ニシヒメアマツバメ Apus affinis

Small swift with a square-tipped tail, broad outer wings and sharply-demarcated white rump. White on the rump extends to the sides and is visible from below. Note the obvious contrasting white throat. Superficially similar to house-martins, but lacks a white belly. Breeds both in natural cliff settings and in urban areas where it nests on buildings. Very vocal, particularly near the nest, giving a constant descending trill.