330 PHILOSOPHICAL TRAKSACTIONS. [aNNO 1747. 



where in open brackish waters they find relief, and at the approach of the spring 

 they retire to their summer recesses. But these cannot be included among those 

 usually termed birds of passage. 



Besides the different kinds of swallows, there is but one kind of European 

 bird that subsists in like manner by catching its food on the wing, that is, the 

 caprimulgus, or goat-sucker, the capacious structure of whose mouth and gullet 

 is formed to receive insects of the larger kinds, as scarabaei, grillotalpae, &c. 

 These are also birds of passage. 



We have made it pretty evident, that summer birds of passage come to, and 

 depart from us, at certain seasons of the year, merely for the sake of a more 

 agreeable degree of warmth, and a greater plenty of food ; both which advantages 

 they procure by an alternate change of climate; but the migration of winter 

 birds of passage, and particularly of fieldfares and redwings, is much more diffi 

 cult to be accounted for; there being no such apparent necessity, either on the 

 score of food or climate, for their departure from us. 



The reason of their coming here in winter is probably for the sake of food, 

 and a more suitable climate than that they leave; but in some severe cold seasons, 

 and when there is a scarcity of berries, they subsist here with difficulty, and are 

 even famished sometimes for want of sufficient food; yet what appears most 

 unaccountable is, that such as have continued with us a whole winter in penury, 

 and should, one would imagine, rejoice at our approaching spring, and build their 

 nests, and couple, on the contrary all depart, as if that mild and pleasant tem- 

 perature, which delights and cherishes most other creatures, were disagreeable to 

 them. We know the places of their summer retirement to be Sweden, and 

 some other countries in that latitude; but, as they would find those countries too 

 cold for their reception, and probably destitute of provision, were they to hasten 

 directly thither when they depart from hence, they journey gradually, and pro- 

 long their passage through the more moderate countries of Germany and Poland, 

 by which means they arrive not at those northern regions, adapted by Providence 

 for their summer abode, and the breeding of their young, till the severity of the 

 cold is so much abated as to render it pleasing to them, and food may be there 

 found. When they visit us again in winter, their return back is after the same 

 manner. 



The winter food of these birds being berries, and particularly haws, as a greater 

 abundance of them grow in this island than can be supposed in the more 

 northern regions, that may possibly be one great allurement to bring them over 

 hither; but the principal reason inducing them to travel southward, is probably 

 the rigour and severity of the cold in those frigid climes, which nature therefore 

 directs them to desert for such as are more temperate. 



A Swedish gentleman informed Mr. C. that observing the use we make of 



