Spectrophotometric Studies of Avian Plumage Coloration with Special Emphasis on the Ultraviolet Range

Abstract

In my survey, altogether 1439 (15 %) of the 9702 bird species listed in Sibley & Monroe’s taxonomic list (1990, 1993) were measured by means of reflection spectrophotometry at different museum collections in Europe, the United States, and Australia. Species of every bird Order were studied in search of birds possessing UV reflecting plumage patches. A significant positive correlation was found between bird Orders with species which are believed to possess VS cone types, and Orders in which most species had their UV maxima between 380 – 399 nm. Orders with species which are assumed to have UVS cone types correlated significantly with Orders in which most species had their UV maxima between 300 – 379 nm respectively. My results suggest that, in general, interspecific variation in plumage color among birds is strongly influenced by the surrounding light and thus my findings support the “Light Environment Hypotheses. The measured birds were assigned to one of four light habitats: No UV phenomena were found in the “night” and the “sea and ice” habitat, few were found in open habitats, whereas 56% of the 425 surveyed species assigned to the forest habitat exhibited UV phenomena. Special emphasis lay on the parrot and hummingbird Orders. All the hummingbird species were surveyed and all of the 347 living parrot species (but 18) were measured at 16 different body regions. Every parrot species possessing chromatic plumage patches exhibited considerable amounts of ultraviolet light in at least one plumage patch. 50 % of the measured species exhibited considerable amounts of UV light in all chromatic plumage patches. Only 29 species of hummingbirds were found to exhibit UV phenomena. No significant correlation was found between parrots which were UV reflective in all chromatic plumage patches and their occurrence in UV-rich habitats such as in high altitudes and forests. Parrots possessing fluorescing plumage regions did not exhibit UV reflections in all chromatic patches to a significant extent. In contrast to the findings of the parrots, my results show a significant positive correlation between hummingbirds exhibiting UV phenomena and altitudinal occurrence. Evidence for altitude dependent evolution of UV reflecting plumage traits could be shown for the first time. To test whether UV phenomena are used in signaling in species with pure UV phenomena, the Satin Bowerbird (Ptilonorhynchos violaceus) was chosen as an example and a behavioral choice experiment was conducted. With the help of feathers of the Crimson Rosella, partly treated with sun block lotion, a procedure previously conducted in UV-related behavioral studies, I was able to demonstrate the Satin Bowerbird’s potential to see ultraviolet light. During their courtship male Satin Bowerbirds are known to collect blue ornaments to decorate their bowers. In my study reflection spectrophotometry revealed significantly more UV-blue compared to unicolored blue collected ornaments, at all bowers surveyed. UV-vision allows the female to judge the quality of male UV colored plumage and bower construction which is believed to have evolved to provide females information about the relative quality of males. My study supports the concept that birds of many more groups may see UV light than have been apparent to date. The large number of species exhibiting UV phenomena indicates their use as potential signals such as is the case with the Satin Bowerbird.