SOLITARY AND GREGARIOUS. 135 



burrowing out chambers in the banks ; but stories 

 are told of the mutual assistance of other quadru- 

 peds, evidently as much overcoloured as that of M. 

 Dupont's swallows. 



The only obvious and decided instance of mutual 

 assistance, which we recollect as occurring among 

 birds, is that of parents feeding their young, keeping 

 them clean and warm, and defending them against 

 enemies. But in order to secure warmth, many 

 species certainly take advantage of the animal heat 

 of their kindred, and we may with some plausibility 

 say, that in most cases this is done by mutual suf- 

 ferance, if not by distinct permission. 



It is one of the most extraordinary, as well as one 

 of the best ascertained facts in the animal economy, 

 though by no means, as yet, satisfactorily explained, 

 that the interior heat of warm-blooded animals va- 

 ries extremely little in the coldest and in the hottest 

 climates. 



This law, by which animal temperature is main- 

 tained at nearly the same degree on exposure to 

 considerable heat or cold, supplies the only known 

 reason why some of the smaller and seemingly ten- 

 der animals outlive the rigours of the severest win- 

 ters. The magpie (Pica caudata, RAY), though rather 

 a hardy bird, has been found having recourse to 

 what is often practised by smaller birds, several of 

 them huddling together during the night, to keep 

 each other warm. A gentleman of intelligence and 

 veracity informed us that he once saw a number of 

 these birds (probably a young family with their pa- 

 rents) on a tree, in a fir plantation, sitting so closely 

 together that they all seemed to be rolled up into a 

 single ball. 



It is a very curious and remarkable circumstance, 

 that many species of birds which are solitary at 

 one period of the year, are gregarious at another ; 

 and though it is possible to account for this in some 

 instances, it becomes not a little difficult in others. 



