Five years ago I had just commenced the final year of my bachelor's degree in Zoology and I was casually scrolling my Facebook timeline in-between lectures. An article from the New Scientist came up: "Drone designers accidentally explain colour of albatross wings". I tend to scroll past most items on my feed, but this grabbed my attention instantly. Albatross wings. Anything to do with birds got a click from me. But this seemed particularly interesting. From when I was a child, I'd been mystified by why birds were the colours they were, and why different species looked the way they did. And now I was looking at an article that might answer one of my questions.
I was living near the coast in Cornwall at the time, and, although it is unfortunately devoid of albatrosses, it does have a lot of gulls (that's "seagulls" but in ornithology-speak). Moving there was the first time I had really noticed them. Sure, I had noticed them before, when they tried to eat my chips at Brighton beach or woke me up in the night by shrieking, but I'd never paid much attention to them. Now I was continually surrounded by them, quite literally. I had an apartment by the docks and they were nesting on the dockyard buildings and on my roof. They were in town when I went shopping and on the beach when I went for walks. But, unlike before, they didn't bother me. Now I noticed what each individual was doing and I was interested to learn more about them.
These gulls were herring gulls - fairly large, but not the largest of the gulls. They had relatively pale, silvery grey backs contrasting against their snow-white bodies. Every so often a lesser black-backed gull would make an appearance. Although these gulls are roughly the same size as a herring gull and look similar, they have a much darker, slate-grey back. They migrate far further than herring gulls, with many spending the winter in Africa. There was also another gull species at the docks - more regular than a lesser black-backed gull but less common than a herring gull: the Great Black-backed Gull, the largest gull in the world. You probably won't be surprised to learn what colour its back is.
The article explained that engineers seeking to improve the efficiency of drones had found that the colour of the surface of the wing was likely to be important. It is well-known that black absorbs radiation from the sun much better than white does - wearing a black T-shirt on a hot day is annoying because it absorbs all the visible light from the sun and converts it into heat whereas a white T-shirt reflects it away. The heat generated by dark surfaces warms the air and makes it flow more freely. The engineers showed that air is less dense when it flows over dark surfaces and that this reduces air resistance, or drag. They hypothesised that albatrosses and other seabirds may have darkly-pigmented wings because it reduces drag and saves them energy when flying. The pigment that gives bird feathers black, grey or brown colours is called melanin; it is the same pigment that gives our skin and hair their colour.
Having read the article, my first thought was "Do great black-backed gulls have such dark backs because they're so bloody big and hefty that they need all the help they can get when flying?" and it became my mission to answer this question. The only problem was I still had a year of my undergraduate course to go, and I didn't have the necessary skills to look into it anyway. After considering studying it for my master's, I settled on the more exciting topic of gull behaviour, but the idea never left my mind.
Fast forward a few years and I had taught myself how to do phylogenetic comparative analyses - that is, to take relatedness between species into account when considering how biological traits might cause one another. Armed with a phylogenetic tree of gull species (which is like a gull family tree but, instead of individuals, there are species) and data on gull body weight, wing size and wing colour, I set out to complete my analysis. My hypothesis was that heavier species would, on average, have darker backs and wings. I was able to show this pattern is true, and, taken together with the knowledge that dark surfaces reduce drag (which has now also been demonstrated in real bird wings), I suggest that the reason is because it helps flight efficiency.
Why not just grow more efficiently-shaped wings to compensate for having a larger body? Longer, more pointed wings are great for when you are in the air but are cumbersome, especially when taking off. Although they are both considered seabirds, a gull is in many ways not like an albatross. Gulls spend a lot of time on land, with a need to take off regularly to avoid ground predators, whereas albatrosses spend a long time airborne out at sea. Small-bodied species can take off with relative ease compared to a large-bodied species like a great black-backed gull, and experience less drag. It costs energy to produce pigment, so it is likely to be produced only if it's useful. Melanin is also important for protecting feathers against damage, which is particularly important in regions close to the equator where ultraviolet radiation from the sun is high.
It's hard to prove anything in science. All we can do as scientists is chip away at the truth, and that is what has been happening with this question. Scientists have already shown that dark wing pigmentation can reduce drag, and this analysis adds to the full picture of wing colour evolution. The ultimate test will be to see whether dark wings reduce drag in living birds.