Physiological and Ecological Correlates of Tunnel Nesting in the European Bee-Eater, Merops apiaster

Bee-eaters excavate tunnel nests to a depth sufficient to blunt the impact of wide variations in surface temperatures on the thermal environment in the nest chamber. Thermal environment of the inhabitants is maintained within or near their thermal neutral zone. High chamber humidity, minimal thermal stress, and succulent insect food combine to stabilize water balance of nestlings. Diffusion of gases through soil and along the nest tunnel can account for maintenance of tolerable levels of O₂ and CO₂; however, the absence of nest sanitation and presence of microorganisms causes significant periodic elevations in NH₃ and CO₂ and a decrease in chamber O₂. This latter circumstance is associated with windless conditions. Wind movements rapidly improve internal conditions by the formation of gas vortices which penetrate via the nest tunnel to the chamber while displacing chamber gas to the exterior. The piston effect of adult birds moving along the tunnels and thermally induced density currents further augment gas renewal. The combination of soil characteristics and nest design provides a generally equable and uniform microenvironment for the developing young, even though external conditions may vary widely.