To all of you with vision-connected explanations of a dipper dip, I point
out all the waterside birds that move only their tails or rear ends up and
down, which I personally think are similar adaptations. But maybe not . . .
Vision hypotheses should be testable, just as social ones are. Reflections
vary a lot, as do other viewing conditions. Time to watch dippers, not
computer screens!

Do wagtails pump their tails more frantically when in social
interactions than when merely foraging? I can't recall. Anyone watched any
wagtails lately (lucky you)? Or Spotted Sandpipers?--there's a good summer
project (social behavior surely described well in the literature, as
Spotteds have been well-studied).

I agree that Rock Wrens bob up and down, as Kelly Cassidy
mentioned, but there's no reason why nonwaterside birds shouldn't do this
(did you get my triple negative?). For example, Palm Warblers don't seem to
be waterside birds. Nevertheless, the up-and-down behavior is still highly
correlated with waterside living.

Another hypothesis to explain bird movements is that they serve to
flush or startle potential prey. The jerky wing-and-tail-out behavior of
redstarts comes to mind, or the movements of a Brown Creeper up a tree
(coupled with its white breast). Does anyone recall seeing this idea in
print?

Dennis Paulson
Slater Museum of Natural History
University of Puget Sound
Tacoma, WA 98416
(206) 756-3798
dpaulson at ups.edu