Loess and bee-eaters I: Ground properties affecting the nesting of European bee-eaters (Merops apiaster L.1758) in loess deposits

The European bee-eater (Merops apiaster L.1758) nests in tunnels in loess deposits. The properties of loess make it particularly suitable for tunnel nesting birds (a major factor is the metastable nature of the ground). The ‘Heneberg Compromise’ operates whereby the conflicting requirements of tunnel stability and ease of excavation dictate the optimum particle size for usable ground. The open structure of loess deposits, due to particle shape and airfall sedimentation, allows gas movement in nesting tunnels. It also allows local compaction during nest building which strengthens tunnel walls. The short range nature of the interparticle bonds in the ground material provides an almost ideal construction environment, ensuring a low plasticity index, which appears to be critical for tunnel building birds. Bee-eaters and sand martins dig tunnels in loess as ‘primary nesters’. These loess tunnels are used by many ‘secondary nesters’. The bee-eater is an efficient ecosystem engineer.Distribution maps of European bee-eater nesting, and of loess deposits, show some coincidence. A concentration of loess and nest regions is observed to the north of the Black Sea where the rivers Dnepr and Don deliver loess material, and to the west of the Black Sea in the Danube basin. The birds nest to the north of the demarcated Meigs arid/semi-arid zones in Africa, but spend winters to the south of these regions. They fly long distances from wintering zones to loess nesting regions, the longest migration of the bee-eaters. Even relatively minor loess deposits on the fringes of the breeding range, as in southern Poland, have their bee-eater inhabitants.