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Great Blue Herons At Willamette Valley Refuges
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

Kestral newsletter, Salem Audubon Society Newsletter, Vol. 38, No. 9, pg 1 May, 2005


Great Blue Heron
Great Blue Heron
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