Dr. A. G. Butler,



L20



(like the somewhat similarly paddle-winged Penguins of our time)

to utilise a depression or burrow in the sand as a site for its eggs.


Fragile eggs lying on rock with no resilient material beneath

them would be very liable to be crushed, so that only the stronger

examples and those best formed to resist pressure would survive :

this may perhaps explain the thickness of shell and form of some of

those placed in such situations; then, again, chips of more or less

pointed wood in the hollows of trees might bruise or pierce delicate

shells, which may account for the hardness and polish of the eggs of

Woodpeckers and others.


Probably the earliest attempt to form a support for its eggs

is exemplified in the case of the Rock-dove, which lays upon a few

stalks or straws placed upon a ledge in the cave which it selects for

its habitation ; its relatives who inhabit trees have advanced a trifle

further, making platforms, of greater or less strength according to

their species, in the forked branches of trees or bushes; some other

groups (as, for instance, the Touracous) have not advanced any

further in nest-construction.


In platforms made of twigs or straws there must always be

more or less defined crevices or openings ; and, to prevent eggs

falling through the wider of these, they must necessarily be calked

in the centre with rootlets, grasses, leaves, or earth ;thus, even at

this early stage of development, one sees some of the materials in

use which are utilised in more advanced structures. When a plat¬

form is constructed in the centre of a many-branched fork, it naturally

tends to become depressed in the middle, thus becoming vaguely

saucer-shaped, and this I imagine was the first step towards the

prevalent cup-like nests of the present time.


The increased stability and safety of eggs deposited in a

saucer-shaped as compared with a level platform would probably

encourage the builder to improve upon this type of nest by building

up the outer margin so as still further to safeguard its contents

during rough weather. As I have pointed out elsewhere, the con¬

struction of most cup-shaped nests is a very simple process. The

parent bird first accumulates a quantity of twigs, straw, bents or


* In my experience the Wood-pigeon generally uses rootlets, which tend to bind

the twigs composing the platform firmly together.



