Observations on Periodical Birds. 135 
periodical birds of summer, and the sleeping 
animals of winter. It is sufficient barely to 
remark, that the former are never found 
slumbering with the latter, near the surface 
of the earth, and deep caverns are proved to 
be unfit for the reception of any creature in 
the torpid season. Consequently the birds 
in question desert the temperate zones at the 
approach of winter, to seek a better climate 
in lower lJatitudes.’”” These conclusions, 
however, will appear to have been formed 
rather hastily, when we consider what num- 
bers of bats become torpid every winter in 
this country, and how rarely they are dis- 
covered in their dormitories. Might they 
not have been derived more satisfactorily, 
from the circumstance of the summer birds 
being seldom or never found abroad, with the 
sleeping animals, during the mild weather 
that we frequently have in winter? Bats, 
field-mice, &c., usually appear when the 
mean daily temperature is about 50°; but I 
am not aware that there is a single instance 
on record, of any of our periodical warblers, 
properly so called, having been observed in 
the cold season, either in a state of active 
existence, or of torpidity.* A few, indeed, 
* Since writing the above, I find that Montagu, in the Supplement 
to his Ornithological Dictionary, asserts that he has occasionally dis- 
