The blue-black grassquit (Volatinia jacarina ) is a small Neotropical bird in the tanager family, Thraupidae. It is the only member of the genus Volatinia. It is a common and widespread bird that breeds from southern Mexico through Central America, and South America as far as northern Chile, Argentina and Paraguay, and on Trinidad and Tobago.
This species is sexually dimorphic; the male is glossy blue with some white under the wing. The female is brown above and pale buff with darker streaks below.
Adult blue-black grassquits are 10.2 cm (4.0 in) long and weigh 9.3 g (0.33 oz). They have a slender conical black bill. The male is glossy blue-black, with a black tail and wings; the white inner underwing is visible in flight or display. Female and immature birds have brown upperparts and dark-streaked buff underparts.
Social monogamous, extra-pair fertilizations, intraspecific parasitism, and quasi-parasitism are commonly found. During the breeding season, males defend small territories, about 13,0 - 72,5 m2, dominant males are normally lighter. The male has a jumping display, often performed for long periods, which gives rise to the local name "johnny jump-up". This is accompanied by a persistent wheezing jweeee call, jumping several times in a minute. The extravagant display also has a cost of call attention of predator, thus displays increase the nest predation. Predation are the main cause of breeding failure, and predator vocalizations can cause immune-related reaction to this species. Nests are small cups of rootlets (diameter about 7.5 cm) found at herbaceous vegetation 10–50 cm high, clustered at landscape, and placed preferably at high complex habitat spots. Nests are built by both sexes.
Blue-black grassquits will often form flocks when not breeding. They eat seeds, mostly on the ground.
