Many birds are dutifully sitting on a nest right now keeping eggs warm. Eggs come in all shapes, sizes and even the contents inside vary.
An egg shell has thousands of pores to allow for air exchange but they are small enough to prevent bacteria from entering
Both the shape and size of the egg are determined by the internal structure of the female. Large birds lay large eggs and tiny birds lay tiny eggs. But when the size of the egg is compared to the size of the adult bird, small birds lay proportionally larger eggs.
For example, it would take 60 ostrich eggs to equal the weight of one ostrich but it would only take nine hummingbird eggs to equal the weight of one calliope hummingbird. Looking at percentages, an ostrich egg weighs 1.8 percent of the adult’s body weight, a robin egg eight percent and a wren egg 14 percent.
An outlier to all these percentages is the kiwi in New Zealand with 18 percent instead of the expected three percent of the adult’s body weight.
One factor influencing the weight of the egg is the yolk. Some eggs have proportionally larger yolks regardless of egg size. The difference results from the type of young hatching from the egg–precocial or altricial.
Hatchlings that are ready to leave the nest within 48 hours of hatching (like waterfowl) are called precocial. They hatch with their eyes open and have downy feathers. The yolks of precocial species average 40 percent of the egg’s weight.
On the other end are hatchlings that require great amounts of care and feeding after hatching and these young are called altricial. Their eyes are closed and they have little to no feathering like baby robins. The yolks of altricial species average 25 percent of the egg’s weight.
Since precocial young hatch ready to leave the nest, they tend to remain in the egg longer than altricial young and therefore need more yolk to sustain them.
Precocial young also tend to synchronize their hatching despite being laid days apart. Young mallards hatch within two to eight hours of each other despite being laid over several days (it takes about one day for a bird to create and lay an egg).
Researchers think baby birds communicate before they are born in order to synchronize hatching. Prior to hatching young birds emit a clicking noise and once one baby bird begins to click in the nest, it speeds up the final development of the other nestlings. This enables all the baby birds to hatch within a minimal time period.
The act of hatching can take several hours to several days depending on the species. Albatross chicks take several days to hatch. When a baby bird is ready to hatch it will assume the hatching position of its head between its right wing and body with its bill pointed at the blunt end.
As the baby bird develops, calcium from the shell is transferred to the growing bones of the bird
A day or two before hatching, the bird punctures the air sac at the blunt end and initiates lung breathing. Shortly after the bird begins making a clicking noise.
The beginning of shell breakage, or pipping, can begin more than 10 hours before emergence. Birds have an egg tooth or horny knob on their bill and also hatching muscles on the back of their head to help with pipping (both which disappear after hatching).
The baby bird strongly thrusts its beak into the shell using its entire body by pushing with its feet. The baby bird rotates within the shell and cracks a ring around the blunt end once or several times until it pushes with its feet and pushes itself out of the egg. Then life outside the egg begins.
