VOICES OF THE NIGHT 
only that sings, and for the purpose afore- 
mentioned. The note of the nightjar, on the 
other hand, is equally uttered by both sexes, 
and both also have the curious habit of 
repeatedly clapping the wings for several 
minutes together. They moreover share the 
business of incubation, taking day and night 
duty on the eggs, which, two in number, are 
laid on the bare ground without any pretence 
of a nest, and generally on open commons in 
the neighbourhood of patches of fern-brake. 
Like the owls, these birds sleep during the 
day and are active only when the sun goes 
down. It is this habit of seeking their insect 
food only in the gloaming which makes 
nightjars among the most difficult of birds 
to study from life, and all accounts of their 
feeding habits must therefore be received 
with caution, particularly that which com- 
pares the bristles on the mouth with baleen 
in whales, serving as a sort of strainer for 
the capture of minute flying prey. This is an 
interesting suggestion, and may even be 
sober fact ; but its adoption would necessi- 
tate the bird flying open-mouthed among the 
oaks and other trees beneath which it finds 
73 
