Fine morphology of experimental tail streamers and flight manoeuvrability in the house martinDelichon urbica

Abstract

Summary 1. Recent studies suggest that long tail streamers (narrow outermost tail feathers) of the barn swallow Hirundo rustica , Linnaeus have initially evolved purely via natural selection for enhanced manoeuvrability. According to an alternative view, streamers have evolved initially solely via sexual selection for costly signalling, and their slender profile is merely an adaptation reducing the aerodynamic cost of a long tail ornament. 2. In order to distinguish between these alternative hypotheses we performed a standardized flightmaze experiment, in which we imitated the initial elongation of outermost tail feathers in a streamer-less hirundine, the house martin Delichon urbica , Linnaeus, contrasting the effects on flight manoeuvrability from adding either a broad or a narrow piece of feather. 3. ‘Narrow-feathered’ house martins (which had initial tail streamers modelled on a natural streamer of the barn swallow) manoeuvred better than ‘broad-feathered’ house martins (which had the natural shape of tips of experimentally elongated outermost tail feathers left unchanged), independent of elongation of the feather. 4. A small elongation of outermost tail feathers did not significantly improve manoeuvrability either in the case of ‘narrow-feathered’ or ‘broad-feathered’ birds. 5. These results suggest that it is the slender shape, but not elongation of streamers that is important for manoeuvrability, and thus streamer elongation is better explained by the sexual-selection than the improved-manoeuvrability hypothesis. We discuss hypothetical scenarios for evolution of hirundine tail streamers.