On Fri, 27 Aug 1999, WAYNE WEBER wrote:

> One problem is that we don't have a really good estimate of the
> numbers of either Bald Eagles or Great Blue Herons in the pre-DDT area
> (or, I would say, prior to about 1970).

In Washington, we don't even have really poor estimates of Bald Eagle
or Great Blue Heron numbers prior to 1975 (later for herons).

> As I recall, there were several studies, from B.C. south to Oregon,
> which showed significant eggshell thinning-- not to the point of
> frequent eggshell breakage (and hence breeding failure),
> but getting close to it.

Don't forget, thinned eggshells often cause desiccation of the developing
embryo so you don't have to have a broken shell to have embryo death.

In my post the other day, I mentioned Don Norman's paper in Colonial Waterbirds
(12(2):215-217) entitled "Bald Eagle incursions and predation at great blue
heron colonies. I was really off in characterizing this work. Norman, Breault,
and Moul observed 8 heron colonies comprised of 712 pairs. They put in 578
hours of observation and recorded 56 incursions (I said two in my post the
other day). Two was actually the number of predation events observed. These
authors admitted the difficulty of viewing what was going on in many of the
colonies they observed. Based on observations by neighbors at one colony,
they concluded that eagle predation was the likely cause for complete failure
of the eleven-nest colony at Powell River, B.C.

I really believe that Bald Eagle predation is having a serious harmful
affect on the productivity of a number of colonial nesting birds. It is
much more persuasively demonstrated by field data than nesting failures or
colony abandonments from human disturbances due to homes, roads, boats,
clam-diggers, or Christmas Carolers. Unfortuntely, most colonies (all of
them in the Puget Sound basin I would venture to say) have been simultaneously
exposed to increasing Bald Eagle numbers as well as increasing human
disturbances as well as declining prey populations.

So.... how do we sort it all out?

Kelly McAllister