Sand martin – Riparia riparia

I love watching them in flight, especially when banking in tight turns, hunting for the insects on the wing.

From the RSPB website:
Sand martins are the smallest European hirundines (martins and swallows), with dark brown upper parts and dark under wings contrasting with otherwise pale under parts divided by a distinctive dark chest bar. Agile fliers, feeding mainly over water, they will perch on overhead wires or branches. They are gregarious in the breeding season and winter. During the past 50 years the European population has crashed on two occasions as a result of drought in the birds' African wintering grounds.