live births in birds

[JNorton@MAILBOX.UNE.EDU]

To the best of my lnowledge, live births in birds would be impossible
because of the structure of bird lungs.  The very small air
capillaries within the fixed-volume avian lung develop embryologically
as fluid-filled extensions of the parabronchi.  They cannot be
inflated at the time of hatching, but must be "dried out" by a period
of air-breathing prior to hatching.  During this period (after
internal pipping but before hatching) the avian embryo is ventilating
the lung, drying out the air capillaries, but still relying on gas
exchange through the chorioallantoic membranes.  Only when the air
capillaries are actually air-filled can the embryo hatch and rely on
its lungs alone for gas exchange.  Such a scenario requires a rigid
egg with a gas permeable shell and an air pocket.  The avian lung
would not be compatible with a live birth and the sudden accompanying
sudden switch to air breathing, because the air capillaries would not
be functional.

If dinosaurs such as sauropods produced live young, they could not
have avian-type lungs, at least those found in modern birds.

James M. Norton, Ph.D.
University of New England
11 Hill's Beach Road
Biddeford, ME  04005
phone: [207]283-0171 x2270
fax: [207]283-3249
email: jnorton@mailbox.une.edu