A Re Interpretation Of The Function And Evolution Of The Tail Streamer In Hirundines.

2221 words - 9 pages

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 other hand benefited from the addition of the tail streamers to the end of their outer tail feathers as indicated by flight performance.The authors argue that the results indicate the barn swallows on average possess tail streamers which are approximately 12 mm longer than that required for achieving the aerodynamic optimum for maneuvering flight. The evolution of the tail streamers can be attributed to natural selection, sexual selection or a combination of the two. The authors maintain that the results argue in favor of the third possibility. They theorize that the elongation of the tail streamers in the barn swallow was originally selected for by natural selection in order to enhance flight maneuverability. This improvement in flight performance in the sand martins with added tail streamers is taken as evidence for this point. Also the fact that shortening of the barn swallow tail streamers beyond the 12 mm length reduces flight maneuverability works to support this hypothesis. They go on to say that the elongation of the tail streamers in the barn swallows beyond the aerodynamic...

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