Sid is a freelance science journalist.

Birds, do your math: The pattern of feathers on the chest of your potential mate might provide a good sense of his or her overall health and well-being. In a new study, researchers find that a single number that describes the complexity of those configurations, a parameter called the fractal dimension, is linked to whether a bird has a strong immune system or is malnourished. (Fractals, possibly most well-known from pop art posters of the 1970s, are incredibly complex patterns that have the same amount of detail at all levels of scale, from the huge to the microscopic.) When scientists restricted the food of red-legged partridges (Alectoris rufa, inset), the feather patterns (details in main image) on their chests had a lower fractal dimension than those sported by their well-fed colleagues, they report online today in the Proceedings of the Royal Society B. The food-restricted birds, on average, weighed 13% less than their well-fed colleagues and had weaker immune systems, which makes fractal dimension an easily recognizable sign of a potential mate's health and vitality, the researchers contend.

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