Bald Eagle trick: turn Pacific Ocean ‘bait ball’ into fast-food treat
Bald Eagle grasping Pacific Sand Lances on May 20, 2009. Photo: David Ellis
The two ornithologists had just visited a Peregrine Falcon (Falco peregrinus) eyrie and were motoring their way to another one. As their inflatable boat rose and fell in heavy swells along the northern Queen Charlotte Islands off Canada’s western coast they saw a flock of whirling Bald Eagles (Haliaeetus leucocephalus).
By Rex Graham
David H. Ellis, Ph.D., of the Institute for Raptor Studies, and R. Wayne Nelson, Ph.D., had seen plenty of Bald Eagles up here before, but the frenzied way the eagles were hitting the Pacific Ocean’s surface was highly unusual. They detoured for a closer look at an event few scientists have ever witnessed.
“We were making our way through a narrow straight when we saw the circling Eagles,” recalled Ellis. “I said, ‘Wayne we’ve got to get out there.’ He was at the helm, simply trying to keep both of us alive.”
Ellis and Nelson not only survived, but they saw a record number of fish taken in a single Bald Eagle feeding frenzy. “Many people had seen such feeding frenzies from afar, but no scientist had previously been close enough to document eagles catching 10-15 fish at a pass,” said Ellis.
Onlooking Auklets
Rhinoceros Auklets feed frequently at "bait balls" of Pacific Sand Lances. Photo courtesy of Langara Fishing Adventures
Ellis and Nelson were impressed by the utter improbability of so many fish being caught and eaten so fast by Bald Eagles – all within a few meters of human observers.
“It looked like the Bald Eagles had shown up at McDonalds and were grabbing fistfuls of French fries,” Ellis said. “I took photo after photo, but the camera’s batteries wore down. There were so many fish being taken so fast by so many Eagles, I just couldn’t take it all in. As the camera batteries were recovering between shots, I instinctively began counting; one-thousand-one, one-thousand-two, one-thousand-three.”
Ellis and Nelson estimated that the Bald Eagles “slapped” at the bait ball about every four seconds. The ornithologists’ first-of-its-kind scientific report of an estimated 2,500 individual fish taken in a 18 minutes on May 20, 2009 was published in a 2010 issue of the Journal of Raptor Research. But Ellis and Nelson are still shaking their heads over the novelty of their observations.
Also watching that day from a discrete distance were Glaucous-winged Gulls (Larus glaucescens), Rhinoceros Auklets (Cerorhinca monocerata), Herring Gulls (Larus argentatus), Black-legged Kittiwakes (Rissa tridactyla) and the occasional Common Murre (Uria aalge).
As 21 Bald Eagles repeatedly grabbed fistfuls of French-fry-sized Pacific Sand Lances (Ammodytes hexapterus), all the onlookers were treated to another, more sublime treat – a glittering shower of fish-scales, with the occasional plop of a whole Sand Lance.
“I have swum with manatees and been hit in the back by a Peregrine, a Prairie Falcon (Falco mexicanus) and a Zone-tailed Hawk (Buteo albonotatus), but that was one of the most amazing wildlife experiences I’ve ever seen,” said Ellis in a telephone interview.
Bait-ball banquet
They noted in their report that nothing close to such a mass fish capture had ever been reported in “the extensive Bald Eagle literature.”
Bald Eagle grabbing Pacific San Lances on July 11, 2008. Photo: Dave Chandler
Comment: “Pretty amazing the numbers of small fish these bald eagles are able to grab at any one time, yet their powerful, almost unbelievable grip coupled with the rough, spiculed surface of their foot pads, seems to be working pretty efficiently here,” said Teryl G. Grubb, a scientist at the Rocky Mountain Research Station in Flagstaff, Ariz.
Bald Eagle carrying Pacific Sand Lances on June 14, 2009. Photo: Dave Car
Comment: “Like Ellis’ lead photo in this series, it appears these adults are about to ‘eat on the wing,’” said Grubb. “Works out well when prey that small would likely be dropped if the Eagle landed, and even better when they’re out to sea and have no choice. Eating on run also allows quick returns for seconds.“
Bald Eagle flying away with Pacific San Lances on June 2, 2010. Photo: Mike Larkin
Comment: “I suspect this eagle is gaining lift by flying close to the water surface like a Brown Pelican, taking advantage of any extra air currents and updrafts, to regain elevation,” said Grubb. “Bald eagles typically do more soaring than active flying: they’re masters of utilizing even the smallest of air currents to their advantage.“
Three Bald Eagles take turns grabbing Sand Lances on July 7, 2011. Photo: Dave Car
Comment: “Group foraging is not all that unusual,” said Grubb. “Sometimes it’s competitive and sometimes there is some obvious group cooperation where several eagles will fly over, one after another, keeping say a diving duck down, till when the poor duck comes to surface for air, the next eagle can grab it.”
Bald Eagles attract a lot of attention no matter what they do. After all, the photogenic predator is a national symbol. A white head, fierce eyes and lethally curved beak adorn the official Great Seal of the United States. Its toes are covered with knobby spicules and tipped by sharp talons. The presidential seal shows an Eagle grasping a 13-leaf olive branch with one foot and 13 arrows in the other, but Bald Eagles have evolved to grab and hold slippery fish.
Bald eagles are one of eight species of sea eagles (Genus Haliaeetus), and as such feed primarily on fish. However, they are incredibly opportunistic and are known to take cats, ground squirrels and rats. Ornithologists have documented them killing White-tailed Deer (Odicolens virginianus) fawns, taking Trumpeter Swans (Cygnus buccinator) and seabird eggs and chicks. They scavenge dead Harbor Seal (Phoca vitulina) pups on the beach when fresh meat is scarce, or feed on live ones.
