438 The Outline of Science 



the sweetest warbler. There are other notes, too, which are not 

 vocal : pigeons, for instance, can clap their wings loudly together 

 in flight, the White Stork rattles the halves of his beak like casta- 

 nets, and the Snipe "bleats" or "drums" in springtime, as we 

 have already remarked. 



13 



NESTING HABITS 

 Primitive Nesting Habits 



If the earliest birds were arboreal, as we have reasons for 

 believing, the primitive nesting sites were doubtless also in trees. 

 The elaborate structures made by many present-day birds, how- 

 ever, are obviously products of a highly specialised habit which 

 has been evolved in the course of ages. At an earlier stage the 

 eggs would be laid in such natural sites as were available with- 

 out the necessity of building, and modern examples of a similar 

 habit are not wanting. A species of White Tern, for instance, 

 inhabits tropical islands and frequently deposits its single egg 

 on the strong horizontal leaf of a palm-tree. As Dr. H. O. 

 Forbes says, "The egg is laid in the narrow angular gap between 

 two leaflets on the summit of the arch of the leaf, where it rests 

 securely, without a scrap of nest ... yet defying the heaving 

 and twisting of the leaves in the strongest winds. The leaf, as 

 in all palms, goes on drooping further and further till it falls, 

 and among the settlers [on Cocos Keeling Island] it is a subject 

 of keen betting, when they see a tern sitting on an ominously 

 withered leaf, whether the young bird will be hatched or not 

 before the leaf falls. The result . . . has always been in favour 

 of the bird ; if the leaf falls in the afternoon, the tern will have 

 escaped from the egg in the morning!" 



Examples of birds which nest in holes in trees, in accordance 

 with the probably ancestral custom, are the Owls, the Parrots, 

 the Titmice, and of course the Woodpeckers. 



