I'm not sure I would buy the evasive story. But I'll offer this up as an
alternative. Since woodpeckers, some corvids, and finches do it, maybe
it's an adaptation to woodland life. It seems that the birds that fly
THROUGH the woods or below canopy have this pattern while other woodland
(warblers for instance...actually I can't recall how a warbler flies!)
who hawk and spend more time in the upper canopy don't. It would of
course be related to wing flapping room.

mmm

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