In “Why do Vultures Spread Their Wings – Feathered Solar Panels” (Max Bird Facts), Maxfield Weakley writes:
Vultures will routinely seek patches of light in the morning before taking flight, and there are three leading theories to this behavior:
1. The vultures use the sun as an energy boost to start the morning.
2. The heat absorbed by the bird’s feathers cleans them of parasites.
3. The sun can help vultures digest more efficiently.
So, we now know that the main reasons vultures spread their wings are to warm up in the morning and remove parasites. There is a tertiary but unsubstantiated theory that it could be related to digestion. However, I think this is mostly to deter predators while the bird is defenseless on the ground.
The black color of the feathers has been selected by natural selection through random mutation over millions of years. Meaning the vultures with darker feathers were more efficient at warming up and getting rid of parasites.
Efficiency is the name of the game for birds. More efficient individuals will be more successful in foraging and finding mates. They then pass on their genes and traits to future generations.
Dark colors absorb the sun’s warmth more readily, so these darker birds would be able to start looking for food sooner than birds with a lighter aspect. This would also mean that the darker feathers absorb more heat and make them less desirable for parasites.
In “Soak up the sun: Understanding the Spread-winged stance” (Sunning Behavior of Turkey Vultures), Grace Wilde, Animal Behavior student, writes:
Turkey vultures (Cathartes aura), though unattractive, are quite a sight to behold when a flock all have their wings outstretched to their impressive length. This behavior is known as sunning; turkey vultures stand in a spread-winged stance to absorb the sun’s radiation. Many birds are known to sun, each with a different purpose, such as drying wet feathers or forcing the removal of parasites. Turkey vultures’ main reason for sunning is to raise their body temperature so they can have enough energy to go about their daily activities.
It is important to note that turkey vultures are one of few birds that intentionally put themselves into a daily hypothermic state each night, lowering their body temperature a few degrees. This has been thought to conserve energy since there is not a constant source of food (carrion) available. In a study on turkey vulture thermoregulation, completed by Zeev Arad, Uffe Midtgård and Marvin H. Bernstein, turkey vultures were found to have a thermoregulation system that allowed for the decrease in internal temperature. The study focused on the changing temperatures of the birds and the occurring behaviors, which included wing extensions.
Thermoregulation in Turkey Vultures. Vascular Anatomy, Arteriovenous Heat Exchange, and Behavior by Zeev Arad, Uffe Midtgård and Marvin H. Bernstein, 1989 (Article linked in the first “study”)
Spread-Winged Posture of Turkey Vultures: Single or Multiple Function? By Robert G. Clark and Robert D. Ohmart, 1985 (Article linked in the second “study”)
Spread-wing Posturing in the Turkey Vulture by Mike Mossman, 1974 (Article linked in “postures”)
See also “Sunning in extreme heat by the critically endangered hooded vulture: a strategy to fight
ectoparasites?” by Jorge S. Gutiérrez, José Pedro Granadeiro, João Belo, Edna Correia, Mohamed Henriques,
Filipe Moniz, João Paulino, André Tomás, Teresa Catry.
“Vulture Facts” by the Wildlife Center of Virginia.