MIGRATION. 157 



own indigenous species are the minutest ; thus the little Wrens cheer 

 us by their presence during the whole progress of ice and snow, pitiless 

 blasts, and shelterless woods, and, with the Titmice and many others 

 equally weak and helpless, brave the greatest rigours of our climate. Now 

 these are chiefly insectivorous birds; but insect food is very scarce in 

 winter, and although there are some few insects, such as gnats, which 

 are seen at all times, almost when the sun is set, this supply would 

 evidently be totally insufficient for the support of so many individuals, 

 and indeed, in hard winters, is often inadequate for our own native 

 songsters ; hence it is wisely provided that as soon as the general supply 

 fails, the bulk of birds that subsist upon it should depart for realms 

 where it is still to be met with. From this it appears that it is chiefly 

 their inability constitutionally to endure our winters, that causes them 

 to migrate, since many of our birds which go through it well enough 

 are insectivorous. All the insect tribes lie concealed in what is called 

 the pupa state, or dormant in transition existence, during winter, either 

 in the . earth, in the bark of trees, or in other concealed situations. As 

 soon as the sun has attained a sufficient altitude in the heavens to render 

 his beams available, by their reflection, as a medium of heat, a stir takes 

 place amongst these vast armies in ambush, and the woods, the plains, 

 and the waters teem with insect transformations. A great proportion of 

 these deposit their eggs in flesh, which is their proper receptacle, and in 

 which they pass the first stage of their existence, and become maggots; 

 and some, as the green flesh-fly, perform their office on the bodies of living 

 animals. There are also some other larger kinds which bear the name 

 of CEstrus, and Bombylim, signifying a large bottle with a narrow mouth, 

 which they resemble in shape; these oviposit in particular kinds of cattle, 

 chiefly of the horse and ox kind. There are vast numbers of the Dipterous 

 and Apterous orders, that is of the fly and flea species, that are blood- 

 suckers, and, in company with the culex or gnat kind, are great pests in 

 warm weather; there are some which sting and some which bite, and these 

 form the two great divisions, for very few are perfectly innocuous. 



All insects are extraordinarily prolific, and it is therefore manifest that, 

 without some counteracting agency, their presence and numbers would soon 

 become intolerable. This difficulty is met by the migration of the summer 

 birds, the bulk of which subsist chiefly upon insect food, and the greater 

 number of the bulk entirely so. Only consider for a moment what an 

 enormous number of insects must be consumed by the Swallow tribe alone, 

 which are on the wing for thirteen hours per day, during their stay of 

 more than five months at a moderate average, and catching some kind of 

 insect continually, as you may easily presume, if you watch them ever so 

 6hort a time. This average of actual activity in search of food, I think, 



VOL. VII. T 



