Water vapour conductance
We used an alternative method for the estimation of the water vapour conductance by using the eggshells following the methodology reported by Portugal et al. (2010), instead of using a whole infertile egg (Ar and Rahn 1985). Fragments from the equator of the eggshell (N=62) from the three species were glued (inside down) onto a PCR tube (SSI, 0.5 ml, Cat. No. 1110-02) filled with 200 μL of distilled water. The tubes were placed in PCR trays for easy handling and the trays placed in a desiccator containing 550 g of colour-indicating silica gel and the desiccator in a controlled temperature room at 25°C. Water loss was measured every 24 hours for a period of three days by weighing the tubes. Eggshells from commercially produced chicken eggs where included in this experiment as control to determine if the values obtained in this experiment were congruent with those reported in the literature. Separately, fragments from different eggshell regions of eggs that were intact enough for each region to be recognised, were used in the same way. The fragments were taken from the blunt end (B), the acute end (A) and the equator (E) (Figure 1B). The purpose for this was to determine if there were differences between the different eggshell regions and according to species.
We determined the daily water loss (\(M_{H_{2}O}\)) by weight loss and calculated the water vapor conductance as:
\(G_{H_{2}O}=M_{H_{2}O}/P_{H_{2}O}\) Eq. 1
Where is the water vapor conductance, and is the pressure difference at standard conditions (1 atmosphere and 25°C). The air cell pressure and nest environment pressure difference has been calculated for most avian species, including burrow nesters (23.77 mg.d-1.torr-1). Therefore, we used this value as the water pressure difference (\(P_{H_{2}O})\).