Predictable evolution toward larger brains and lower hand-wing indices in long-tailed birds

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 behaviors, 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.