heat or cold, and indeed 

 (so far as we can judge) by most climatic influences, provided only their supply 

 of food is not affected thereby (c/. Max Schmidt, Zoolog. Garten, 1865, pp. 

 330-340). 



^ The only animals which approach Birds in the extent and character of their 

 migrations are Fishes, of which there is no need here to say anything. 



