Dear Tweeters,

In a private posting, David Chelimer asked whether Great Blue
Herons had increased when Bald Eagles were scarce during the "DDT
era", and whether it seemed likely that numbers of eagles and herons
had re-established, or would soon re-establish, some kind of natural
balance.
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). However, at least in B.C.,
Great Blue Heron numbers certainly did not show a significant increase
when Bald Eagles were scarce. One of the reasons is that Great Blues,
as well as eagles, may have been suffering from DDT; both are
fish-eating species which tend to accumulate DDT residues in their
tissues.
In the Northwest, it appears that PCBs were an even bigger problem
for Great Blue Herons than DDT. Both PCBs and DDT have one effect in
common: they interfere with calcium metabolism, and can cause serious
eggshell thinning. 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. Also, there was at least one Great Blue
colony on Vancouver Island which suffered serious contamination by
dioxins from a nearby pulp mill.
Efforts to clean up chemical pollution in Canada and the U.S. have
resulted in dramatic decreases in levels of both PCBs and dioxins in
Great Blue Heron eggs. However, herons continue to face other threats
besides eagle predation. One of the worst is the continuing loss of
colony sites because of urban expansion (especially around Vancouver)
and conversion of former colony sites into housing developments.
Because of threats from habitat loss and chemical pollutants, Great
Blue Herons have for a long time been on the "blue list" in B.C.-- a
vulnerable species, one step above "threatened" status. Even though
heron numbers so far seem to have been fairly stable in south coastal
B.C., as the birds have managed to find alternative colony sites,
their status is still somewhat precarious, and future population
declines are likely.
The sad fact is that as long as human populations keep growing, we
will continue to see negative impacts on more and more bird species--
if not indirectly through chemical pollutants, then directly through
habitat loss. Nothing can be expected to stay the same as long as our
numbers keep increasing.

Wayne C. Weber
Kamloops, B.C.
wayne_weber at bc.sympatico.ca