The beautiful white-necked jacobin is usually seen on its own in the forest, although occasionally small groups may gather, alongside other hummingbirds, at flowering trees (6). At these trees, the hummingbirds feed on nectar, probing the flowers with their long, specialised bills to obtain the sugar-rich liquid (5), and sometimes becoming aggressive as they compete for this valuable resource (2). The white-necked jacobin also feeds on flying insects, making short flights from its perch to snatch one from the air, or hovering in front of a swarm and picking them off one by one. It may also pluck its prey from leaves or branches (2) (6).
During the breeding season, the fantastic plumage of the male white-necked jacobin is most prominent, as it performs an attractive territorial display (6). A male will shoot up into the air, suddenly fan out its white tail, and then slowly descend as it steadily turns, displaying the tail in its full glory to a watching female (6) (7). Such displays often take place at considerable heights (6), up in the forest canopy (2). The white-necked jacobin nests in the forest understorey, one to three metres above the ground, where it constructs a shallow nest of soft vegetation and cobwebs on the surface of a broad leaf, which is sheltered from above by another large leaf (2).