In most birds, incubation does not begin until the final egg of a clutch has been laid thus, observers should not be alarmed when a songbird begins laying but appears to pay no attention to her nest. This allows for optimal exposure to external heat and to water-laden, energy-rich albumen inside the egg.

Within the egg-white are the chalazae, two twisted strands of mucin (a protein) that hold the yolk and early embryo in place at egg center-even when the egg is turned during incubation. This air cell fills with oxygen that is then available when the embryo begins breathing on its own just prior to hatch. Between the two membranes at the egg's 'base' is an air cell that increases in size as moisture and carbon dioxide depart through the eggshell. These, too, are semi-permeable and help keep out bacteria. Immediately inside the eggshell (see drawing above) and surrounding the albumen are two thin but tough membranes made mostly of keratin protein. Although fish and amphibians are obligated to lay eggs in more or less aqueous environments, bird and reptile eggs MUST be deposited in dry locales lest water seep in through cuticle and shell, eventually suffocating or drowning the developing embryo.Įgg anatomy drawing courtesy

The shell also has a outer coating (bloom or cuticle) semi-permeable, it helps repel dust and bacteria while also allowing passage of gases and some liquids. The eggshell itself-surprisingly strong for its weight and thickness-is made almost entirely of crystals of calcium carbonate (CaCO3) it is permeable, meaning air and moisture can pass through its pores (above). In many birds a new ovum is released at the same time an egg is being laid, with the process repeating until a clutch is complete.Īll text, maps, charts & photos © Hilton Pond CenterĬhicken eggshell photo courtesy ShutterstockĪ bird's egg is a completely self-contained unit, requiring nothing from the parents except protection and heat. This whole process from ovum release and fertilization to actual egg laying takes about 24 hours regardless of bird size. (Unfertilized ova are either resorbed in the reproductive tract or laid as infertile eggs.) Each fertilized egg-replete with an embryo astride the yolk-travels down the oviduct and passes through several substructures that sequentially produce the watery egg-white (albumen), various membranes, and-finally-a rubbery calcium-laden eggshell that hardens before moving through the vagina and out of the cloacal opening. (Click on the image below right to view a larger version of a female bird's reproductive tract, courtesy W.H Freeman and Co.) Once ripe, these ova are released-usually one per day-into the infundibulum, a funnel-like structure where fertilization occurs if a female has mated. (We also received a couple of notes insisting that bunnies really DO bring eggs.) As follow-up, our current photo essay below deals with that topic, from the time a bird egg is laid until it hatches.Īmong wild female birds at Hilton Pond Center and elsewhere, a rush of hormones-typically in spring-causes dormant ova (unfertilized eggs) to begin maturing in the bird's ovaries there, each receives a deposit of fat- and protein-rich yellow yolk. NOTE: After we posted 'This Week at Hilton Pond' installment #653 about the way bird ova are fertilized and hard-shelled eggs form in the female's oviduct, we received several e-mail inquiries about how embryos actually develop after laying.

Bird incubator hatching eggs