Spectral Data of Avian Plumage

Abstract

Plumage coloration of museum bird skins provides significant morphometrical data, although it is difficult to objectively access the latter. Among the different methods of analyzing coloration, reflection-spectrophotometry is the most effective means to collect such data, coping with the feather’s property of often reflecting ultraviolet light. Using coincident illumination and reading fibers of a conventional reflection-spectrophotometer, I advise positioning the latter at a perpendicular angle to the surface as measuring geometry dramatically affects the quality of obtained data. This measuring geometry on average provides both, the brightest reflections and the least variability in the resulting data. Plumage coloration of museum bird skins has been evaluated with regard to the reliability of the spectral information. Under appropriate storage conditions, the structural iridescent coloration of hummingbirds can be maintained unaltered for more than a hundred years. In contrast, some specimens are subject to variability in their coloration. Whenever dealing with spectral data, a potential a priory variation in plumage coloration has to be taken into account. <br /> Museum specimens exposed to light, dust or insect pests are in danger of alteration to their spectral properties and hence, become unsuitable for spectral analysis, either in the first place or due to acquired color changes. Most disadvantageous are the frequently occurring alterations in the ultraviolet as these remain undetectable to the human eye. However, even in the visible spectrum alterations might elude the observer and, in particular, small reflectance peaks could easily be ignored. Furthermore, at low levels of overall brightness and chroma in both, natural dull feathers or bleached specimens, slight variations in the reflectance spectrum might be entirely annihilated. It is essential to consider this disadvantageous variability in spectral data when analyzing avian coloration, as this variability does not represent actual differences within a population. If only overall brightness is reduced, a sample might still be suitable for taxonomic research as it might contain valuable information concerning hue. As the entire spectral property of a feather may be involved in avian signaling, only unaltered feathers are suitable for analysis, if behavioral or ecological topics are involved. <br /> For the first time, the crucial significance of preservation agents for the spectral properties of museum bird skins has been clearly demonstrated. Artificial fluorescence in museum bird skins originated in fluorescent compounds in preservation agents as well as from the remains of body fats due to unsatisfactory preservation techniques. As this affects the results of reflection spectrophotometric measurements, skins must be observed under black light illumination before collecting data. <br /> In my study, to my knowledge, the most diversified analysis has been conducted involving the distribution of fluorescence in avian plumage. 181 bird species in 14 families with fluorescent plumage parts have been confirmed and hence, avian fluorescence is far more widespread than it was previously assumed. The ecological reasons cannot yet be ascribed to particular context. Evidence for a dependency of both, light environment and biogeographical region was obtained at species level but could not be perpetuated at family level, suggesting an ecological cause of fluorescence.