Unless yet another freak of nature occurs, it appears that the world’s largest, and perhaps most studied, Caspian tern nesting colony will produce zero chicks this year at the lower Columbia River estuary’s East Sand Island.
The East Sand nesting site, enabled by human manipulations intended to reduce the birds’ consumption of migrating juvenile salmon, has been besieged this spring and early summer by other avian predators, most commonly eagles and gulls.
Last week the tern colony on East Sand had a high count of 253 birds, down dramatically from a count of 8,931 the previous week. The birds more than likely have left to forage elsewhere until they head south for the winter. There were no fledglings to make the trip.
By late this week only a handful of terns remained.
“All tern nest with eggs failed due to gull predation during several disturbances by peregrine falcons and bald eagles; Caspian terns were unsuccessful in rearing any young at East Sand Island this year,” a weekly research update for July 25-31 said. The research is a collaborative project between Oregon State University, Real Time Research Inc., and the USGS-Oregon Cooperative Fish and Wildlife Research Unit. Started in the late 1990s, the work aims to assess the impact of avian predation on recovery of ESA-listed salmonids.
“This is the first season since we started monitoring the colony that is has not produced eggs,” researcher Allen Evans said. Given the fact that it is late in the season, and most of the terns have left, there is little chance of tern production.
“It might be too early to conclude that, but I think it’s highly unlikely,” said researcher Don Lyons. It takes about 28 days for a Caspian tern to lay and incubate an egg, then another 28 to 36 days before a young bird can fly.
“We’re definitely to the point that most birds would be leaving the colony, even in a normal year,” Lyons said.
But it’s been anything but a normal year for the terns, who numbered more than 10,000 pairs in 2008 and has averaged about 9,000 pairs since 2000 at East Sand. The entire Columbia River estuary Caspian tern colony settled on East Sand in 2000 after being lured there by researchers.
The previous nesting grounds had been upstream at Rice Island. The terns’ favored bare sand nesting area at Rice was covered with vegetation to dissuade breeding there, and six acres at East Sand was prepared to the birds’ liking.
The theory, and it has proved out, was that if the terns were moved closer to the ocean they would eat more marine fishes and fewer protected salmon and steelhead. All of the salmon and steelhead from the Columbia-Snake river basin, including 13 listed stocks, swim down through the estuary on their way to the Pacific.
Soon after the terns began laying eggs, a two-pronged attack began in mid-May with eagles swooping down to snatch adult terns, which would “flush” the entire colony. When the terns took flight and abandoned their nests, even if briefly, gulls that share the island pounced on the tern eggs.
“Repeated evening/night-time disturbances by bald eagles to the East Sand Island Caspian tern colony occurred throughout the week, contributing to complete colony failure (i.e., no remaining tern eggs or chicks) on 1 June; thousands of terns are still roosting on East Sand Island and some terns have attempted to re-nest but most tern eggs are depredated by gulls within 30 minutes of laying,” a May 30-June 5 update said.
“…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.
Many of those terns kept trying, but the predation continued.
Lyons said that the terns in past years would not all be flushed or spooked off the colony when an eagle flew over during the daytime. But this year’s dusk to early nighttime raids made them extremely flighty.
“They cope much better with daytime predators that they can see,” Lyons said.
This year’s tern reproductive failure follows a year when researchers estimated that only 425 fledglings were produced at the East Sand Island tern colony, which held 8,283 nesting pairs at the peak of nesting activity in 2010. The average number of young raised per breeding pair in 2010 was 0.05, which was the lowest productivity recorded in 10 years of study.
“Nesting success at the East Sand Island Caspian tern colony peaked in 2001 and has trended downward since then,” according to the “Research, Monitoring, and Evaluation of Avian Predation on Salmonid Smolts in the Lower and Mid-Columbia River Draft 2010 Annual Report.” “Two factors likely have contributed to declining productivity of the East Sand Island tern colony: ocean conditions and nest predation.”
Two consecutive years of reproductive failure should not hurt the overall population of Caspian terns, Lyons said, because they are very long-lived. The terns have been known to live well over 20 years and start to reproduce at age 4 or 5, so most of the birds will have many more chances.
The researchers are not sure what factors have led to what seems to be an increase in eagle predation, whether it’s related to the food supply or simply changed eagle behavior. With a huge snowpack feeding the Columbia and Snake this year the flows pushing into the Pacific were higher than normal. That likely caused a reduction in the number of marine forage fish that were close at hand as prey for the terns.
“Maybe there wasn’t a lot of food for eagles either,” Lyons said, so they turned to the terns.
The updates are posted on Bird Research Northwest’s web site: