The ‘egg cycle’ of all birds is loosely similar regardless of the size of the egg: a clutch of eggs is laid, incubation follows, and, after a specific period of time (which varies according to species), young hatchlings emerge from the eggs to begin life in the outside world.
But what factors affect how long an egg needs to be incubated? And do eggs survive if they are not incubated? If you’d like to know more about the fascinating hatching process, then please read on.
After a period of incubation – anything from 9 days for a white-eye to 80 days for a royal albatross – newborn chicks emerge from eggs, pecking their way out from inside the shell. Incubation is undertaken by the mother alone, by both parents in rotation, or in rarer cases, by just the male.
Artificial incubation is also used in commercial settings, with thermostat-controlled incubators maintaining the required temperature for eggs of chickens, ducks, quails and pheasants to hatch successfully.
Royal Albatross eggs can be incubated for up to 80 days
Once an egg has been sufficiently incubated and the embryo inside has developed fully enough to be able to survive, the tiniest hole will appear in the shell, made by a sharp cap on a hatchling’s beak known as an egg tooth. This temporary anatomical feature allows the chick to tap from inside the egg and cause a crack to appear. The crack in the eggshell ultimately widens, creating the chick’s exit point.
Hatchlings generally do not need any intervention while hatching, and it may take some time between the first cracks appearing and the young bird being ready to emerge, but it will usually have the strength to do this by itself.
Keep reading if you’re interested in learning more about the egg-hatching process, and look into whether eggs grow in size as the embryo inside develops.
Recently hatched chicks of a Fieldfare
The timeline from an egg being laid to hatchling is relatively universal. Once a clutch of eggs has been laid, incubation follows, and although the length and source of heat may differ in various species, the aim and outcome are the same.
From inside the egg, birds show the initial signs of hatching once they are developed enough to stand a good chance of survival.
The major steps followed in the egg-hatching process are as follows:
Once fertilized eggs have been laid, they need to be kept at a constant temperature for a certain number of days. Incubation periods vary from species to species, but are on average 12 to 15 days for most songbirds, and double this for larger birds, such as owls and kestrels.
To aid with temperature regulation, the incubating bird develops what is known as a “brood patch,” a featherless area on a bird’s abdomen which is filled with blood vessels, and is adapted to provide extra warmth to eggs that are being incubated.
Female American Robin sat on the nest incubating her eggs
The physical act of hatching of a bird is triggered by development of the embryo reaching completion inside the egg. As the unhatched embryo forms, an anatomical feature called an “egg tooth” develops; this sharp cap on the edge of a hatchling’s beak helps the chick to break out of an egg, in a process known as “pipping”.
The first external sign of hatching is when a tiny crack or hole appears on the eggshell, as the embryonic chick begins to pip its way out from the inside. By pecking in a circular motion from inside the shell, the weakest point is located, and the tiny crack opens wider, allowing the bird’s head to emerge, followed by the rest of its body. This stage can take more than 12 hours in some species.
Duckling hatching from their eggs
Once the hatchlings have fully emerged from the eggs, it’s common for the mother bird to remove all the eggshell fragments from the nest, to free up a bit of living space for the growing chicks.
The incubation time needed for birds’ eggs to hatch successfully varies from species to species, with small songbirds’ eggs hatching after a shorter time than those of larger, heavier bird species.
The shortest incubation period is claimed by the white-eye or silver-eye, whose young hatch after just 9 to 10 days. Northern cardinals and dark-eyed juncos’ eggs are incubated for 13 to 14 days before hatching.
At the opposite end of the incubation length scale are larger birds, such as barn owls (29 to 32 days) golden eagles (41 to 45 days), California condor (53 to 60 days), and the Royal albatross (up to 80 days).
Oriental White Eye chicks usually hatch within just 10 days
In some bird species, the eggs are incubated solely by the father, including emus, cassowaries and emperor penguins.
Jacanas, dotterels, and phalaropes are polyandrous species, meaning that the females mate with more than one male. For such birds, incubation and raising of young is undertaken by the male alone. However these species are relatively rare and comprise around only 5 percent of all bird species.
Birds’ eggs do need to be incubated with an external heat source to hatch, and in the wild, this is by either parent bird. One exception is cuckoos’ eggs, which are never hatched by the mother cuckoo, as they are stealthily laid in other brooding birds’ nests.
Another highly unusual anomaly in the bird world is the maleo, an extremely rare shorebird native to Sulawesi, Indonesia. Instead of being incubated by a parent bird, eggs are laid in a pit dug by the mother in the hot ground, where they are ‘baked’ for between 60 and 90 days, before hatching, with the newborn chick needing to tunnel up to the sand’s surface.
Poultry farmers rely on artificial incubators set at a specific temperature to successfully hatch eggs laid by chickens, ducks, quails, geese, and pheasants. In these conditions, no mother birds are present and the set-up allows the eggs to incubate for the necessary amount of time.
Maleos bury their eggs in the hot ground
If incubation has been successful, the chick will break out from inside the egg without any intervention from its parents. Once the embryonic chick has reached full term and is sufficiently developed to be able to survive in the outside world, it will begin pipping at the eggshell, and gradually emerge through the crack it makes with its beak.
Once an egg is laid, it does not increase in size, and it actually decreases in weight as the days pass, due to moisture loss through the permeable eggshell. Interestingly, a newly hatched chick is lighter than the egg that was laid.
Sandhill Crane sat on the nest awaiting two more eggs to hatch, with one hatched chick nearby
Eggs need to be kept at a constant temperature a few degrees lower than a bird’s average body temperature for normal development to occur. The optimum temperature for the eggs of many bird species is 98.6 degrees F (37 degrees C), which is roughly the same as normal human body temperature.
If the temperature in the nest drops too far below this for any length of time, then there’s a chance that the clutch will not hatch successfully.
Conversely, if the nest temperature is too high, then there’s a risk that eggs will hatch too early and newborn chicks will not be fully developed and unable to survive.
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