My schedule today allowed being home from my UW lab earlier than usual;
so, I checked on the new Des Moines urban Great Blue Heronry. And, I
witnessed a direct approach and disturbance of these three new nests by
an adult Bald eagle. The eagle was getting intense pressure from a flock
of crows to leave the area. I could not determine whether the eagle got a
chick (not sure there are any yet), but I did see the adult herons rising
from their nests and milling about.

The Bald eagle had been declining over the last half century or so, and
the Great Blue herons may have been able to occupy niches and prime sites
that heretofore were also feeding grounds for the eagles. The account in
Bent's Life Histories of GBH enemies, mostly based on eastern and central
USA observations, does not even mention the eagle, though gulls, crows,
ravens, and on rare occasion, Buteos are the major heronry predators.

Perhaps, the NW GBH populations HAVE historically been part of the eagle
diet. With selective pressure off because of many fewer eagles, the heron
populations may have greatly increased and passive individuals were
favored no less than the fierce, more aggressive ones. Bent's account
tells of the courage and dangerous defensive behaviors of the GBH toward
nest predators. I did not see the herons successfully threatening the
Bald eagle today!

I think what's happened in the behavioral ecology/population dynamics of
these two species is that for the short term the eagles are in "fat city."
They'll eat out the local, somewhat defenseless 'modern-minded' GBH's, and
I suspect there'll be a drastic decline of our NW coastal herons. Thais
Bock mentioned recently to me during a phone chat that a large, old south
King County heronry, thriving in April, has been totally decimated, not
one surviving nest! So, it has started, the consequences of this recent
change of the balance of power between two predatory bird species.

What'll happen next? Sounds like a wonderful opportunity for a
longitudinal study. (Question: Bald eagles and heronries also co-exist
now in Florida. Does anyone have any knowledge about the relationships
these two species have in that area? )

P.S. Today, I was sort of rooting for the crows and the herons, not very
objective. While the Bald eagle seems to have recovered enough from the
brink of extinction to become a serious threat to certain other birds in
the short term, its continued survival, given its level in the food chain,
is solely at our collective, capricious, human whim.

And so it goes,
Maureen Ellis me2 at u.washington.edu Univ of WA and Des Moines, WA, USA