160 THE FLIGHT OF BIRDS 
recuperate after the labours of the day. For most 
birds, however, the afer is reduced to a minimum. 
While they sleep perched on a branch the weight of 
their bodies keeps their legs bent at the ankle, and 
when the ankle is bent the toes grip automatically. 
Were it not for the automatic grip of the toes, the 
bird would fall and be at the mercy of any prowling 
enemy. 
¥ Call-notes and Song. 
When the bird became able to move with speed and 
make long journeys, some new power was required 
to bring the sexes together and to prevent a pair 
that had mated from becoming separated. Hence 
it is that birds are loud-voiced, though reptiles are 
nearly all of them dumb. To a mere call-note, a 
very humble origin, it is probable that we can trace 
the beautiful songs of the Nightingale and the 
Blackcap. For many of the migrant birds in par- 
ticular a loud, easily recognisable call-note was an 
urgent need. After a flight of some thousands of 
miles cock and hen must somehow find each other. 
The Nightingale, when he has passed the Mediter- 
ranean and reached his northern home, trumpets 
forth to all the world the fact that he has arrived. 
His future mate hears the call, and together they set 
about the all-important business of nesting. And 
thus we see that all a bird’s activities must be viewed 
in connection with the fact that he has wings and 
is capable of long flights. 
THre END. 
