Subject: [Tweeters] Bald Eagles help salmon by disrupting Columbia R.
Date: Jun 10 17:11:02 2011
From: Ray White - rw at seanet.com



<http://www.cbbulletin.com/ThemeImages/CBBulletin/Themes/Default/cbb-log
o-print.gif> Bald Eagles' Predation Decimates Columbia's Salmon-Eating
Tern Colony; Cormorants Also Hit Hard



Harassed in recent weeks by bald eagles, the world's largest Caspian
tern colony for the past decade "collapsed entirely" last week with the
last of some 5,000 nests plundered.



Repeated evening/night-time disturbances at the East Sand Island colony
over the past few weeks by a bald eagle or eagles has made the terns
fidgety and willing to flush frequently. When the terns take flight and
abandon their nests, even if briefly, gulls that share the island have
been swooping in and robbing the nests.



"The real damage is caused by terns leaving their nests," said Ken
Collis of Real Time Research, who is co-principal investigator for the
long-running research project along with Dan Roby of Oregon State
University.



The lower Columbia River island's double crested cormorant colony, which
is also believed to be the world's largest, has also been besieged this
spring by bald eagles, peregrine falcons and great horned owls.



".these disturbances and nest predation events coupled with heavy
rainfall on 23 May resulting in significant flooding on the colony has
caused unprecedented nest failure at the tern colony; the number of
active tern nests with eggs has declined from ca. 5,000 to less than 500
over the past two weeks," according to a research update for the week
ending May 29. The updates are posted on Bird Research Northwest's web
site:

http://www.birdresearchnw.org/ <http://www.birdresearchnw.org/>



The last remaining egg-laden nests were looted "during a series of
mid-morning flushes" on Wednesday, June 1, according to information
updated through June 5.



"Individuals are still laying eggs, but nests rarely persist for more
than 30 minutes."



"Gull pressure on newly formed nests has been very intense and one
individual was seen taking an egg from under an incubating tern," the
June 6 update report says.



The level of "disturbance" caused by the bald eagles is unprecedented,
according to researchers who have been monitoring the island since the
late 1990s. The adult terns being taken by the eagles are being
"completely consumed" except for the wings and head, Roby said.



Bird Research Northwest gathers data on the size, distribution, status,
productivity, diet composition, smolt consumption, and factors limiting
the size and productivity of various piscivorous waterbird colonies
located in Oregon, Washington, and California. A primary goal is to
evaluate the birds' impact on salmon and steelhead stocks that are
protected under the Endangered Species Act.



Aggregations of birds, Caspian terns and double-crested cormorants in
particular, have found a home in the lower Columbia on manmade islands
such as Rice and East Sand. Both islands were created with the
deposition of materials dredged from the Columbia River navigation
channel, which is maintained by the U.S. Army Corps of Engineers.



The tern colony has grown significantly since it was first documented in
1984. Caspian terns have concentrated in the estuary because historic
nesting sites have been lost elsewhere in the Pacific Coast/Western
region and human-created dredge-spoil islands in the estuary offered
stable nesting habitat close to abundant supplies of fish.



A total of 8,300 tern pairs nested at the island in 2010, which was down
slightly from the 2000-2010 average. The birds wing in each spring to
nest and leave with their young in summer.



At East Sand they have had an average annual (2000-2009) consumption of
5.3 million juvenile salmon and steelhead, including an estimated 5.3
million last year. The proportion of juvenile salmonids in tern diets
during the 2010 nesting season was 33 percent, which was similar to
recent years.



Researchers won't make a salmon consumption estimate until after the
nesting season, but it does appear that the terns are eating more salmon
and steelhead as a percentage of their diet that they do typically. East
Sand island salmon consumption usually peaks at about 60 percent of the
birds' diet in mid-season. But researchers this year have seen diet
composition spike to 75 percent.



Roby said that because of the larger than normal and continuing runoff
that is flushing down the Columbia, the estuary and plume of the mouth
of the river is more diluted with freshwater than usual. That could mean
marine forage species are not coming within range of the terns in normal
numbers, so the birds are relying more on salmon and steelhead smolts.



"The estuary is not the way it normally is," because of the heavy runoff
and cold, wet spring, Roby said.



That high salmon rate of consumption will likely be balanced by the fact
that there have been fewer terns at the island throughout the season.
Counts peaked at about 12,000 terns during the second and third weeks in
May, as compared to the 10-year average of 14,000 for that time frame.
The count dropped from the peak to an estimated 4,100 May 31.



"Colony attendance has fluctuated greatly, but many individuals are
clearly still in the estuary and at least attempting to roost at East
Sand," the most recent update says.



The tern chick hatching season on average has been completed by the end
of May. This year, the first two chicks were observed on May 31 "but
both were gone within an hour," the victims of predators, the most
recent update says. As of Sunday monitors could see no tern chicks on
the island.



"I think it would be premature to write the epitaph" for this year's
East Sand tern reproductive season, Roby said. The researchers have seen
chicks hatched out as late as the first half of July so persistent terns
could still complete their task.



"The food has to be available and the predators have to cut them some
slack," Roby said.



The island's double crested and Brandts cormorant colonies took a
beating too in recent weeks.



"Groups of eagles observed walking through the colony during the day to
take cormorant eggs," the May 30 update says. Up to 70 percent of the
double-crested cormorant nests west of a tower blind were abandoned,
including almost all nests in satellite colonies; all cormorant nests on
the rip rap east of the tower blind have also been abandoned.



"Double-crested cormorants are attempting to re-nest in these areas but
face continued pressure from bald eagles causing adult cormorants to
flush and leave nests unattended for entire day," the May 30 update
says.



But, last week "disturbances were relatively uncommon" at the cormorant
colony and many of the big birds began to resettle at their original
site on the west end of the island and at the West Jetty, which had not
been used for several years.



The double-crested cormorant colony has shown steady growth in recent
years and has become the island's largest presence. Last year an
estimated 13,600 breeding pairs nested on the island and they gobbled up
about 19 million young salmon and steelhead, according to preliminary
estimates made by the research team.



The research project is a collaborative project between Oregon State
University, Real Time Research Inc., and the USGS-Oregon Cooperative
Fish and Wildlife Research Unit. Support for this research project is or
has come from the Bonneville Power Administration; the U.S. Army Corps
of Engineers - Walla Walla District; the U.S. Army Corps of Engineers -
Portland District; the Bureau of Reclamation; the U.S. Fish and Wildlife
Service - Pacific Region, Migratory Birds and Habitat Programs; NOAA
Fisheries; and the Northwest Power and Conservation Council.





>From The Columbia Basin Bulletin at www.cbbulletin.com.