Quick response to Ray White's query on grebe flocks:

Ray asked if his observations of Western Grebe rafts totalling 900 birds in Penn Cove and 350 birds at
Discovery Park mitigate my concern over possibly dwindling numbers.

At this point, I have only anecdotal evidence, but the anecdotes don't look good. My impression is that
the number of large rafts (over 100 birds) has dwindled precipitously over the past ten years. There
used to be many such flocks, now the few Ray identified are not supplemented by so many others. I
have been unable to locate any sizable flocks at various spots between Everett and Bellingham, nor
have I found good numbers around the S end of Whidbey Island. What about between Seattle and
Olympia? Vashon Island, West Seattle & Bainbridge have had flocks in past years. But the numbers just
don't seem close to numbers like 26,000 on Bellingham's 1991 CBC or 40,000 in Feb 1969 in
Bellingham Bay.

This may be a classic case of "shifting baselines" (see http://www.actionbioscience.org/environment/
olson.html). As the environment slowly changes, we set arbitrary baselines in our minds, usually
related to our own experience, on what is "normal" or reasonable for population numbers. Rafts of 400
Western Grebes seem like good-size rafts now, but in fact they bear little resemblance to what our local
waters supported not even a generation ago.

Western grebes contrast with Red-necked Grebes in their winter distribution in Puget Sound: Westerns
tend to lump into somewhat dense cohesive aggregations that used to number in the thousands. Red-
necks keep their distance from each other and never seem to aggregate into cohesive flocks. Gary
Shugart suggests that this is related to their diet: while Western Grebes chase schooling baitfish, red-
necks forage on bottom-dwelling flatfish. There may be different population trajectories for their prey
affecting current trends of predators differently. What if there are threshold effects, such that smaller
flocks of grebes hunt less efficiently (per individual) than larger flocks, even aside from the effects of
changes in prey population densities?

I worry that Western Grebe numbers are collapsing under our very noses while the best data we have is
the winter snapshot that CBC counts provide. Wouldn't it be great to harness the skills and record-
keeping obsession of the tweeters membership to target monitoring at species of current concern?
Such tools are already used in the form of ebird records, but on a regional level, such observations
could be more focused. I'll keep you posted as we think about how to set this up.

Best,

Dan



Date: Sat, 31 Mar 2007 10:03:50 -0700
From: Ray White <rw at seanet.com>
To: tweeters at u.washington.edu
Subject: [Tweeters] Western Grebe rafts, Penn Cove, Whidbey Island

March 30, about 2 p.m. -- Two large rafts of Western Grebes in Penn Cove.
Seen from Madrona (Drive?). One raft contained roughly 420-450 birds
(estimate made by counting 30 on one side, then sequentially applying the
area that they covered as a template on the rest of the raft. The other
raft appeared to be of about the same size.

A recent Tweeters posting lamented poor abundance of Western Grebes in
Puget Sound this spring, compared with previous years. Do my observations
of rafts off Seattle's Discovery Park (about 350 birds in total) on March
28 and the rafts in Penns Cove (800-900 birds in total) change that
impression? Of course, these could have been the same flocks, which had
moved 70 miles north and maybe picked up more birds. Are the flocks still
present at Discovery Park?

Ray White
Edmonds, WA
425-672-8268

******************************
Daniel Froehlich
Burke Museum, U. of Washington
Box 353010
Seattle, WA 98195-3010
206-595-2305
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