Abstract

Diverse plumages have evolved among birds through complex morphological modifications. We investigate how the interplay of light with surface and subsurface feather morphology determines the direction of light propagation, an understudied aspect of avian visual signalling. We hypothesize that milli-scale modifications of feathers produce anisotropic reflectance, the direction of which may be predicted by the orientation of the milli-scale structure. The subject of this study is the African Emerald Cuckoo, Chrysococcyx cupreus, noted for its shimmering green iridescent appearance. Using a spherical gantry, we measured the change in the directional reflectance across the feather surface and over a hemisphere of incident lighting directions. Using a microCT scanner, we also studied the morphology of the structural branches of the barb. We tracked the changes in the directional reflectance to the orientation of the structural branches as observed in the CT data. We conclude that (i) the far-field signal of the feather consists of multiple specular components, each associated with a different structural branch and (ii) the direction of each specular component is correlated to the orientation of the corresponding structure.

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Cuckoo feather reflectance data and visualization software

Images and Videos

Anisotropic reflectance from the brilliant green plumage of Chrysococcyx cupreus (African Emerald Cuckoo), Male.  Photographed by Dr. Hugh Chittenden in Eshowe, KwaZulu/Natal, South Africa on 4 Jan 2010 at 14h30. Copyright Dr. Hugh Chittenden. Reproduced with kind permission.

Shimmering anisotropic reflectance from a male Chrysococcyx cupreus (African Emerald Cuckoo) tertial feather illuminated by a light source orbiting the feather (one-dimension of movement).  Throughout the movement the light maintains a 30 degree elevation towards the feather's tip (in a plane 30 degrees above the midline of the feather vane).  The video begins with the light located at the right side of the screen and at a grazing angle to the feather. The light immediately proceeds to cross in front of the feather towards the left side of the screen. Upon reaching the left side of the screen the light orbits back towards the right side. The video concludes with the light located approximately midway. The rami and distal barbules can be seen illuminated in the zoom region.  This one-dimensional movement of the light does not illuminate the proximal barbules.

The microCT reconstruction of the Chrysococcyx cupreus (African Emerald Cuckoo) tertial feather vane rotating around an axis in the plane of the vane and perpendicular to its rami.  The inclined bases of the barbules form a three-dimensional herringbone zigzag in a plane orthogonal to the longitudinal axes of the rami. Our geometrical model correctly predicts the direction of the reflection from the orientation of the zigzagged structure.

Acknowledgements

Thanks to Dr Ellis Loew, Dr John Hermanson and Dr Susan Suarez, Cornell University; Dr James Harvey, CREOL, University of Central Florida; and Dr Richard Prum, Yale University for their guidance and review of this manuscript; Kalliope Stournaras, University Freiburg, for her translation of the Durrer and Villiger manuscript; Josh VanHouten, Yale University, and Mark Riccio, Cornell University, for microCT measurements; and Dr Donald Greenberg, Dr Jon Moon, Dr Jaroslav Krivanek, Wenzel Jakob, Edgar Velazquez-Armendariz and Hurf Sheldon, Cornell University, for their contributions in Computer Graphics. This research was supported by funding from the National Science Foundation (NSF CAREER award CCF-0347303 and NSF grant CCF-0541105).