I haven't seen synchronous diving in grebes. Most of the grebes we see
around here (northern Idaho, eastern Washington) are solitary critters.
However, recently, I have observed synchronous diving in mergansers, both
Hooded and Commons, and in other diving ducks, such as Canvasbacks and
Redheads.
Yesterday, November 12, I visited a small reservoir in northern Idaho,
Mann Lake. Three Hooded Mergansers were diving synchronously in very
shallow water. The three, two females and a male, would swim with their
bills and face up to the eyes in the water. Then, suddenly, they would dive
so their whole bodies were just below the water (I could see the ripple
marks left by their bodies just under the surface). Then they would bob up
and resume swimming. They did this for at least an hour (that's how long I
was there) and were moving rapidly along the shoreline. I am incline to
believe this was a feeding behavior rather than a courtship manuever for two
reasons: 1) there were more females than males (usually multiple males court
single females) 2) they were being closely followed by a Ring-billed Gull.
I don't think sincronized diving was an anti-predator defense because there
wasn't any predators around.
Several weeks ago, I was at the same lake and observed Common Mergansers
diving repeatedly and rapidly in a tight group. The diving wasn't really
synchronized; some birds were under the water while others were above the
water. However, they were acting as a unit. They were all diving in the
same direction, coming up in the same direction, and moving along the shore
rapidly. For a brief period, a Western Grebe joined in the group, diving
with the mergansers in the same general direction. Again, I think this was
feeding behavior because they were being followed closely by the gulls. No
birds surfaced with fish in their bills. The merganser continued this
behavior for at least twenty minutes and worked their way almost all the way
around the lake. The also went up the small inlet; I thought they might be
driving the fish into the shallower water of the inlet (which is now mostly
dry).
I have also observed American White Pelican flocks feeding cooperatively
in southern Idaho at Twin Lakes (Franklin County). They were swimming in a
line directly towards the shoreline, poking their bills in water at regular
intervals. Suddenly, they picked up the pace and swam swiftly to shore,
tightening the circle. When they reached shallow water, the fish began to
splash on the surface of the water and the pelicans quickly gulped them
down. My mother and I watched in amazement as they repeated the process
several times.
Interestingly, mated pairs of diving ducks don't seem to synchronize
their diving. The female spends much more time under the water than above
it while the male spends more time below the water. Grebes may also show a
difference in diving behavior based on sex but it is much more difficult to
tell since they are monomorphic. I one time watched a mated pair of Western
Grebes on the Snake River. One was diving continuously while the other was
only diving occasionally. (I determined sex based on behavior, the female
diving constantly.) Whenever the female surfaced, the male would rush over
to her, calling. She would swim towards him, calling, then would dive and
he would resume swimming lazily on the surface. At one point, a Clark's
Grebe drifted down the river to where the pair were feeding. At the time,
the male had been underwater. He surfaced near the Clark's and began
calling loudly, swimming towards it. The Clark's Grebe ignored him as if he
wasn't there. The Clark's dove and the male Western also dove. The female
Western surfaced, called, and then dove. The Clark and the male Western
surfaced and he called to the Clark's incessantly. The Clark's ignored the
Western; it called occasionally but not directed at the Western. The call
was clearly different to me (and to the Clark's) but the Western didn't
appear to perceive a difference. At one point, the female Western surfaced
and called to the male and swam over to him and he finally ended his pursuit
of the Clark's. The Clark's Grebe floated off downstream. Clearly, the
Clark's Grebe could tell he wasn't her/his type but the Western couldn't!
Okay, slightly off topic but I thought it was interesting.

Deb

Deborah K. Beutler
Dept. Zoology
Washington State University
Pullman, Whitman Co., WA

dbeutler at wsunix.wsu.edu

-----Original Message-----
From: Bonnie Stout <bestout at sfu.ca>
To: tweeters at u.washington.edu <tweeters at u.washington.edu>
Date: Wednesday, November 11, 1998 9:12 PM
Subject: fishy grebe question


>
>(OK, so Tweeter's isn't for fun. Whatever. Anger can always be justified.
> There is no excuse for rudeness. Debate can be civil, if we try.)
>
>NOW! A question to those of you (who haven't unsubscribed in disgust) who
>know a little bit about coastal fishes or other marine life suitable for
>consumption by GREBES.
>
>Last year at this time I observed flocks of HORNED and WESTERN GREBES in
>Semiahmoo Bay (White Rock, BC). (separate flocks Horned Grebe flocks and
>Western Grebe flocks) These flocks were swimming together in a fairly
>tight group (more so than typical resting flocks) and DIVING TOGETHER. All
>or nearly all the birds in the flock would dive nearly simultaneously, over
>and over and over. The flocks also covered a lot of water, moving back and
>forth across and underneath the water as if in pursuit of something. Last
>weekend I saw this behavior again (really quite interesting to watch the
>birds synchronize their behavior).
>
>It would seem that these birds are following some sort of schooling fish
>(or maybe some other type of organism?). Does anybody have any ideas about
>what the grebes might be chasing? What kind of small fish would be
>schooling in 5-16 m of water in late October and November?
>
>I would also be interested to hear if anyone else has observed this
>behavior in grebes or other diving birds.
>
>Thanks!
>
>Bonnie Stout
>Burnaby, BC
>bestout at sfu.ca
>
>
>
>
>
>Bonnie Stout
>Department of Biological Sciences
>Simon Fraser University
>Burnaby, BC V5A 1S6 Canada
>ph:(604)-294-9355
>bestout at sfu.ca
>
>
>
>