Abstract
Soil penetrability resistance was found to be crucial for nest site selection of all three Central European burrowing bird species-Sand Martins (Riparia riparia), European Bee-eaters (Merops apiaster), and Eurasian Kingfishers (Alcedo atthis). Soil penetrability resistance measurements were used to find out whether increased hardness of unexcavated banks is the key factor affecting the presence of burrowing birds.
All three species avoided banks composed of too compact or too loose soils. Birds discriminated not only between high- and low-quality breeding banks, but also between different soil strata within banks.
In banks with generally low penetrability resistance, Sand Martins preferred soil strata with the highest available penetrability resistance and compactness to avoid hole collapses. There was a preference for hard soil below holes to serve as a resistant platform when birds begin to dig their holes.
In Sand Martins, the penetrability resistance level affected physical characteristics of holes such as tunnel length and dimensions of the orifices. Excessive compactness-and probably not high talus presence-was a major cause of abandonment of Sand Martin localities.
A high penetrability resistance is the crucial factor for site selection in Sand Martins.