The trend noted by Kelly Mcallister of increasing attacks by Bald
Eagles on Great Blue Herons is also evident in south coastal British
Columbia, especially on the Gulf Islands. Dr. Rob Butler of the
Canadian Wildlife Service, who has done extensive research on Great
Blue Herons, has documented frequent attacks by eagles on heronries,
and, I believe, colony abandonments resulting from attacks. This is a
serious problem for some, but by no means all, heronries. I would
expect that heronries close to salt water are most vulnerable to
attack, while those located farther inland (where there are fewer
eagles) are less vulnerable.
I have sent a copy of this message to Dr. Butler in case he wishes
to offer a brief summary himself.
One of the reasons for the increase in eagle attacks on heronries
is the great increase in Bald Eagles-- I would say at least a tenfold
increase in both breeding and wintering populations in the Georgia
Strait/Puget Sound area-- over the last 30 years. (Herons may occupy
their colonies as early as late February, so are vulnerable to attack
even from wintering eagles.) This increase in eagles, I believe, is a
somewhat delayed but direct result of the banning of DDT in 1972. I
suspect that the increase in eagles is a return to pre-DDT
populations, rather than an increase in numbers beyond the
pre-settlement carrying capacity. (The latter has happened, for
instance, with Glaucous-winged Gulls, because of a vast increase in
food supply-- garbage dumps, fishing vessels, etc.)
Whenever one species increases greatly like Bald Eagles have done,
one or more species is likely to suffer as a result because of
predation, competition, or some other factor. In this case, Great Blue
Herons may be one species that suffers somewhat. Speaking of gulls,
they are also frequent targets of eagle predation (as anyone who has
tried to find a Slaty-backed Gull near the Vancouver garbage dump in
Burns Bog knows.) I can remember when eagle attacks on gulls were a
novelty, but they are now so common that birders sometimes have
trouble finding the flocks of loafing gulls, or at least looking them
over thoroughly before they are spooked by an eagle.
I suspect that eagles have had negligible effect on gull
populations, but another species that is quite vulnerable is the
Double-crested Cormorant. Even an eagle flyover, without an attack,
can cause serious mortality in Double-crest colonies. "Eagle panics"
caused by a flyover can cause dozens of eggs to be dislodged from
nests when the adults take off in a panic. (The sudden appearance of a
human can do the same thing.) I don't know if there are recent data
showing a decrease in Double-crest numbers in the Gulf of Georgia
(they were increasing till recently). If so, an increase in eagles
could be one of the causes.
The "balance of nature" is a delicate thing, and even something as
welcome as the increase of Bald Eagles in the Northwest has the
potential of being bad news for some other species.

Wayne C. Weber
114-525 Dalgleish Drive
Kamloops, B.C. V2C 6E4
Phone: (250) 377-8865
wayne_weber at bc.sympatico.ca


-----Original Message-----
From: Kelly Mcallister <mcallkrm at dfw.wa.gov>
To: tweeters at u.washington.edu <tweeters at u.washington.edu>
Date: Thursday, August 26, 1999 8:08 PM
Subject: Re: Eagle predation..was Various threads



>> On Thu, 26 Aug 1999, Herb Curl wrote
>
>> > I understand that increasing Bald Eagle predation on heron
rookeries has
>> > had a significant effect. Can anyone confirm that?
>
>There is substantial cause for concern about Great Blue Herons in the
>Puget Trough. Bald Eagle predation rates are high at nesting colonies
>and abandonments of large colonies are occurring at unprecedented
levels
>(though the history of recording such things is rather short).
>
>Jane Hadley's article in the Times quoted quite a few of the people
with
>strong concerns. I expressed similiar concerns but the only quote
from
>me that she used was the one I tried to use to temper some of my
other
>comments, which was "Great Blue Herons are still a widespread and
abundant
>bird". I have been doing some thinking about alternate ways to look
into
>the possibility of a marked decline in the Puget Sound region. The
heronry
>abandonments are pretty strong evidence in themselves but there needs
to
>be some corroborating evidence that the lack of production of young
is
>producing a population decline.
>
>I thought about Christmas Bird Count data but I haven't done any
summaries.
>The Web Page for this data would make such an analysis relatively
easy.
>
>I talked Dave Nysewander of the Puget Sound Ambient Monitoring
Program and
>he summarized some of their flight census data since 1992. Appears to
>show a general downward trend (if you throw out the first year when
they
>were not very experience - obviously). If I remember right, the high
count
>was about 1700 herons in 1993 and the last two years the numbers have
been
>800+. Unfortunately, these surveys are not designed to sample during
similar
>tide levels so there is a potentially large source of variability
produced
>by that factor alone.
>
>It's really an important monitoring need.
>
>
>On Thu, 26 Aug 1999, Tom Foote wrote:
>
>> Jim Pruske knows a volunteer at the Nisqually Wildlife Refuge who
>> counted 26 eagles attacking the heron rookery on McAlister
>> Creek. He said the eagles cleaned it out..mostly predating on
>> the nestlings.
>
>Now, what is this "26" all about? Are we talking about 26 separate
attacks
>documented over the course of a month or two (reasonable) or 26
individual
>eagles attacking in blood-crazed flock (not so reasonable). Since
there
>is a nesting pair of eagles very close to this heronry, I would be
surprised
>if any eagles other than the resident pair got much of a chance to
get into
>that heronry. The resident birds would kill them.
>
>Kelly McAllister
>