Decision making in burrowing birds: Sediment properties in conflict with biological variables

Distribution and properties of loess deposits were recently suggested to drive the presence of burrowing birds: sand martins Riparia riparia, European bee-eaters Merops apiaster, and kingfishers Alcedo spp. Packing density with very low CN (such as those of loess) was suggested to be the most suitable for sand martins and bee-eaters. Correlating the distribution range of particular sediments with nesting areas of the particular burrowing bird species is a great argument in a recent discussion on whether the availability of nest sites is limiting or not. However, biological variables should be also taken into account. Each of the particular species tends to occupy its own ecological niche, avoiding direct competition for space, food, or other resources. Tight adherence to loessic slopes would severely limit two of the three Central European burrowing bird species in their distribution range, even though they are physiologically adapted to utilize a broader range of nest bank substrates. While the European bee-eaters (and perhaps most of the other bee-eater species) are limited to the fine-grained loess and silt, the other two species utilize broader textural range requiring either sufficiently high penetration resistance (Eurasian kingfisher Alcedo atthis) or digging nest holes of limited cross-sectional area (sand martins).