86 MIGEATION. 



recollect, not witliout regret, that in June, 1746, I was 

 visiting for a week together at Spalding, without ever being 

 told that such a curiositj was just at hand. Pray send me 

 word in your next what sort of tree it is that contains such 

 a quantity of herons' nests ; and whether the heronry 

 consists of a whole grove or wood, or only of a few trees. 



It gave me satisfaction to find we accorded so well about 

 the caprimulgus ; all I contended for was to prove that it 

 often chatters sitting as well as flying, and therefore the 

 noise was voluntary and from organic impulse, and not 

 from the resistance of the air against the hollow of its mouth 

 and throat. 



If ever I saw anything like actual migration, it was last 

 Michaelmas-day.* I was travelling, and out early in the 



* The subject of migration appears to have been a very favourite one with 

 our author, occupying the greater part of many of his subsequent letters, and 

 evidently often the subject of his private thoughts. He sometimes seems 

 puzzled with regard to the possibility of many of the migrating species being 

 able to undergo the fatigue of long or continued journeys; and often wishes 

 almost to believe, though contrary to his better judgment, that some of these 

 enter into a regular torpidity. We find torpidity occuiTing among animals, 

 fishes, the amphibiie, and reptiles, and among insects ; but we have never 

 found any authenticated instance of this provision taking place among birds. 

 Their frames are adapted to a, more extensive locomotive power ; and the 

 change to climates more congenial to their constitutions, preventing the 

 necessity of any actual change in the system, is supplied to those animals 

 deprived of the power for extensive migration, by a temporary suspension of 

 the most of the faculties which, in other circumstances, would be entirely 

 destroyed. Biids, it is true, are occasionally found in holes, particularly our 

 summer birds of passage, in what has been called a torpid state, and have 

 revived upon being placed in a warmer temperature ; but this, I consider, has 

 always been a suspended animation, where all the functions were entirely 

 bound up as in death, and which, by the coniinuance of a short period, would 

 have caused death itself — not torpidity, where various functions and secretions, 

 capable for a time of sustaining the frame, are still going on. 



The possibility of performing long journeys, as we must believe some 

 species are obliged to do before arriving at their destination, at first appears 

 nearly incredible ; but, when brought to a matter of plain calculation, the 

 difficulty is much diminished. The flight of birds may be estimated at from 

 50 to 150 miles an hour ; and if we take a medium of this as a rate for the 

 migrating species, we shall have little difficulty in reconciling the possibility of 

 their flights. This, however, can only be applied to such species as, in their 

 migrations, have to cross some vast extent of ocean, without a resting-place. 

 Many that visit this countiy, particularly those from Africa, merely skirt the 

 coast, crossing at the narrowest parts, and again progressively advancing, until 



