What coloration marks characterize the upper and lower mandibles of the Willow Flycatcher bill?
Answer
Upper mandible dark, lower mandible pale
The bill structure and coloration serve as a significant field mark, often overriding confusion caused by the body plumage. The coloration is highly specific: the upper mandible, known as the maxilla, is dark, ranging from dusky brown to blackish. In direct contrast, the lower mandible is notably pale, presenting colors that can range from yellowish-orange or pinkish to brownish-white. This clear differentiation between a dark upper bill and a light lower bill is a key characteristic used for positive identification, assuming adequate lighting conditions for observation.
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