78 “THE EGG 
tinued laying of our domestic fowls is a familiar illustration of the 
results following unnatural stimulation of the ovaries caused by per- 
sistent robbing. 
Size of the Eggs.—Primarily, the size of the egg depends upon the 
size of the bird that lays it. Hummingbirds, the smallest birds, lay 
the smallest eggs, while Ostriches, the largest birds, lay the largest 
eggs. This scale, however, is not always maintained, the eggs of preco- 
cial birds being relatively larger than those of altricial birds of the same 
size, obviously because of the more advanced, larger young they hold. 
Compare, for example, the eggs of a Meadowlark with those of an 
Upland Plover, or a Robin’s eggs with those of a Spotted Sandpiper. 
The Colors of Eggs.—The colors of eggs are due to pigments deposited 
by ducts while the egg is receiving its shell in the lower or uterine 
dilation of the oviduct. One or all the layers of the shell may be pig- 
mented, and variation in color may be caused by a superimposed 
stratum of carbonate of lime, producing lilac tints and ‘clouded’ or 
‘shell’ markings. In the earlier stages of their descent from reptilian 
ancestors all birds doubtless laid white eggs, as do all reptiles, and color 
is believed to have been acquired for purposes of protection, as the 
bird has departed from the reptilian habit of covering its eggs or laying 
them in covered situations and has adopted a more or less open nest. 
Hence, where the eggs are still placed in holes or hollowed trees, they as a 
rule are white, and where they are laid in exposed situations they are 
usually protectively marked. Compare, for example, the eggs of Kirfg- 
fishers, Woodpeckers, and Owls, with those of Terns, Snipe, or Plover. 
There are, however/ numerous exceptions to this rule; for instance, 
Doves lay white eggs in frail, open nests. But here both sexes incubate 
and the eggs are rarely exposed. Again, Grebes lay white or whitish 
eggs in flat, open nests, but they generally cover them with nest 
material before leaving the nest. 
While we cannot so readily explain why Chickadees or Cliff Swal- 
lows lay pigmented eggs, it is obvious that the significance of the colors 
of eggs can be ascertained only by asttidy of them where they were 
laid. 
Shape of Eggs.—Birds’ eggs are usually ovate, but may be ellip- 
tical, spherical, pyriform or conical. The pyriform egg of the Murre, 
when moved slowly, describes a circle about its own point, and is 
therefore less apt to roll over the narrow, rocky ledges on which Murres 
usually lay, than an oval egg. The conical eggs of Snipe and Plover 
are placed in the nest point downward or inward, thus fitting together 
so closely that they can be easily covered by the comparatively small- 
’ bodied parent. Thus with form, as with color, it is clear that the egg 
should be studied where the bird placed it. ~ 
Variations—Although, generally speaking, the eggs of the same 
species resemble one another, there is a wide range of variation in color, 
size and shape, and to a lesser degree in number. Doubtless these varia- 
tions are in the main an index of the physical condition of the bird 
