The lesser goldfinch (Spinus psaltria) is a very small songbird of the Americas. Together with its relatives the American goldfinch and Lawrence’s goldfinch, it forms the American goldfinches clade in the genus Spinus sensu stricto.

The American goldfinches can be distinguished by the males having a black (rarely green) forehead, whereas the latter is (like the rest of the face) red or yellow in the European goldfinch and its relatives. North American males are markedly polymorphic and 5 subspecies are often named; at least 2 of them seem to represent a less-progressed stage in evolution however.

This petite species is not only the smallest North American Spinus finch, it may be the smallest true finch in the world. Some sources list more subtropical Spinus species as slightly smaller on average, including the Andean siskin. This species ranges from 9 to 12 cm (3.5 to 4.7 in) in length and can weigh from 8 to 11.5 g (0.28 to 0.41 oz). Among standard measurements, the wing chord is 5.5 to 7 cm (2.2 to 2.8 in), the tail is 3.9 to 4.7 cm (1.5 to 1.9 in), the bill is 0.9 to 1.1 cm (0.35 to 0.43 in) and the tarsus is 1.1 to 1.2 cm (0.43 to 0.47 in).[4] There is a slight NW-SE cline in size, with the largest birds from Mexico and south being up to one-fifth larger than the smallest from the extreme NW of its range; this effect is more pronounced in females. There is also considerable variation in the amount of black on head and back in males, and thus three subspecies have been proposed. But this variation too seem to be simple and clinal changes in allele frequency, and thus the “subspecies” might be better considered morphs or geographical forms.

Ear region is usually dark in typical psaltria.
Males are easily recognized by their bright yellow underparts and big white patches in the tail (outer rectrices) and on the wings (the base of the primaries). They range from having solid black from the back to the upper head including the ear-coverts to having these regions medium green; each of the back, crown and ear regions varies in darkness rather independently though as a rule the ears are not darker than the rest. In most of the range dark psaltria birds (Arkansas goldfinch) predominate. The light birds are termed hesperophilus and are most common in the far western U.S. and northwestern Mexico.

The zone in which both light and dark males occur on a regular basis is broadest in the north, and extends across the width of the Rocky Mountains and Sierra Madre Occidental ranges. It reaches the Pacific coast in southern Sonora to northern Sinaloa, roughly between area of Ciudad Obregón to Culiacán. In the United States, the most diverse array of phenotypes can be found in Colorado and New Mexico. East of the 106th meridian west in southwestern Texas as well as in most of Mexico, almost all males have black backs. Spinus psaltria colombianus, east and south of the Isthmus of Tehuantepec, is richer yellow below in males. This as well as the even yellower S. p. jouyi from the Yucatán Peninsula and adjacent regions and S. p. witti from the Islas Marías off Nayarit require more study, especially as at least the former two seem also to be significantly larger and longer-billed.

Females’ and immatures’ upperparts are more or less grayish olive-green; their underparts are yellowish, buffier in immatures. They have only a narrow strip of white on the wings (with other white markings in some forms) and little or no white on the tail. They are best distinguished from other members of the genus by the combination of small size, upperparts without white or yellow, and dark gray bill. In all plumages this bird can easily be taken for a New World warbler if the typical finch bill isn’t seen well.

Like other goldfinches, it has an undulating flight in which it frequently gives a call: in this case, a harsh chig chig chig. Another distinctive call is a very high-pitched, drawn-out whistle, often rising from one level pitch to another (teeeyeee) or falling (teeeyooo). The song is a prolonged warble or twitter, more phrased than that of the American goldfinch, often incorporating imitations of other species.

– wikipedia