Why Do Birds Sit on Their Eggs? Incubation Explained

Birds are observed sitting on their eggs, a common behavior that might seem simple but is fundamental to their reproductive success. This behavior, known as incubation, is a complex and finely tuned process. It involves more than just keeping eggs warm; it is a dedicated effort by parent birds to ensure the delicate development of new life within the egg.

Maintaining Optimal Temperature

A primary reason birds sit on their eggs is to provide a consistent, optimal temperature for embryonic development. Bird embryos require a specific temperature range, generally between 37°C and 39°C (98.6°F to 102.2°F), for proper growth. Temperatures outside this narrow range, even by a few degrees, can be detrimental or lethal. If eggs become too cold, development can slow down or stop entirely. Conversely, excessive heat can denature proteins within the egg and disrupt vital biochemical processes, leading to developmental abnormalities or death.

To facilitate efficient heat transfer, incubating birds develop a specialized area on their belly called a “brood patch.” This patch of skin becomes featherless, either through automatic shedding or plucking by the bird, which removes the insulating barrier of feathers. The brood patch also develops an increased blood supply, bringing warm blood closer to the surface of the skin. This allows for direct and effective conduction of the parent’s body heat to the eggs, maintaining the precise thermal conditions needed for the embryo to grow.

Protecting and Nurturing the Developing Embryo

Beyond temperature regulation, a parent bird’s presence on the nest offers vital protection and nurturing for the developing embryos. The parent acts as a physical shield, guarding the vulnerable eggs from predators that might otherwise consume them. This presence also conceals the eggs from view, adding another layer of defense.

Parent birds also shield their eggs from harsh environmental elements such as strong winds, heavy rain, or direct, intense sunlight. In hot conditions, some species may even stand over the nest to create shade or wet their belly feathers to cool the eggs through evaporation. This careful management of the nest microclimate helps prevent the eggs from overheating or drying out.

Regularly turning the eggs is another behavior during incubation. Birds turn their eggs multiple times a day, sometimes as often as every eight minutes or up to six times daily. This action prevents the developing embryo from sticking to the inside of the shell membranes, which can cause deformities or impede hatching. Turning also helps distribute heat evenly throughout the egg and ensures the embryo has access to fresh nutrients and oxygen. The eggshell contains microscopic pores that allow for the exchange of gases like oxygen and carbon dioxide, which is necessary for the embryo’s respiration.

What Happens If Eggs Aren’t Incubated?

If eggs are not properly incubated, the consequences for the developing embryo are serious. Without consistent warmth, the embryo’s development will slow, cease, or result in death. Embryos are sensitive to temperature fluctuations, and prolonged exposure to temperatures outside the optimal range can lead to mortality.

Lack of incubation also makes eggs susceptible to desiccation. Eggs require 50-65% relative humidity to prevent excessive water loss through their porous shells. Without the parent’s presence, the internal contents can dry, leading to the embryo’s demise.

Unattended eggs are vulnerable to predators, which can easily access them without the parent’s protection. Eggs left unincubated too long, even if fertile, will not hatch. Some bird eggs can survive a few days without incubation, especially if development is minimal, but their viability decreases rapidly.