68 OBSERVATIONS ON THE CUCKOO. 
that they have been described by several authors as 
a distinct species. In the colours of their plumage, 
and in their eyes, they bear some resemblance to 
young Kestrils; while the old birds, in both these 
particulars, are very similar to the male Sparrow- 
Hawk after the third or fourth moult. As young 
Cuckoos do not acquire their mature plumage while 
they remain in this country, though they are fre- 
quently seen here in September, two months later 
than old birds, and as they are never found in their 
first feathers on their return im spring, they must 
moult during their absence, which clearly proves that 
they are migratory, as it is hardly possible that they 
should acquire fresh feathers in a state of torpidity. 
This fact is further corroborated by the early depar- 
ture of the old birds, which takes place when the 
temperature is approaching the maximum for the 
year, and consequently when it is much higher than 
at the time of their arrival; and it is evident that 
they cannot become torpid with an increasing tem- 
perature: indeed the young birds, which stay so long 
after them, instead of displaying symptoms of debi- 
lity and torpor, continue to advance progressively in 
growth and vigour. Cuckoos, at a mean of fifteen 
years’ observations, appear in the neighbourhood of 
Manchester on the 20th of April, when the tempera- 
ture of the air is 47° in the shade, and quit it on the 
27th of June, when the temperature is 59°. 
It has been asserted that Cuckoos sometimes incu- 
