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Birds at the head of a flock are the more efficient, tactical fliers

American Association for the Advancement of Science

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IMAGE: Group of white storks flying. This material relates to a paper that appeared in the date, issue of Science, published by AAAS. The paper, by A. Flack at Max Planck... view more 

Credit: Photo by Simone Rosenfelder. Copyright MaxCine, Max Planck Institute for Ornithology Radolfzell

Detailed tracking of a flock of juvenile storks - birds known for lengthy migrations sustained by thermal winds - reveals very different flight tactics among those leading the group and those following; notably, leaders were more efficient at harnessing thermal winds, and they also flapped less. The results, say the authors, suggest that how individual storks find and exploit thermals is related to an underlying initial variation in flight performance. To date, whether individual storks work collectively to exploit thermals to migrate great distances is not well understood - in part because of the challenge of simultaneously tracking single birds within flocks. Here, Andrea Flack and colleagues tagged 27 juvenile white storks (Ciconia ciconia) with high-resolution tracking devices, monitoring their movements over roughly 1,000 kilometers during the first five days of their migratory journey; they focused on this period in part to evaluate whether individuals used each other to find and exploit thermals during their very first migration. Their data ultimately suggest that how individuals find and exploit thermals is not influenced by individual features such as sex or body size, or by conditions before the birds develop wings strong enough for flight. Rather, it is related to differences in initial flight performance. Specifically, they found that juvenile white storks with the lowest proportion of wing flapping activity tended to fly ahead of birds that flapped their wings more. The individuals in the lead were able to find thermals and circle to higher altitudes compared to those birds who made up the rear of the flock; instead of searching for thermals on their own, these birds followed the leaders directly to them. Followers not only spent considerably more time flapping their wings, but also spent less time riding thermal winds (thermalling), which saves energy during flight. Followers also finished thermalling earlier, at a lower altitude, likely to avoid being isolated from others - thus seemingly failing to exploit the full potential of thermals, the authors say. Gabrielle Nevitt discusses this study in a related Perspective.

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