What structural feature of the Willow Warbler's bill is characteristic of an insectivore specializing in catching small, aerial insects?
Answer
The bill is relatively fine and pointed, with the lower mandible usually paler at the base.
The bill structure of the Willow Warbler is highly specialized for its diet of small, aerial insects. It is described as being relatively fine and pointed, suited for picking tiny invertebrates from foliage or snatching them from the air during short sallies. Furthermore, there is a distinct colouration pattern on the bill: the upper mandible is dark, whereas the lower mandible typically exhibits a paler coloration, often being yellowish or horn-coloured specifically at its base. This fine structure contrasts slightly with the bill of some other warblers in the *Phylloscopus* genus.
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