(noun) The short feathers under the base of a bird's tail. These feathers surround the cloaca (vent), the opening through which birds expel both liquid and solid waste.
Undertail coverts are often useful in proper bird identification because they can be easily seen and may have unique colors that help distinguish similar species. For example, the bohemian waxwing and cedar waxwing are very similar, but the bohemian waxwing has rich cinnamon or rust-colored undertail coverts whereas the cedar waxwing's undertail coverts are white.
Markings on the undertail coverts can also help with identification, such as with house finches and purple finches. The house finch has brown streaks on its undertail coverts, whereas the purple finch's undertail coverts are plain white. These types of differences – both colors and markings – can make field identification for many birds much easier, since the undertail coverts can be quickly seen even when the bird may not present adequate views of other field marks.
Photo – Bohemian Waxwing Undertail Coverts © pbonenfant
Crissum, Undertail Feathers, Under Tail Coverts