(noun) The sides of a bird's body between the edge of the wing and the legs or hips, framing the belly on each side. A bird's flanks often have unique markings such as barring, striping, streaks or a color wash or patch that can assist with proper identification. In some species, the color and markings of the flanks will match those of the chest and abdomen, but the lack of any distinguishing marks can be equally useful for identification. Field marks on the flanks are often easiest to see on perched birds.
When looking a bird's flanks for identification, note any different colors or markings on the flanks when compared to surrounding areas of the bird's body. Especially note how sharp or blurred colors and markings are, and how far along the flanks those markings may extend.
All types of birds have some species with distinct flank markings, though these types of field marks are more common with a number of warbler and sparrow species. For example, yellow-rumped warblers have streaked flanks with a yellow patch. In some birds, comparing the flanks of two similar species can help identify them clearly, such as the slightly barred flanks of the lesser scaup compared to the plain flanks of the very similar greater scaup.
Photo – Yellow-Rumped Warbler Flanks © marlin harms