A re-interpretation of the function and evolution of the tail streamer in hirundines.

Essay by darwin666University, Bachelor'sA, March 2003

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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 related species, the sand martins, who lack the tail streamer, by the use of a flight maze. Eighty four barn swallows and fifty seven sand martins underwent tail streamer manipulation and successfully made it through the flight maze. The manipulations consisted of removing various lengths of tail streamers in the barn swallows, and the addition of similar lengths of streamers to the ends of the outer tail feathers of the sand martins using an adhesive.

The flight maze consisted of an eighteen meter enclosure which presented the birds with nine meters of obstacle free acceleration zone followed by a nine meter maneuverable obstacle zone. The obstacles consisted of successive rows of strings suspended from the roof which were spaced in increasingly tighter rows towards the end of the maze. In order to get through the maze, the birds would have to perform increasingly tight turns around the strings and hence required excellent maneuverability on their part. The time taken for the birds to get through the maze as well as the number of strings they encountered during flight were taken as co-varying measures of each birds flight performance. The results indicated that the barn swallows benefited from increasingly shortened tail streamers up to a maximum of approximately 12 mm at which point further shortening actually decreased flight performance. The sand martins on the...