Predictable evolution towards larger brains and lower hand‐wing indices in long‐tailed birds

Abstract The elongated tail increases body drag and alters aerodynamic efficiency, resulting in a decrease in flight performance that is closely associated with the hand–wing index (HWI). Enlarged brains provide cognitive benefits including tool‐use behaviours, learning and decision‐making that increase individual fitness. By enhancing individual fitness, they ultimately contribute to a reduced risk of species extinction. Therefore, predictable evolution towards larger brains and lower HWIs may be expected for antipredation in long‐tailed birds. Here, we assembled brain and HWI data for 2473 bird species, including new brain measurements for 103 species, to test this hypothesis. We found that long‐tailed birds had larger relative brain sizes and smaller HWIs than their normal‐tailed counterparts, indicating that high cognitive capacity resulting from an enlarged brain probably offsets the negative effects of weak flight capacity, especially a higher predation risk. This is supported by the finding that long‐tailed birds with equivalent brain size faced a higher extinction risk than both normal‐ and short‐tailed birds. Our results suggest that the evolutionary trade‐off between brain size and HWI has sustained long‐tail evolution in birds, thus providing new insights into understanding natural selection and adaptive evolution. Read the free Plain Language Summary for this article on the Journal blog.