Comparison of multiple-angle spectrometry of plumage versus individual feathers for the assessment of sexual dichromatism in the long-tailed finch (Poephila acuticauda)

Reflectance spectrometry of avian plumage colour is a commonly used technique in ecological, behavioural and taxonomical studies. This method enables visualization of different characteristics in the ultraviolet part of the spectrum. Different research groups currently active in this field use diverse set-ups to assess avian colouration and compare results – without taking methodological differences into consideration. In this reflectometric study, we show significant differences between colour assessment results obtained with three different incidence and observation angle geometries (45°/45°, 45°/90° and 90°/90°) used for the sexual colour characterization of the long-tailed finch (Poephila acuticauda) feather coat. Furthermore, we show that results obtained from measuring the plumage and individual feathers lead to different conclusions. Both plumage and individual feathers revealed sexual dichromatism but, occasionally, at different angle geometries. This study shows how some plumage characteristics may be overviewed if single unidirectional reflectance spectrometry is used.