The male Common Sunbird-asity takes flower nectar and insects from crevices with its hooked bill, but its blue eye wattle clinches its avian uniqueness.
By Rex Graham
The blue skin of this tiny songbird endemic to Madagascar is not due to pigments. It is a “structural color,” produced by the way light reflects from its skin. Scientists have made recent progress in understanding the microscopic features of feathers, bills and eyes that make they appear to us as intense blues and greens. About 2.5% of bird species have these structural colors, which are produced by the way light reflects from “quasi-ordered arrays” of collagen fibers.
Common Sunbird-asity blueness
Other birds with blue skin patches include the Ruddy Duck, Helmeted Guineafowl, Satyr Tragopan, Whistling Heron, Toco Tucan, Bicolored Antbird, and the Madagascar Paradise-flycatcher. (The aqua-blue feet of Blue-footed Boobies is due to a dietary pigment.) Transmission electron micrographs of the upper layer of these birds’ blue skin reveals arrays of spaghetti-like collagen fibers, with all the noodles perfectly lined up and stacked within the top layer of the eye-wattle skin. Instead of randomly scattering light, the size and spacing of the semi-transparent collagen noodles reflect blue light “coherently.” Other colors of light scatter randomly or are absorbed in a deeper layer of skin.
The result is a bright patch of blue against a dark background of Tropical forest shade.
The structural colors of bird skin vary, depending on the size and spacing of the collagen fibers. Ornithologists are gaining a better understanding of the underlying mechanisms of structural colors in birds’ skin, feathers, bills and other structures.
Blue feathers employ several different nano-structures that deliver similar results. Some feather barbs are filled with a Swiss cheese-like matrix of mostly tiny air-bubble holes surrounded by keratin. Under the microscope, it looks spongy. Other blue-feather barbules have interior sandwich structure of air-filled channels alternating with keratin channels. Richard Prum, an ornithology professor at Yale University, found that the Swiss cheese and sandwich structures generate blue reflected light by a process called “coherent scattering.”
‘Quasi-ordered’ collagen arrays
Since blue skin has evolved independently in distantly-related birds, each one must take advantage of the innate structure and stacking tendency of collagen. It is one of the most common biological polymers. Indeed, blue skin also has evolved in a few primate and marsupial species. Richard Prum, an ornithology professor at Yale University,
Quasi-ordered arrays of dermal collagen fibers in skin are thought generate blue feather colors in a similar coherent light-scattering process. “Genetic variation in the nanostructure of integumentary collagen may occasionally create visible variations in reflectance that could become subject to subsequent natural selection , sexual, or social selection for structural color production,” said Prum, a co-author of the book Bird Coloration, Volume 1: Mechanisms and Measurements.
Prum and his colleagues were the first to note that that structurally colored skin is prevalent in forest interior birds, especially cuckoos, trogons and many songbird (passerines) and is likely to have evolved multiple times to enhance the conspicuousness of the birds intent on sending visual signals. The shaded interiors of these forests have diverse lighting environments. Depending on the kinds of trees present, their spacing, and the height above the forest floor, the lighting can be deep forest shade, somewhat brighter woodland shade, shade with small gaps, or sections with large gaps created by a fallen tree, river or other break in the canopy.
Visual signalling research
Visual signalling by birds in these environments is one of the most exciting areas of ornithological research, and the Common Sunbird-asity may play a key role. Currently, very little known about its mating habits, preferred forest micro-habitat, and feeding behavior.
The species is believed to breed August through January in Madagascar’s eastern forest canopy. The small bird has almost no tail. Maybe tail feathers would just get in the way as it busily probes for the nectar of Impatiens, showy mistletoe blooms, stunning red flowers in the Ginger family and flowering herbs, shrubs and trees in the Melastomataceae family. The male’s stunning eye wattle draws attention away from its special hair-like feather ornaments and iridescent blue- and yellow-fringed feathers and streaking on its 10-cm (4 in) body. Females are olive and yellow.
The photographic possibilities with so many flowers in the picture are remarkable, but actually taking one of the fast-moving bird presents challenges. It is most reliably see in Madagascar’s national parks and reserves, including Ranomafana National Park, Anjanaharibe-sud Special Reserve, and Marojejy National Park, according to Handbook of the Birds of the World. On a birding tour to one of these parks during this time of year would yield many other endemic Madagascar birds, lemurs, chameleons and other unique creatures.