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.
Pictureguy/Shutterstock (Horned grebe nest)
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.
Amelia Martin/Shutterstock (American coot eggs)
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.
D and D Photo Sudbury/Shutterstock (American robin eggs)
Changing their spots
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.
Juris Kraulis/Shutterstock(Blue jay eggs)
Stirred, not shaken
During incubation, parents will infrequently 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.
Chrislofotos/Shutterstock (Canada Goose eggs)
Shape of things to come
Almost all shorebirds, such as the killdeer, lay large, "pyriform" eggs, meaning pear-shaped or tapering to a point at one end. They're large because shorebird chicks are born well-developed, or ”precocial” —almost ready to leave the nest. So, to adequately cover these big eggs during incubation, the parents take advantage of their tapered shape and orient them in the most compact arrangement, with the pointy ends facing in and down.
Pictureguy/Shutterstock (Killdeer eggs)
Fastest egg-layer in the nest
The female brown-headed cowbird never makes a nest but lays her eggs in the nests of other species and lets them do the work of raising her young. Called a brood parasite, she may lay an egg a day for up to 40 days, in different "host" nests. 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.
Agnieszka Bacal/Shutterstock (Indigo bunting nest with brown-headed cowbird egg)
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.
Sophia Granchinho/Shutterstock (Semipalmated plover eggs)
Ready for action
The male red-winged blackbird is frequently polygamous, and may have several nests on the go, each being incubated by a different female. The avian Casanova won't generally provide for his broods, but will protect them aggressively, dive-bombing intruders, including human passers-by.
Michael Siluk/Shutterstock (Red-winged blackbird nest)
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.
Leah Strople/Shutterstock (Herring gull nest)