Conductance of Water Vapor in Eggs of Burrowing and Nonburrowing Birds: Implications for Embryonic Gas Exchange

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Abstract
The physical dimensions, water vapor conductance ( ), permeability (Pm), and effective pore area (Ap) of eggs from a tunnel nester, the bank swallow (Riparia riparia), and an open nester, the barn swallow (Hirundo rustica), were compared. Development of a chorioallantois (CA) was found to increase by as much as 100% the magnitude of , Pm, and Ap in both species. Bank swallow eggs without a CA present have a higher , Ap, and Pm than barn swallows. This greater conductance of bank swallow eggs would not only facilitate water loss in the humid burrow microenvironment but would also enhance CO₂ loss and O₂ uptake through the shell. An improved ability to exchange gases would be advantageous for the eggs of these fossorial nesting birds since they are also exposed to low O₂ and high CO₂ tensions within burrows. A simple steady state model shows that the eggs of bank swallows may be exposed to a gaseous environment as extreme as 17% ;O₂ and 3% CO₂ and still have the same air-cell gas tensions as those in the eggs of nonburrowing species.