Vertical embankments and mounds serve as suitable habitats for burrowing birds, such as the Sand Martin (Riparia riparia). Sand Martins have decreased in many countries during the last two decades, possibly because of the decline in suitable nest sites. Therefore, it is important to understand the factors affecting nest burrowing and nest hole characteristics for the Sand Martin. A smaller entrance hole would be beneficial for regulating the internal environment of the nest, whereas deeper nests are more advantageous against nest predators and parasites. We examined the general structure of Sand Martin colonies and determined if particle size, pH, or electrical conductivity (EC) of the soil and the location of the colony affects the morphology of Sand Martin nest holes. We hypothesized that the climate of near lakeshore and non-lakeshore differs; consequently, we predicted that Sand Martins would construct wider nest tunnel entrances in more humid environments near the lakeshore than further from the lake. We also hypothesized that a lower pH of clay loam soil would result in an increasing level of exchangeable aluminum (Al) and acidity, which in turn would promote soil aggregation. Because soils with a low EC are more stable and less prone to flooding or erosion, we predicted that Sand Martins in such soils would have deeper nesting burrows. A total of four colonies were located in the study area in Turkey. They contained 2510 burrows, of which 91.83% were used for breeding. The mean colony size was 627. We measured the soil and the nest burrow characteristics from the 80 nest bottoms used for breeding by the Sand Martin. The mean pH was 8.8, and the mean EC was 171. Tunnel depth was longer in nests with greater pH and EC and in lakeshore than in non-lakeshore colonies. The distance between nest holes increased with the EC, and nests were located nearer to each other in the lakeshore colonies than in non-lakeshore colonies. The width of the entrance opening increased with soil particle size and was wider in nests located at the lakeshore areas. Our results indicated that Sand Martins will avoid sites with too compact or loose soils for nesting, probably to avoid nest collapses. Vertical lakeshore embankments offer good nesting sites for Sand Martins and should, therefore, be protected. Because soil particle size, pH, EC, and distance from the lakeshore influenced the nest hole characteristics of the Sand Martin, conservation and management efforts should take these variables into account when maintaining or establishing suitable soil conditions for the Sand Martin.
Keywords: Bank Swallow; Collared Sand Martin; nesting strategies; soil structure.