white-rumped spinetail Zoonavena sylvatica

A small swift with short square tail (rounded when spread), a relatively large head, black upperparts, and a white rump patch. It has a distinctive wing shape with a hooked outer wing and a bulging mid-wing that is pinched at the base. Underparts grade from a brown-gray throat to a pure white belly without any sharp demarcation. Spines on the tail are minute and hard to discern in the field. Flight action, with rapid wingbeats and short glides, is different from that of Little Swift. Mostly seen in flocks, sticking to woodlands and hills, though it descends to the adjoining plains just after showers.