PERCHING BIRDS. 175 
nightjar cannot effect any grasp of the bough as it sits 
lengthwise, and therefore the necessary firm position is 
obtained in other ways. It will be seen by any one who 
will take the trouble to examine it, that the serratures 
on the centre claws are therefore placed in exactly the 
best position for preventing the foot from slipping side- 
ways; and this is still further provided against by the 
hind toe projecting forwards and inwards—so much so, 
indeed, that it has the appearance of being inserted on 
the inner side of the foot ; and the whole organ is thus 
admirably adapted for its designed use. 
It is worthy of remark, as tending to corroborate my 
theory, that there are one or two Australian species 
(Podargus and Agotheles), and at least one in South 
America, which have the middle claw smooth and the 
hind toe directed backwards. This variation in struc- - 
ture leads, as might be expected, to a corresponding 
difference in the use of the organ; and we consequently 
find that they do not perch sideways, but across, and 
also hop from bough to bough. 
I do not put this view forward as the undoubted solu- 
tion of the disputed point, but think that from both 
positive and negative evidence the probabilities are in 
its favour. 
There is a peculiarity about the jarring note of the 
nightjar which I have never seen mentioned by any 
writer. This peculiar sound consists of two notes, ut- 
tered alternately—one a third lower than the other— 
the highest being evidently made when the breath is 
expelled, and the lowest when it is inspired ; and thus 
the jar is continued without intermission for a much 
longer time than would otherwise be possible. 
Mr, Jesse, in his Gleanings, says, “ they continue their 
