In the article "The function and evolution of the tail streamer in hirundines", the authors attempt to explain the mechanisms behind the morphology and evolutionary development of tail streamers in the barn swallow. They argue that the tail streamers represent a "trait shaped by the trade-offs between aerodynamic efficiency and reproductive benefits mediated through female preference" (Rowe, Evans and Buchanan, 2001). The effects of tail streamer length manipulation is tested in barn swallows and a closely
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hunt larger, more maneuverable prey would be of great asset to lend support of one the two hypotheses. Where as the authors maintain both natural and sexual selection to be the mechanisms behind the tail streamer development in barn swallows, I believe pure sexual selection is sufficient to explain the bulk of the data.
References: Rowe L, Evans M, Buchanan K, 2001. The function and evolution of the tail streamer in hirundines. Behavioral Ecology: V12, 2:157-163.