Volcanic Sulfur May Color Barn Owl Feathers Red

Exposure to sulfur-rich volcanic soil may increase production of pheomelanin, a pigment that gives a reddish tint to the feathers of barn owls living on remote volcanic islands

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Barn owls, Tyto alba, are the most widely distributed species of owl in the world and are one of the most widespread of all species of birds, being found nearly everywhere, except for polar and desert regions, Asia north of the Himalayas, most of Indonesia, and a few Pacific Islands. Throughout their vast range, their body size differs by location as does their plumage coloring, which varies from almost completely white to a rich coppery-red.

This plumage coloration is entirely due to melanins, because no carotenoids or other pigments have been isolated from barn owl feathers (ref). In particular, plumage color on the belly varies from white to dark reddish corresponding to different amounts of the yellowish-red pigment, pheomelanin, being deposited into growing feathers. Barn owls’ belly plumage is especially variable, ranging from ghostly white to chocolate brown, and can also either be immaculate or heavily spotted with black spots of variable sizes created by the deposition of eumelanin (ref).

In general, mainland populations of barn owls are darker than those that dwell on isolated islands. But in 2021, environmental ecologist Andrea Romano, a Research Fellow at the University of Milan, and an international team of collaborators were surprised to see that barn owls living on some islands are redder than mainland populations (ref). This made them curious to learn more about the sorts of environmental conditions might influence plumage color, and why owls living on some small, isolated islands can be visibly darker than mainland populations?

Dr Romano and his collaborators took a closer look and found that many of these islands are volcanic in origin, which means the soil contains high concentrations of sulfur due to volcanic eruptions. Might sulfur be the answer to their question, wondered Dr Romano and his collaborators? Although many environmental factors are known to affect plumage color (humidity, rainfall, temperature and elevation for starters), they were aware that sulfur plays a crucial role in the biochemical synthesis of some melanin pigments, particularly pheomelanin, which incorporates sulfur-containing compounds into its structure, and that pheomelanin creates a reddish or copper color. They also knew that some previous studies had linked sulfur-rich diets or artificial sulfur sources like pollution to plumage and fur color. So the team proposed that a volcanic environment full of sulfur might trigger barn owls to produce more pheomelanin, thereby making their plumage darker.

To test this hypothesis, Dr Romano and his collaborators examined study skins of more than 2,000 barn owls specimens that had been collected throughout the previous decades from more than 50 isolated islands or island groups (Figure 1) and are held either in the collections of several museums or by private citizens.

The team used a spectrophotometer to measure the relative redness of the owls’ belly plumage, and averaged these spectrophotometric values measured for all the owls from each geographic location.

Dr Romano and his collaborators found that geographic origin influences barn owl plumage color. Specifically, they found that owls that inhabit large islands near a mainland are generally paler in color than those living on the mainland. In contrast, barn owls from remote islands composed of sulfur-rich volcanic soils or with recently active volcanoes had darker, redder plumage than those from islands lacking volcanos (Figure 2).

Dr Romano and his collaborators proposed that the reason for the barn owl’s coppery-red plumage may possibly be protective: the birds deposit excess sulfur, which is toxic, into their feathers to keep it metabolically isolated.

But at the same time, Dr Romano and his collaborators estimate that volcanic sulfur actually explains less than 10% of the measured barn owl plumage color variation, whereas genetics have a major influence on plumage color. For example, one gene, MC1R, is responsible for as much as 70% of the owls’ plumage color variation. The MC1R gene encodes instructions for making a protein receptor, the melanocortin 1 receptor, located on the surface of melanocytes, which are specialized cells that manufacture melanin pigments. Thus, MC1R plays a central role in normal pigmentation.

Other roles for darker feathers include helping the owls to better blend in with humid, shadowy vegetation. Nevertheless, this study’s findings are among the first evidence that environmental sources of sulfur — such as the soil — can profoundly influence the color of integument like fur or feathers.

Source:

Andrea Romano, Robin Séchaud, Luca Montanarella, and Alexandre Roulin (2023). Island volcanism predicts pheomelanin-based plumage colouration in a cosmopolitan raptor, Journal of Biogeography | doi:10.1111/jbi.14596

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