Egg incubation
Egg incubation is the process by which an egg develops into a young animal inside the egg after it is laid. It needs a favorable environment — warmth, moisture and time — and is usually done by the parent or by some other heat source.
Temperature and humidity are key. Birds generally need a steady, species-specific temperature for the embryo to grow. In many reptiles, the temperature can influence the sex of the offspring. Humidity matters too: if the air is too dry, the egg can lose too much water and fail to hatch. As incubation proceeds, the egg becomes lighter because water evaporates and the air space inside grows. The shell’s minerals are partly absorbed by the developing chick to help form its skeleton.
In birds, who sits on the eggs and for how long varies a lot. The work is often shared or done mostly by one parent. Some common patterns: in many species the female does most of the incubation (for example, some canaries and robins), in others the male does most of it or even all of it (such as cassowaries). Some species, like the whooping crane, take turns. In a few cases, both parents cooperate or helpers assist with later broods. Incubation periods also vary greatly: from about 11 days for small birds to around 85 days for some large birds. The emperor penguin has one of the longest uninterrupted incubation times, while many birds hatch faster if they have more suitable conditions. The energetic cost can be high for adults; for example, albatrosses can lose a large portion of their body weight during incubation.
Megapodes are unusual among birds because they do not rely on body heat. They build mounds of rotting vegetation or use heat from the sun to incubate their eggs, creating a natural heat source outside the body.
Certain behaviors show fascinating diversity. The Namaqua sandgrouse shields its eggs from the daytime heat by standing over them and shading them with its wings. Crab plovers use sun warmth, while in other species incubation patterns are more traditional.
Reptiles have many incubation strategies. Sea turtles bury their eggs in warm sand, snakes may lay eggs in communal burrows to share warmth, and some lay their eggs in heat-retaining mounds or in holes they dig. Some reptiles, like alligators and crocodiles, rely on decaying vegetation or soil heat to incubate eggs. Fish generally do not incubate eggs, but some species mouthbrood their eggs, holding them in the mouth until they hatch. Some amphibians brood their eggs as well; for example, certain salamanders care for eggs with their bodies, and some frogs have eggs in skin pouches or other protective structures.
Among mammals, only the platypus and the echidna lay eggs. In the platypus, eggs spend most of development inside the mother, with only a short period of external incubation after laying.
Eggs can be stored before incubation to slow development. For many domestic birds, refrigeration at about 5°C can keep eggs viable for around two weeks.
Birds and other animals may start incubation with the first egg or wait until the clutch is complete, causing chicks to hatch at different times or all at once, depending on the species’ strategy.
In short, incubation is a wide and varied part of life, with many species using different temperatures, humidity levels, timing, and caregiving strategies to turn laid eggs into healthy young.
This page was last edited on 2 February 2026, at 09:35 (CET).