BIRDS OF PASSAGE. 
203 
from an invariable temperature, would have consisted, in the 
certainty of not being frozen ? for the same decree of cold 
which disposes them to sleep in autumn, would evidently per- 
petuate their slumbers in these situations? unless we sup- 
pose them to be roused to action by the calls of hunger, 
which is a precarious and treacherous cause. For the sense of 
want would not fail in many instances to invite these animals to 
certain death in the midst of frost and snow, at an earlier season 
than the commencement of spring. If we suppose our known 
sleepers, or any other animals suspected of torpid habits, to 
retire to a depth less than 80 feet, but to a distance from the 
surface which is sufficient to conceal them, in damp and dreary 
grottos, from human observation ? the supposition will not 
remove the difficulty. For the time when our periodical qua- 
drupeds, birds, and reptiles disappear, coincides with the 
maximum of temperature in such places, and they are seen 
abroad again when the same temperature is at the lowest. 
Very few arguments will be now required to demonstrate 
the impossibility of the analogy which is supposed to connect 
the 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 butter climate in lower latitudes. 
The migration of our summer visitors being established upon 
authentic facts, I intend to proceed in the next place, to give a 
theory of their annual motions derived from natural causes. 
All the birds constituting the migrating tribe feed upon insects, 
which disappear and become torpid, either in a perfect state or 
under the form of embryos, soon after the autumnal equinox. 
This circumstance refuses the animals under consideration a 
farther supply of proper aliment in the higher latitudes. They 
are therefore compelled, by the apprehension of starving, to use 
their wings and retire southwards into more genial climates, 
where the rigours of winter do not lock ' up the sources of 
their natural food. The manners of the winter birds of pas- 
sage favour the last conclusion? for the jack-snipe, the red- 
wing, the woodcock, and the fieldfare, with some other spe- 
/ . ~ ' cies. 
Birds are 
never found 
in the shallow 
retreats, and 
the deeper are 
unfit. 
The migration 
of birds is well 
established. 
They follow 
their food ,-viz. 
insects which 
disappear dur* 
ing cold. 
