Dave Bevan / Slow motion clip of a Sand martin (Riparia riparia) feeding chick at entrance to nest hole, Carmarthenshire, Wales, UK, May.

Dave Bevan / Pair of Sand martins (Riparia riparia) at nest hole in bank of coal waste, with another trying to land at the same hole, Carmarthenshire, Wales, UK, April.

Dave Bevan / Pairs of Sand martins (Riparia riparia) at entrances to nest holes in bank of coal waste, Carmarthenshire, Wales, UK, April.

Dave Bevan / Flock of Sand martins (Riparia riparia) preening, perched on tree roots adjacent to nesting bank, Carmarthenshire, Wales, UK, April.

Dave Bevan / Slow motion clip of a Sand martin (Riparia riparia) digging nest hole in a bank of waste coal, Carmarthenshire, Wales, UK, April.

Dave Bevan / Slow motion clip of a pair of Sand martins (Riparia riparia) at nest hole, chasing away an intruder, Carmarthenshire, Wales, UK, April.

Dave Bevan / Slow motion clip of a Sand martin (Riparia riparia) flying to nest hole and starting to dig before mate arrives to protect entrance, Carmarthenshire, Wales, UK, April.

Ralf Kistowski/ BIA / Sand Martin (Riparia riparia), North Rhine-Westphalia, Germany

Ralf Kistowski/ BIA / Sand Martin (Riparia riparia) at nest cavity, North Rhine-Westphalia, Germany

Ralph Martin/ BIA / Sand Martin (Riparia riparia) chicks in nest cavity, Baden-Wurttemberg, Germany

Hans Glader/ BIA / Sand Martin (Riparia riparia) at nest cavity, North Rhine-Westphalia, Germany

Phil Savoie / Sand Martin (Riparia riparia) in flight catching insects over the Madison River, Montana, USA. May.