Effects of Nestling Age and Burrow Depth on CO₂ and O₂ Concentrations in the Burrows of Bank Swallows (Riparia riparia)

Gas samples were taken from the nest chambers of bank swallows (Riparia riparia) and analyzed for CO₂ and O₂ content. The mean CO₂ content was 2.62% with a maximum value of 5.58%, and the mean O₂ content was 17.83% with a minimum value of 14.61%. There was a positive and significant correlation of increasing CO₂ content with both increasing nestling age and increasing total metabolizing mass (adults plus young). With increasing metabolizing mass there is a linear increase in CO₂ content, which suggests no active regulation of gas concentrations. Burrow depth also affected CO₂, particularly in burrows with older nestlings. The effect of depth is most likely due to the relative importance of convective exchange rather than to diffusion. The increase in CO₂ was proportional to the decrease in O₂ with a ratio of CO₂:O₂ equal to 0.86.