The occurrence of villi (outgrowths, projections) on the bases of the basalmost downy barbulcs of breast-feathers is documented with scanning electron microscopy. The presence of these structures is thought to constitute a synapomorphic character for Passeriformes, Trochilidac, Capitonidac, Indicatoridae, Ramphastidae, and Picidae, which once more casts suspicion on the monophylv of Wetmore's order Piciformes.
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Tim G. Brom (1990) studied this question.