ON THE INSTINCTS OF BIRDS. 121 
beyond the latter end of July or the beginning of 
August, the strenuous but unavailing exertions they 
make to prolong them sufficiently proving their 
silence not to be a matter of choice, but of necessity. 
This circumstance, together with the extreme diffi- 
culty they experience in recommencing their songs 
in spring, clearly demonstrates that their delightful 
warblings depend upon the energy of those muscles 
which contribute to form the voice—an energy which 
appears to be influenced chiefly by food, temperature, 
health, and the exercise of the reproductive functions ; 
for by due attention to the regulation of these parti- 
culars the vocal powers of caged birds may be called 
into action or circumscribed at pleasure. Of this 
fact, persons who have the management of breeding 
Canaries may easily satisfy themselves; and female 
birds in a state of captivity, when brought into high 
condition, are known occasionally to assume a song 
somewhat resembling that of the male. That Jonston 
must have been deceived in supposing he heard the 
Nightingale in Scotland is evident, as that Warbler, 
it is well known, is never found north of the Tweed 
in Great Britain. It has been ascertained, too, con- 
trary to the opinion of Linnzus, that young Cuckoos, 
before they come to maturity, utter a feeble cry only ; 
they cannot, therefore, acquire the calls of their spe- 
cies while they remain in this country. No wonder, 
then, that the conclusion Dr. Darwin arrived at was 
erroneous, when the premises on which his reasoning 
is grounded are so inaccurate. 