Needlefish not on menu?
But in all the previously documented 2,000 total fish eaten by Bald Eagles, not a single Sand Lance was listed on the menu.
Ellis and Nelson had no reason to doubt what they had seen that windy afternoon in 2009. They were convinced that mass fish captures may be common. But they needed more observations to be sure.
Another opportunity came two weeks later in the same area. The ornithologists followed a Bald Eagle from shore to another bait ball that quickly attracted 13 additional Eagles.
“Somehow, often thought from their keen eyesight in association with subtle variations in flight behaviors, Eagles seem to be able to key on each other from great distances to ‘communicate’ exceptional foraging opportunities like these Sand Lance balls,” said Teryl G. Grubb, a scientist at the Rocky Mountain Research Station in Flagstaff, Ariz.
But this time the birds circling about 1 km from shore came up empty-footed after four attempts to grab Sand Lances. Rather than waste their energy, the Eagles flew back to shore.
“We then motored very close to this pod of fish and saw that it consisted of much smaller fish than we’d seen earlier,” Ellis said. Something else was different: the smaller Sand Lances were clustered in a narrow serpentine line at the water’s surface rather than in a dense ball of full-sized adults.
Boiling panic of fish
Humpback Whales feeding on Pacific Herring on June 29, 2004. Photo courtesy of Langara Fishing Adventures
Pacific Sand Lances, also called Needlefish, along the Coast of Washington, British Columbia and Alaska are also favorites of Salmon, Humpback Whales (Megaptera novaeangliae) and diving birds such as Rhinoceros Auklets.
Huge schools of Sand Lances are found in areas with sandy bottoms. If Sand Lances can’t escape by burrowing into the sand, they swim to the surface in dense masses. Sections of the ocean’s surface can turn into a boiling panic of fingerlings.
Ellis and Nelson, both retired, are convinced that “under the right conditions,” Bald Eagles in the Pacific Northwest are harvesting thousands of Sand Lances or more in binges similar to the one they witnessed on May 20, 2009.
Indeed, photos provided by Langara Fishing Adventures that were taken in the same vicinity by the company’s clients before and after the observations of Ellis and Nelson show Bald Eagles feasting on Sand Lances.
“From afar, I’ve seen the Eagles fly out to the bait balls,” said Mike Randall, marketing manager of Langara Fishing Adventures, a sport-fishing company on Langara Island, British Columbia, Canada. “Some of the people on our trips have provided photos of Bald Eagles and other birds feeding on Herring and Needlefish.”
A variety of gull species take advantage of a Sand Lance "bait ball" on July 24, 2011, off Canada’s northern Queen Charlotte Islands. Photo courtesy of Langara Fishing Adventures
The fish-rich Pacific about 500 km south of Juneau, Alaska, attracts salmon, Humpback Whales, Dolphins, Porpoises, gulls and diving birds. Some of the predators corral large schools of Sand Lances and Pacific Herring (Clupea pallasii) at the water’s surface. If Ellis and Nelson are right, Bald Eagles routinely join the feast.
Eagles, DDT and lead
Egg-shell thinning caused by pesticides such as DDT and Dieldrin decimated Bald Eagles and other apex avian predators in the lower-48 states in the 1960s and 1970s. Swiss chemist Paul Hermann Müller won a Nobel Prize in 1948 for discovering the pesticide’s high efficiency against arthropods, and Rachel Carson’s 1962 book “Silent Spring” led to the banning of it in most places. Carson’s vision went beyond DDT by questioned the wisdom of releasing large amounts of any environmentally devastating chemical without fully understanding the effects on ecology or human health.
Ornithologist David Ellis
Eagles in Alaska and Western Canada were mostly unaffected by DDT, but to understand Eagles’ diet elsewhere, researchers have measured the concentration of DDT and other environmental contaminants in the remains of prey under their nests,as well as from blood and feather samples collected during banding operations.
With DDT fading from the environment, annual Bald Eagle surveys in many parts of the United States such as in the Pacific Northwest (Oregon and Washington), Great Lakes region (Michigan, Minnesota), Arizona, New York, Florida and other states have documented soaring numbers of breeding pairs. But Bald Eagles aren’t out of the environmental woods in the Lower 48.
“With an ever increasing human population, especially along marine and aquatic shorelines, habitat loss is one of the greatest threats to bald eagles,” said Grubb. “Other contaminants such as PCBs and mercury remain pervasive in the environment. Collisions with vehicles, power poles, and transmission lines continue, as do electrocutions. And now a new threat is appearing on the landscape – wind farms, with giant turbine blades that are already documented to be killing a disturbing number of golden eagles. Sadly, direct persecution in the form of shooting, poisoning, and trapping, is further exacerbated by secondary lead poisoning.”
Bullet fragments in rifle-killed deer carrion are blamed for lead poisoning and death in Bald Eagles, Golden Eagles (Aquila chrysaetos), California condors (Gymnogyps californianus), and other avian scavengers. Recent research has shown that a single bullet will produce a “spray” of thousands of microscopic fragments from a single bullet impact.
Researchers with The Peregrine Fund in Boise, Idaho and Washington State University in Pullman warn that such exposure to lead from bullets and shotgun pellets poses a danger to all avian scavengers.
Short URL: http://birdsnews.com/?p=647
Posted by Rex Graham
on Oct 30 2011. Filed under Canada, Eagles.
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