eagle glided in toward a Great Blue Heron rookery of about 30 birds. The adult herons loudly vocalized their objections, but that did nothing to influence the eagle that deftly plucked a nestling from its sanctuary near the top of a Douglas fir. Apparently this is not so rare an occurrence.
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| Great Blue Heron |
In a May 16, 2001 article in the Seattle Times, reporter Caitlin Cleary writes about eagles harassing herons, and the response by King County wildlife officials. Great Blue Heron colonies throughout western Washington, from Point Roberts to the Nisqually are under attack from bald eagles. A combination of eagles followed by crows is resulting in nest predation of chicks and eggs. In some cases the herons are abandoning nests altogether.
Obviously herons and eagles have been co-existing for millennia, but the apparent high incidence of heron predation by eagles is a good indicator that there is likely an imbalance in historic relationships. A number of factors can contribute to this situation, including diminished open space, which can lead to competition for nesting among eagles, Great Blue herons and crows. The population density in King County has increased over the past two decades by 37%. The conversion of undeveloped lands to urban or industrial lands in Washington state was an estimated to be 70,000 acres between 1992 and 1997 according to a U.S. Department of Agriculture study.
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| Great Blue Heron |
Another possible reason for increased pressure from eagles on herons is the decline in salmon fisheries.
Salmon have declined in abundance for many reasons, including loss of habitat. On the positive side, it is possible the are more eagles, which appear to be recovering from depressed levels due to pesticide thinning of eggshells. Biologists are finding that historic Great Blue heron colonies of 100 to 200 nests are now reduced to perhaps 30 to 40 nests. Some colonies are not producing chicks, as found recently at the Nisqually Wildlife Refuge.
The abundance and distribution of Great Blue herons in western Washington is not well known, and thus the effects of eagle predation are also not quantified. But there is increasing concern among biologists and birders that what is being observed recently needs to be carefully monitored and better understood.
