Abstract
This paper presents an investigation of the sediment used by the Bank Swallow (Sand Martin) Riparia riparia for the construction of breeding tunnels. Grain-size distribution, bulk density and hydraulic conductivity of breeding-tunnel sediment were investigated at four localities in western Sweden. The investigation shows that the Bank Swallow was consistent in using layers composed of a narrow range of fine and medium sand. Ninety percent of the investigated breeding burrows were located in fine to medium sand (0.125–0.5 mm) and 10% in coarse sand (0.5–1.0 mm). No breeding tunnels were found in sediment finer than fine sand or coarser than coarse sand. The fine to medium sand fraction has the properties to hold stable walls and keep dry tunnels even during rainy periods with heavy infiltration. The hydraulic conductivity of the sediment was in the range of 10–4 to 10–3 m/s and the bulk density 1,510–1,575 kg/m3. In Sweden, artificially excavated slopes in gravel and sand pits have long been the dominating breeding locality for the Bank Swallow. However, during the last two decades, four major factors have led to the decrease of breeding localities: (1) a decreasing demand of aggregate resources, (2) landscaping of gravel and sand pits and stabilization of eroding slopes, including river banks and shorelines, (3) a change to quarries as a source for aggregate production, and (4) concentrating gravel and sand exploitation to fewer and larger pits. It is concluded that these factors are important for the decline of the Bank Swallow population in Sweden and possibly elsewhere in Europe and North America.
Highlights
The Bank Swallow Riparia riparia is the smallest bird in the swallow family and it is distributed as a breeding bird in much of North America north of Mexico and reaches the Artic Sea in the northern Mackenzie Delta region
The nest sites are found in a great variety of sediments, from coarse gravely to clayey soils, but there is a strong concentration to fine-medium sand with a clay content less that 10% (Petersen 1955, Svensson 1969, Heneberg 2001)
With the appropriate combination of fine to medium sand that is easy to dig and yet stable enough to give safe walls, they have found the ultimate strategy in their nesting
Summary
The Bank Swallow Riparia riparia is the smallest bird in the swallow family and it is distributed as a breeding bird in much of North America north of Mexico and reaches the Artic Sea in the northern Mackenzie Delta region. The typically breeding ground for the Bank Swallow in Sweden is within the uppermost 100 cm, sometimes only about 40 cm below the ground surface and 4 to meters up on the shaft wall in an active gravel pit where the vegetation layer has been removed. Because the vegetation cover is removed the evaporation is very small and during rainy periods, like in Sweden in July 1998 or 2000, more than 100 mm of rain may fall during 72 hours. This means that a 100 mm thick water body will move down through the soil profile. The purpose of this paper is to give the hydrogeological background to a clever and successful breeding strategy of the Bank Swallow and to highlight factors important for the decline in the Swedish Bank Swallow population
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