The crimson sunbird is a species of bird in the sunbird family which feed largely on nectar, although they will also take insects, especially when feeding the young. Crimson sunbirds are tiny, only 11 cm long. Male 11·7–15 cm, 4·8–9 g; female 10 cm, 5–6·9 g. Male nominate race has forehead to centre of crown glossed purple-green. They have medium-length thin down-curved bills and brush-tipped tubular tongues, both adaptations to their nectar feeding. The adult male has a crimson breast and maroon back. The rump is yellow and the belly is olive. The female has an olive-green back, yellowish breast and white tips to the outer tail feathers. Most species can take nectar by hovering like a hummingbird, but usually perch to feed most of the time.
Diet is Insects, spiders (Araneae) and nectar. Forages in pairs and singly, also in family parties, usually low down but occasionally in canopy. Various forest types, including mangroves, heavily disturbed and regenerating secondary forests. Seasonal altitudinal movements in Himalayas. Sight record from Pakistan (Islamabad) in winter, Resident. Not globally threatened (Least Concern). Generally fairly common to common throughout wide range; uncommon to rare on Butung (off SE Sulawesi). Occurs in many protected areas.