Distinctive small swallow, with blue-black upperparts, a big white rump patch, and snow-white underparts. Juvenile in late summer and autumn is duller above and dingier below than the adult. Compare with other swallows; note the relatively shallowly forked and streamerless tail, as well as jerky and fluttery flight. Breeds colonially, frequently building mud nests under the eaves of buildings; forages over varied open and semiopen habitats, especially water bodies and adjacent fields. Gives a soft burbling trill.