tter how sensitive or how well
attuned the instinctive response of the parent may be, it will avail but
little in the presence of unfavourable conditions in the environment.

Everything turns upon the question of the effect of exposure. And in
order to ascertain how far extremes of temperature are injurious, I
removed the nests of various species containing newly hatched young,
and, placing them in surroundings that afforded the customary amount of
protection from the elements, I made a note of the temperature and the
atmospheric conditions and then observed the condition of the young at
frequent intervals. Details of these experiments will be found at the
end of the chapter.

The experiments with the Blackbirds and the Whitethroats gave the most
interesting results. Both broods of each species were respectively of
much the same age, yet one brood of Blackbirds survived for five, and
the other only for two and a half hours, and one brood of Whitethroats
lived for twelve hours whilst the other succumbed in a little over an
hour. This difference is rather remarkable; and it seems clear that the
power of resistance of the young diminishes rapidly when the temperature
falls below 52 deg. F. It must be borne in mind, however, that the
conditions under which the experiments were made were, on the whole,
favourable--the weather was dry, the temperature was not unusually low,
nor was the wind exceptionally strong or cold; and even in those cases
in which the young succumbed so rapidly, the atmospheric conditions
could by no means be regarded as abnormal.

What, then, would happen in an unusually wet or cold breeding season?
For how long would the young then survive? In the spring and early
summer of the year 1916, I was fortunate in observing the effect of
exposure under natural but inclement conditions. I happened to be
watching the Yellow Buntings on Hartlebury Common--200 acres of Upper
Soft Red Sandstone, profusely overgrown with cross-leaved heath (_Erica
tetralix_), ling (_Calluna vulgaris_), and furze (_Ulex_)--in one corner
of which eight males had established adjoining territories covering some
fifteen acres of ground. The males obtained mates towards the end of
March or at the beginning of April; nests were built in the middle of
May, and the successful pairs hatched out their young in June. On the
10th June the weather became exceptionally cold, and during the next ten
days the temperature fell at times to 40 deg.