(noun) The lower part or underside of a bird's body. The underparts include the chest, abdomen and flanks, though the chin, throat and undertail coverts can also be considered part of the underparts. The term underparts is typically used when all of a bird's plumage has similar colors and markings distinct from the upperparts, such as the rust and white underparts of the eastern bluebird or the whitish underparts of the northern mockingbird. In some bird species, the upperparts and underparts may both be similar, such as the gray plumage of the American dipper or the bright yellow coloration of the yellow warbler.
When using a bird's underparts for identification, note the "base" color of the plumage and describe any additional colors or markings, such as spots on the breast, a color patch on the throat or streaks on the flanks, that might be distinctive to help with proper identification.
Photo – Black-Necked Stilt © Windell H. Oskay, www.evilmadscientist.com