BRITISH BIRDS. 75 



extraordinary one ? is it migratory impulse, not the want of any thing ? and 

 what may migratory impulse be ? Is it safety ? is it want of light ? is it to 

 breed ? is it wind which causes it ? Spawning has a great deal to do with 

 piscine migration ; and I agree with Dr. Jerdon, in his Introduction to the 

 ' Birds of India,' p. xxii, vol. i., " Cuvier's explanation, the variations of the 

 atmosphere," is not an efficient reason alone for migration. It may arise 

 from all these causes : various men do the same thing from many motives. 

 Each species of bird may migrate from its own motive. Sometimes it is 

 hunger, for example ; strictly. Larks, Snipes, Woodcocks are moved by frost 

 and nival impulse. Insect-eating birds, as a rule, perhaps come under the 

 head of food rather than cold, though cold destroys their food. Yet the 

 Nightingale goes when it would appear that its food is abundant. 



'AvTia-Topyrj (that wonderful change from intense affection to coercive 

 violence, so necessary for Nature's balance) is the cause of the appearance of 

 most of what are called rare birds, which are those of the year seeking new 

 habitations. Huntsmen know a travelling fox ; and we, not only a travelling 

 bird, but more — a lost bird. Mice and rats sometimes lose their way, as do 

 birds. I recall to my mind now instances which have come under my notice 

 of each. Of excessive multiplication, from a combination of circumstances, 

 in a peculiar season favourable to production, we have a notable example in 

 the irruption of Syrrhaptes paradoxus from beyond the Great Chinese Wall, 

 1863, and, of insects, in the swarms of S. pyrastri (a fly) which appeared in 

 Brighton some years ago. 



Regarding migratory impulse one speaks tentatively and cautiously. I 

 apprehend it to be a restless, uneasy sensation, driving the bird into action — 

 the converse of the sitting impulse, a warmth in the breast which causes a 

 wish to incubate and creates an aversion to movement : — at one time forced to 

 sit, by the ease obtained by sitting ; at another urged to travel, by a feeling 

 similar to that which causes lambs and foals to disport themselves. The 

 evidence of one very competent dealer and respectable man to me is this : " I 



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