Egg Incubation Physiology - Anatomy & Physiology

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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 chrioallantoic 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.

Heat Transfer to the Egg

  • 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.

Development of Physiological Functions