1. Recent studies in Scotland suggest that the outermost tail feathers of Barn Swallows ( Hirundo rustica Linnaeus) may be around 10–12 mm longer than the aerodynamic optimum, with sexual selection for long tails accounting for this extra length. 2. To test this hypothesis, we shortened the outermost tail feathers in male and female Barn Swallows in southern Spain by cutting 1, 11 or 21 mm from...