Complex optical structure in the ribbon-like feathers of the African open-bill stork

The structural origin of the weak iridescence from the very peculiar ribbon-shaped feathers of the African open-bill stork, Anastomus lamelligerus (Ciconiidae) is investigated, using a combination of spectrophotometry, electron microscopy, and theoretical modelling. The cortex of these feathers can be described as a slab of keratin, transformed into a multilayer by the insertion of thin parallel planes containing harder nodules, disposed sideby- side and oriented along the feather axis. These nodules each show a sperically capped cylindrical shape. An empty cylindrical channel - the vacuole - occupies the long axis of the nodule. These nodules act in a collective and individual way to produce the frequency selection giving rise to the observed dark-green coloration of these special feathers.