| Great blue herons are the largest
herons in North America, standing about four feet tall. They
are among Oregon's most recognized birds, commonly found near
bodies of water and in farm fields. They feed on fish, amphibians,
rodents, and almost any other animal they can swallow whole.
Beginning birders are sometimes surprised to learn that these
long-legged birds nest primarily in trees. Like many other wading
birds, they nest colonially in large stick nest in trees, usually
close to water.
In the Willamette Valley, herons frequently nest in cottonwood
and ash trees in riparian area. They lbegin visiting the nest
colony, defending nest sites, and courting in February. Herons
often return to the same nesting colony from year to year,
and enlarge existing nests by adding sticks. Eggs are usually
laid in March, and eggs hatch in April and May. By early April,
nests become obscured by emerging leaves and the nests are
more difficult to see. Herons can be seen flying to and from
the nest until the young fledge between June and August.
Nest colonies are often monitored to determine trends in
nesting populations. The large conspicuous nests make monitoring
fairly easy. Herons are easily disturbed early in the nesting
season, so care must be taken to observe them from a distance
to prevent nest abandonment.
On the Willamette Valley National Wildlife Refuge (NWR) Complex,
there are several "heronries," or nesting colonies
of great blue herons. Each spring, refuge staff and volunteers
monitor the colonies to determine how many nests are occupied
by herons. A small colony is present on the Snagboat Bend
Unit of W.L. Finley NWR. This year, there are three active
heron nests (with a great horned own nest nearby).
Herons formerly nested along Muddy Creek at W.L. Finley NWR,
but the colony was recently abandoned, which sometimes occurs
for no apparent reason.
Another heronry with over 20 nests is on a U.S. Fish and
Wildlife Conservation Easement along the Santiam River. This
year, 22 of these nests were active in mid-March.
Last April, a colony of nesting great blue herons was discovered
on Baskett Slough NWR, in a re-mote part of the refuge closed
to public use. I was surveying the area for other birds when
a heron flew through the trees squawking. Since herons are
not usually seen in this habitat except when nesting, I searched
the trees in the area. One nest was seen, with at least four
birds present, but observation was difficult because the birds
were in a dense stand of trees, and they could only be seen
from almost directly below.
The nests were located in Douglas fir trees in an area in
which these trees are overtopping oaks. A heron eggshell was
found on the ground in the general vicinity of the nests.
The colony will not be monitored because it is impossible
to observe nests without disturbing the birds. Habitat restoration
work in the area around the nests will take into account the
presence of the colony.
As with many other species, habitat loss is the largest threat
to heron populations. The U.S. Fish and Wildlife Service is
involved in wetland and riparian restoration on private land
in the Willamette Valley as Well as on the local refuges.
Although herons are still common and have adapted well to
human alteration of the environment, they are extremely sensitive
to disturbance at nest sites and seek out remote areas with
little human activity. Protection and restoration of wetlands
and riparian areas on public and private lands will help ensure
the long-term survival of this species.
Researched and written by Karen Viste-Sparkman, 2005
Bibliography:
Kestral newsletter, Salem Audubon Society Newsletter, Vol.
38, No. 9, pg 1 May, 2005
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