White-necked Jacobin

The white-necked jacobin is 11 to 12 cm (4.3 to 4.7 in) long. Males weigh 7.4 to 9.0 g (0.26 to 0.32 oz) and females 6.0 to 9.2 g (0.21 to 0.32 oz). The male is unmistakable with its dark blue head and chest and white belly and tail; the tail feathers have black tips. A white band on the nape separates the blue head from the bright green back and long uppertail coverts. Females are highly variable, and may resemble adult or immature males. Most females have green upperparts, a blue-green throat and breast with white "scales", a white belly, and a mostly green tail with a blue end. Immature males vary from female-like, but with more white in the tail, to male-like with more black there. Immature females also vary, but usually have less white in the tail and are somewhat bronzy on the throat and chest.

The nominate subspecies of white-necked jacobin, F. m. mellivora, is found from southern Veracruz and northern Oaxaca, Mexico, through southern Belize, northern Guatemala, eastern Honduras and Nicaragua, eastern and western Costa Rica, and Panama into South America. In that continent, it is found in much of Colombia and Ecuador, eastern Peru, northern Bolivia, most of Venezuela, the Guianas, the northwestern half of Brazil, and the island of Trinidad. F. m. flabellifera is found only on the island of Tobago. The nominate has been recorded as a vagrant in Argentina, on the islands of Aruba and Curaçao, and on Isla Contoy off the coast of the Yucatan Peninsula. The white-necked jacobin inhabits the canopy and edges of humid forest and also semiopen landscapes such as tall secondary forest, gallery forest, and coffee and cacao plantations. It is usually seen high in trees, but comes lower at edges and in clearings. In elevation, it usually ranges from sea level to about 900 m (2,950 ft), but rarely has also been seen as high as 1,500 m (4,920 ft).

The white-necked jacobin's movement pattern is not well understood. It apparently moves seasonally as flower abundance changes, but details are lacking. It feeds on nectar at the flowers of tall trees, epiphytes, shrubs, and Heliconia plants. Several may feed in one tree and are aggressive to each other, but they are otherwise seldom territorial. Both sexes hawk small insects, mostly by hovering, darting, or sallying from perches. This species breeds in the dry to early wet seasons, which vary across their range. The nest is a shallow cup of plant down and cobweb placed on the upper surface of a leaf where another leaf provides a "roof". It is typically 1 to 3 m (3 to 10 ft) above ground and sometimes near a stream. Males display and chase in the canopy and along edges during the breeding season. Females use a fluttering flight to distract predators. The white-necked jacobin is not highly vocal. Its song is "a long series of high-pitched, single notes, repeated at rate around 0.7–1.0 notes/second 'tseee....tseee....tseee....tseee....'." Calls include "a short 'tsik', a longer, high-pitched 'sweet', and a descending 'swee-swee-swee-swee' in antagonistic interactions."