John Chandler asked

>Received a call from a gentleman who has worked at St. George's School on
>Vancouver's westside for 20 years. He has noticed that 40 - 60 of the
>Great Blue Herons that usually roost in Pacific Spirit Park have started to
>roost in trees N of the school, at the edge of Camosen Bog. He was
>wondering whether disturbance of their usual roost site in the Park might
>have caused this change of behaviour.

John, I don't have a definite answer, but perhaps a direction to point you
in where you hopefully might find one. *Just* the other day I purchased a
copy of Robert Butler's 1997 book, _The Great Blue Heron_. Rob is a
biologist with CWS and a professor at Simon Fraser; the book is published
by UBC Press. Since I've only read the Forward, Preface, and Introduction
so far, I tried to skim through to find an answer to your question.

I didn't find anything specifically about roost sites, but he does mention
the Pacific Spirit Park colony: "More recent censuses [since 1985] showed
that there were five large nesting colonies with over 100 pairs in British
Columbia between 1988 and 1992...: Holden Lake on Vancouver Island, Sidney
Island in the Gulf Islands, Pacific Spirit Park in Vancouver, Point Roberts
in the Fraser River delta, and the Canadian Forces Base near Chilliwack in
the Fraser Valley. ..."

And I found this about nesting colonies (citing studies done in Ohio and
Pennsylvania):

"The numbers of fledglings raised in heron colonies with frequent foot
traffic was significantly lower than at colonies with no disturbance or
mechanical [such as farm equipment, vehicle traffic, trains, or low-flying
aircraft]. These results suggest that some herons are very sensitive to the
presence of humans in their colonies. Some authors have found that
individual herons habituate to non-threatening human activities over
time... For example, herons nesting in Stanley Park are unperturbed by
throngs of people on the ground below. ..."

And in another place: "Thus, within British Columbia about half of the
herons exhibit a nomadic lifestyle, roaming between sites within and
between breeding seasons, and about half nest at stable sites each year.
Many small colonies use a site for only a few years, and the number of
nesting pairs shows wide swings between years..."

My impression in the past has been that herons may periodically change nest
sites for unknown reasons. Perhaps it has to do with a change in the trees
they used? Something to do with favored foraging areas? Perhaps some level
of disturbance or complicating factors of which we have no concept?

Another impression I've had is that there is increasing interest in Great
Blue Herons as environmental indicators, and more studies have been
conducted or begun in recent years than ever before. Perhaps a call to CWS
will provide a specific answer as to the current status of the Pacific
Spirit colony. I know that the Oregon Dept. of Fish and Wildlife monitors
(or at least used to) nesting colonies from year to year, and one task I
was given as a volunteer was to search for some of those colonies before
the trees leafed out, and to determine access for future followups. (We're
talking lots of private farmlands along the Willamette River here). I was
unable to locate some known sites, and discovered one new one. The city of
Portland has adopted the Great Blue Heron as its symbol, and there is one
long-term, successful nesting colony near the Columbia Slough --
immediately adjacent to the Heron Lakes Golf Course ;)

So it doesn't sound like there's an easy answer. The Pacific Spirit herons
may be used to some level of disturbance; never having been there, I have
no idea of the landscape condition around the colony.

Perhaps other Tweeters (M. Price? Don B.?) can provide more insight.

-- Janet Hardin
Port Townsend, WA
wings at olympus.net