April 18, 2024

Why Do North American Vultures Have Red or Black Heads?

A common question asked of ornithology professors, birders and bird guides has a surprisingly complex answer

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As a former bird guide, one question I have often been asked is why do turkey vultures have red heads and white beaks, while black vultures have black heads and beaks? To the eyes of most casual observers, these two vultures look very similar, except for the colors of their heads, which provide a form of color coding to quickly distinguish the two species. Why do they have this obvious color difference? Are there advantages to one color over the other?

From an adaptive perspective, color can serve several functions in animals, and often represents a trade-off among different evolutionary drivers. These trade-offs can vary from individual to individual within the same population, and color can even change to serve very different purposes, such as thermoregulation and communication.

It is generally accepted that the skin color of vultures is black, Coragyps atratusas a result of skin cells containing eumelanins, a group of brown-black pigments in the upper layer of skin (or epidermis) that act as sunscreens that can effectively absorb UV photons, but how red skin coloring is created in turkey vultures, Cathartes auraand what adaptive purposes it serves remain a mystery and unexplored (ref).

Adding to the confusion, scientists have discovered a variety of biochemical mechanisms and processes underlying the creation of colors that look alike, but are not at all the same. For example, birds create red skin color in different ways. They may, for example, deposit a variety of red pigments (carotenoids) in their skin, or they may blush and thus turn their skin red by passing highly oxygenated blood near the surface of the skin – sometimes in combination with carotenoids (ref). As an added bonus, skin blushing is useful for communicating with bird social partners and mates, and some bird species can use it in dominance or courtship displays, too (more here ).

To explore the anatomy and physiology of skull skin colors in these two species of New World vultures, a team of scientists from Auburn University and Mississippi State University they collaborated to use different techniques to investigate this question.

To do this work, the researchers captured seven wild black vultures and seven wild turkey vultures at two sites in Mississippi. In addition to obtaining a small blood sample, they collected a small skin sample from each bird. Back in the lab, the researchers then measured the color of the skin samples using spectroscopy, examined the structure of the skin samples using light microscopy, and quantified the concentration of carotenoids present in the skin and blood of each species. Spectroscopy identified that eumelanin was responsible for the black coloration of black vulture skin, as predicted, and that hemoglobin is primarily responsible for the red coloration of turkey vulture skin.

When pigment tissue concentrations were assessed and compared between the two species, the researchers found that turkey vulture skin accumulated significantly more carotenoids than black vultures. But once again, they discovered that hemoglobin – and no carotenoids – the main pigment responsible for the red coloration of the turkey vulture’s head skin, and eumelanin is the source for the black head skin coloration of the black vulture.

This might make you wonder why adult turkey vultures have a higher concentration of skin carotenoids compared to adult black vultures if these molecules are not used to create red color? The increased concentration of carotenoids in turkey vulture skin may compensate for the lack of melanin in their skin cells and help protect against the harmful effects of the high levels of UV radiation that turkey vultures are exposed to when soaring in direct sunlight for long periods of time. In contrast, black vultures have high levels of eumelanins in their skin cells, which also rise for long periods, which act as biological sunscreens, so they are already protected from UV radiation and probably do not benefit from the additional protection of carotenoids.

There may be other evolutionary functions for the red head color of turkey vultures. For example, turkey vultures do not develop their bright red color until the bird reaches adulthood; juveniles have a dark gray head and are sometimes mistaken (by birders) for black vultures. This red color may indicate to other turkey vultures that a particular bird is mature. In addition, because turkey vultures are very social, often roosting in large numbers, they can use the blushing of the skin of their heads as a non-violent way to communicate with others.

Source:

Nicholas M. Justyn, Matthew J. Powers, Geoffrey E. Hill, Kayla Alexander, Adrián Naveda-Rodríguez, and Scott A. Rush (2022). The mechanisms of color production in black versus red skin on the heads of New World vultures, Avian Research 14:100071 | for me:10.1016/J.av.2022.100071


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