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Can you tell what a bird eats from the shape of its head? For many years, naturalists suspected that you could.
Bird head shapes were thought to evolve over time to be best adapted to eat the available food.
Charles Darwin himself took notes on the variety of beak shapes he found among finches in the Galápagos, and saw that they seemed to change in response to food availability.
But a new paper, published in Royal Society journal Proceedings B, is shedding light on the relationship between a bird's diet and how it looks.
It's the latest in a string of discoveries from a team led by Prof Anjali Goswami, who is studying skull shapes at the Museum, and it rewrites our understanding of how ecosystems influence evolution.
A recent analysis has suggested that a bird's diet is not necessarily a primary driver of skull shape. Instead, phylogeny and allometry (how the characteristics of living creatures change with size) might be more significant factors in skull evolution.
Anjali says, 'Darwin went to the Galápagos and saw finches with different beak shapes that correlated to what they were eating. That observation sparked an idea that things adapt their environment.
'However, Darwin was lucky to see that pattern - things aren't quite a simple as that. Our research has found that there isn't a simple relationship between diet and how bird skulls look.'
Dr Ryan Felice, a researcher at UCL and a co-author of the study, adds, 'If we apply Darwin's conclusion for different kinds of birds who primarily eat fish, then pelicans and penguins should have exactly the same head and beak shape, as they both use their beaks to eat fish. However, pelicans have a long beak and large throat pouch, while penguins' beaks are comparatively small.
'Although they eat the same thing, pelicans and penguins acquire their prey in different ways, demonstrating the important role behaviour plays in skull evolution.'
Bird diets do reveal one thing, however: how quickly a skull shape might change.
Anjali explains, 'When we looked at how shapes evolved and how fast they evolved, we found that birds with certain kinds of diets changed really quickly.
'If you are a seed eater (like a finch) or a nectar eater (like a hummingbird) then your skull shape evolves really fast, but carnivores such as eagles and owls show relatively slow evolution of their skulls.'
That's partly because birds that eat seeds and nectar use their beaks to process food, whereas carnivorous birds rip meat apart with their talons. Seed eaters may need to adapt to a changing environment much more quickly.
Anjali says, 'Birds that eat nectar or seeds are going to experience lots of competition for resources and must evolve in order to survive.
'So it does seem that although diet doesn't have a significant correlation with what you look like, it does impact how fast you evolve. Darwin did have it right, in that there is an impact of diet in the evolution of birds, but it is not quite as simple as he thought it was.'
Anjali's team examined a huge range of birds, many of them from the Museum's extensive research collection at Tring.
They built high-resolution 3D digital models of the bird skulls, allowing them to plot many more points on the skull than previously possible.
Anjali says, 'When people have studied this before, they tend to use really simple measures of shape, looking at just the curve of the beak. We use a much more intensive procedure.'
The team's next step is to expand their analysis to other groups of animals - and ultimately to understand all of the different factors that have shaped skull evolution through time.