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.