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 prseco- 

 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 King- 

 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 a study of them where they were 

 laid. 



Shape of Eggs. Birds 7 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 



