Bill, Kate and Tweeters:

There's an interesting writeup (in rather uninteresting telegraphic style)
on bobbing and its possible functions in the American Dipper species
account written by Hugh E. Kingery (1996) in the Birds of North American
series published by the American Ornithologists' Union and the Philadelphia
Academy of Sciences, which I'll try to type below without too many typos (I
hope; I'm not much of a typist). It includes some facts and ideas I'd not
known/thought of. Some may generate even more theories.

"Dipping Behavior. In up-and-down movement of entire body, bird bends legs
to crouch, then stands. During dip, it flexes or stretches wings and
sometimes depresses tail. Similar to bobbing of Rock Wren (Salpinctes
obsoletus), different from teeter of Spotted Sandpiper (Actitis macularia);
American Dipper moves entire body vertically (Bent 1948). Older birds dip
less frequently than younger birds (Steiger 1940; contra Bakus 1959a).
Blinks white eyelid while dipping. Dipping is also characteristic of the 2
Eurasian dipper species. South American species do NOT dip [!], and rarely
or never swim and dive, possibly because of competition with Torrent Ducks
(Merganetta armata) in the Andes (Tyler and Ormerod 1994).

American Dipper dips during courtship, feeding, and agitation, up to 40-60
times/minute. Dips more frequently when excited or disturbed. Does not dip
while preening.

Dipping intensifies with state of arousal of bird. Contexts include
feeding, competing for roost site, territorial advertisement, territorial
defense, and alarm over human or predator intrusions. Because of linear
territory [along stream] and high background noise, dipping is perhaps
detectable at greater distances than vocal sounds (Fite 1984).

Theories about function of dipping vary from communication to vision
enhancement, e.g., communication as to location or as warning to cohorts
(agitated dipping; Bakus 1959a), but solitary birds dip, as do groups of
fledglings; as intraspecific advertising or contact signal because of
stream noise (Tyler and Ormerod 1994); better vision into water (dipping
while feeding;HEK); triangulation (eye moves up and down with the dip) to
help bird measure its position in a complex moving environment (C.Bock
pers. comm.)."

Beside the above theories, I have seen these additional ones:

1. A dip suggests to a predator that is trying to creep up on a Dipper that
the Dipper is about to launch itself (take off), so the predator makes a
premature lunge at the Dipper in an attempt to prevent its escape, when the
predator really needed to get closer or move to an otherwise better
position to be successful. The result is that the predator misses the prey.

2. Dipping camouflages the Dipper because he is simulating the motion of
stream and thus blends in. This idea is the opposite of one of those
expresed in the Kingery article where dipping was thought to make the
Dipper MORE visible. Interesting how the same bird behavior can generate
such totally opposite ideas.

Richard (Dick) E. Johnson
Curator, Conner Museum
Washington State University
Pullman, WA 99164-4236
johnsonre at wsu.edu


>Tweeters -
>We were watching a spotted sandpiper this morning, and started wondering
>what purpose the bobbing body action served. Has anyone seen any theories
>on this?
>Thanks
>Bill and Kate, Lake Sammamish
>w_rietveld at hotmail.com
>
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