Smallest organisms
With a mass of approximately and a length of 5 centimetres (2.0 in), the bee hummingbird ("Mellisuga helenae") is the world's smallest bird species and the smallest warm-blooded vertebrate. Called the "zunzún" in its native habitat on Cuba, it is lighter than a Canadian or U.S. penny. It is said that it is "more apt to be mistaken for a bee than a bird". The bee hummingbird eats half its total body mass and drinks eight times its total body mass each day. Its nest is 3 cm across.
Lesser goldfinch
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 in length and can weigh from . Among standard measurements, the wing chord is , the tail is , the bill is and the tarsus is . 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.
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.