Egg Incubation Physiology - Anatomy & Physiology

Changes in Composition of the Egg during Incubation

As the embryo grows within the egg, the composition of the egg changes.
Both yolk and albumin diminish.
- Yolk providing energy for growth and maintenance of the embryo.
- Albumin providing protein for the embryo and giving up water, which is lost through pores in the shell.
Water is produced when yolk fat is oxidised.
Water lost from the egg is replaced by air, creating the 'air cell' at the blunt end of the egg.
Embryo punctures the chorioallantoic and internal shell membrane.
- Able to begin pulmonary ventilation, rebreathing the 'air cell' gas.
- Transition from diffusive respiration through the chorioallantois to breathing through the lungs.

Transfer of heat from the body to the egg is the most important contribution of the incubating adult to incubation.
Most birds develop a seasonal bare patch of skin known as the 'brood patch' on part of the thorax and abdomen.
- 'Brood patch' is in direct contact with the egg to permit a greater heat transfer than if there were feathers between them.
At the same time, there is an increase in the size and number of blood vessels in the bare skin.
- Serves to increase heat transfer, as hot blood runs closer to the surface.
Adult can adjust the rate of heat transfer by:
- Standing or leaving the egg.
- The closeness with which the 'brood patch' is applied to the egg.
- Responding physiologically to the temperature of the egg, thus increasing or decreasing metabolic heat production.
'Brood patch' temperature is 34.9-42.4ºC.
- This is 1.1-5.5ºC higher than the egg temperature.

Heat loss from the egg is prevented by a thin boundary layer of air adjacent to the shell.
If the egg is in a nest, the nest will also provide resistance to heat loss.