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 int< 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. | 
