Owl be seeing you...in UV

Arguably, birds are some of the most colorful organisms on the planet. The source of their colors is almost as diverse as the colors themselves. Some of these colors are the results of certain pigments, while others are the result of the physical structures of feathers and skin tissues themselves. Even the pigments themselves originate from many different sources. For example, some pigments are naturally made by the body, while others are derived from dietary sources.

Porphyrins are one group of pigments that life might not exist without. Porphyrins are responsible for plants being capable of carrying out photosynthesis (chlorophyll) and for many animals to transport oxygen for cellular respiration (the heme in hemoglobin). However, porphyrins used as external pigments is relatively rare in many vertebrate species. This is partly due to the fact that porphyrins are photosensitive and break down quickly when exposed to light. In some birds, it has been shown that these porphyrin pigments can break down in just 12 minutes of exposure to direct light.

One aspect of porphyrins that is really fascinating is that they fluoresce under ultraviolet (UV) light, and one study has shown that this UV fluorescence can potentially be used to determine the age of some owl species. The idea is that newer feathers incorporate the porphyrins in the keratin structure as the feathers grow and develop, and therefore should fluoresce brightly when exposed to a UV light. Older feathers, grown in previous years, should not fluoresce as brightly because their porphyrins are older, and will have broken down more through exposure to light. For some species of owls, they may take 3-6 years to completely molt their flight feathers; losing and replacing some feathers one year, and then other feathers in subsequent years. So, depending on which feathers exhibit bright fluorescence and which feathers are less fluorescent, you can potentially determine how old the bird is.

Not only are porphyrins potentially useful for indicating the age of a bird, but they may also be indicators of health to other birds. One study on Eurasian Eagle Owls found that the feather concentration of a porphyrin derivative (Coproporphyrin III) corresponded to the quality of the breeding sites from which they fledged. Birds with higher porphyrin concentrations were raised in habitats with an abundance of quality food, and as a result those fledglings also tended to have a greater body mass. It was suggested that the fluorescence of the porphyrins could be indicators to other individuals of how healthy the young owl as a result of where they fledged.

Another possibility is that the porphyrins may also convey some sort of advantage that helps keep birds healthy. Some studies, for example, have illustrated that porphyrins have antibacterial and antiviral properties, which might protect feathers of birds, and potentially be an indicator of health.

In order for UV fluorescence to be an indicator of intraspecific health, not only do the feathers need to reflect UV flight, but other birds have to be able to detect the reflected light with their eyes. For this, many bird species have special cone cells on their retina that are able to detect short wavelength light in the ultraviolet (UV) or violet (V) end of the spectrum, as well as the short (blue), medium (green), and long (red) wavelength cones like those that we possess. Additionally, each cone has a droplet of oil, pigmented with carotenoid pigment sequestered from their diet, that helps the cells absorb the correct wavelengths of light and filtering the other wavelengths. Unlike most birds, owls’ eyes have many more rod cells, good for detecting light as opposed to color, on their retinas than they do cone cells, and they lack the cone cells necessary to detect UV wavelengths. This makes sense given that owls are most active when there is less light in which to discern colors. The many rod cells help them to detect the smallest amounts of light so they can see in the dark. So, if owls lack cone cells to detect UV wavelengths, why have porphyrin pigments in their feathers that reflect UV wavelengths? It turns out that owls’ eyes are very good at transmitting UV light, and in spite of the fact that they lack cone cells capable of detecting UV wavelength light they are still able to detect it with rod cells.

The field of bird coloration is rapidly changing as scientists utilize new tools to examine why birds look the way they do, and ask new questions of how bird coloration potentially impacts how they interact with their environment and one another.

Special thanks to Dr. Blake Mathys of Ohio Dominican University who is doing research on the owls of central Ohio