Alain Jacot, researcher of the Max Planck Institute for Ornithology in Seewiesen and colleagues analyzed the yellow coloring of breast feathers of great tits. Like other birds with their splendid red, orange or yellow plumage, the color of breast feathers of great tits is based on carotenoid pigments.
These are natural colorant substances, supplied by food. Research mostly divides plumage color into pigment based and structural color. Structural color develops from the specific structure reflecting light (i.e. keratin structure). “We want to know whether plumage color consist of pigment based or structural colors” says Jacot. The researchers combined fieldwork with lab work and computer simulation. In the field study, the researchers changed the environmental conditions by providing half of the brood with additional carotenoids and by manipulating the brood size - i.e., the number of siblings per brood. “We know from other studies that both factors affect the development of nestlings,” says Jacot.
To alter brood size, the researchers swapped partially the newly hatched chicks among nests and so broods with few or many chicks were created. Within the next days, half of the chicks of a brood were repeatedly fed with small carotenoid-filled pellets while the other nestlings received a placebo pellet. Shortly before the nestlings fledged, the researchers plucked a couple of breast feathers, which were analyzed at a later stage in the lab.
Nestlings that received carotenoid-pellets developed a more intense yellow feather coloration. In contrast, nestlings from small broods had much brighter feathers, while there was no effect on how yellow the feathers were. “We demonstrate in our study that the brightness of a feather depends on its structure and not on the amount of incorporated carotenoid pigments”, says Jacot. “These results show that most, if not all, yellow, orange, and red carotenoid-based colors also incorporate a structural component”. The longstanding division of plumage colors into structural and pigment-based colors is therefore not correct, corresponding to the researchers.
This study also shows that a single feather may be a multi-component signal that reflects different aspects including growth conditions or health state. A great tit with a very bright plumage most likely grew up with few siblings and one with very yellow breast feathers grew up in a territory with carotenoid-rich food. The question why birds from large broods have a less shiny plumage is still unsolved. “It is possible that the nestlings growing up in large broods have to fight more for their food, which causes more abrasion on their feathers” says Jacot. Another reason could be that nestlings from small broods are better fed and therefore are able to develop a thicker feather structure reflecting more light and thus look glossier. [AJ, SP]Original work:
Dr. Sabine Spehn | Max-Planck-Institut
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