The Microsphere Layer On The Eggshells Of Eared Grebes (Podiceps Nigricollis) And Its Impedance Of Water Vapor Conductance Through The Eggshell

Eggs of the Eared Grebe are laid on floating nests made from aquatic vegetation and may come into contact with or be partially submerged in water. Unlike eggs of most other bird species, water does not cross the eggshell of the Eared Grebe. The outermost Grebe eggshell layer contains calcium phosphate microspheres (0.69 ± 0.29 mm; mean ± SD; n=418), which are hypothesized to prevent the influx of water; however, the effects of microspheres on gas conductance across eggshells are unknown. The total external pore surface area (S pore area) and eggshell thickness of 181 eggshell fragments from 3 eggs were used to estimate the gas conductance. When eggshell gas conductance rates were normalized to fragment surface area, the mean conductance across eggshell fragments with microspheres (5.15 ± 5.79 mg H2O/day-torr/mm2) was significantly lower than those without microspheres (8.25 ± 5.72 mg H2O/day-torr/mm2; t180= -17.4, p < 0.001). Estimates of gas conductance through minimum pore diameters as determined via polyurethane casts (n=44) were similar. Ongoing experiments are being conducted to determine whether the effect on gas conductance is due to the microspheres or simply the change in eggshell thickness.