All about birds’ eggs
When the egg cell, or ovum, with its attached food supply of yolk, is ready to be fertilized, it moves into mama bird’s oviduct, where things happen as if on an assembly line: After fertilization, the ovum is coated in cushioning layers of albumen, or egg white; next, it is encased with a thin shell membrane; and finally, the whole delicate bundle is coated in secretions of calcium carbonate (the same stuff as in limestone and chalk) that harden to create the porous eggshell. The entire process takes several days (with several egg cells maturing in a sequence); however, the shell itself is usually formed in only one day, just before mama lays her egg.
Unfragile as an egg?
An egg’s structure and shape give it strength. A chicken’s egg can reputedly withstand 10 tons of pressure per square inch if the force is applied evenly at its ends.
Shells come in almost every colour, thanks to only two pigments – blue/green and red/brown/black – produced by the “shell gland” in the bird’s oviduct. If blue/green pigment is secreted, it will be deposited throughout the eggshell, giving it a bluish or whitish base colour. The other pigment is responsible for the scrawls, spots, or splotches characteristic of a species’ egg (if the red/brown/black is uniformly applied to the surface, it will darken the base colour too). If the egg is not moving when the second pigment is added, the colour will appear as spots or speckles. If it’s moving, there’ll be scrawls or swirls. The markings are often densest at the blunt end of the egg, which usually emerges first.
Most birds lay their eggs soon after dawn, possibly because their inactivity during the night encourages egg development. They commonly lay them in one- to two-day intervals, though some species wait up to eight days between eggs.
Cleaning up the evidence
Once the chick has broken out of the shell, its parents will often eat the fragments (extra calcium for mother) or carry them a distance away so as not to attract predators’ attention to the hatchling. Birds with precocial young, such as the common merganser, don’t need to do this as their young are born ready to vacate the nest almost immediately.
Not all eggs are created oval
Scientists have identified four basic egg shapes, but a species’ egg often ranges between two shapes; e.g., elliptical to subelliptical, or oval to pyriform: ¬ Elliptical, meaning they’re widest in the middle, with equally rounded ends (heron egg); some are almost spherical (owl egg). Subelliptical, longer and more tapered towards the ends (merganser egg) ¬ Oval, broader, with one rounded end and the other more tapered (cedar waxwing egg) ¬ Pyriform, pear-shaped, with one quite pointed end (killdeer egg). Whether an egg is oval or pyriform is determined by the shape of the female’s oviduct in a particular species. But natural selection will choose variations of the shape that best suit the nesting environment.
Pip, pip, cheerio
The chick in the egg is well equipped to make its breakaway. It has a small hard point at the tip of its upper mandible, called the egg tooth, which it uses to chip its way out of the shell. The first cracks are called “starring” and the first tiny hole is called the “pip.” After a little while, the hatchling loses its baby tooth.
Stirred, not shaken
During incubation, parents will frequently stir and turn their eggs, as many as 11 times an hour, with their beaks. In a large clutch, the eggs are shifted about a little to ensure they get equal heating. In a small clutch, say with a single large egg, the parent has room to turn it completely so it gets warmed all over.
Fastest egg-layer in the nest
Since she doesn’t want to be caught in the act, the brown-headed cowbird takes a mere four to 60 seconds to drop her egg in a host’s nest. Other birds require up to an hour to deliver; in the loon’s case, with a great deal of straining and panting.