Bald eagle swoops in for the kill – and misses

American Bald Eagle fall mating ritual

Image via Wikipedia

Currents of shore birds lifted above the Brazoria National Wildlife marshes on a cold February day. Thousands of sandpipers, dowitchers, yellowlegs, stilts, and ducks fled the hunting bald eagle. A blue-winged teal flew across the marsh, now high, then low, with the eagle in pursuit. This one teal had become the target and was fleeing for its life. It was likely to succeed because it was fighting for its life, while the eagle was simply fighting for its dinner, as Richard Dawkins put it so clearly in The Blind Watchmaker. The other reason the teal was likely to live was that this was a second year eagle, mottled white on its breast. It was clearly still learning when to give up the chase and move on. I wouldn’t exactly say it was toying with this duck, or that one, as it coursed across the marsh, but the eagle didn’t seem to be too serious either.

I looked over at our students, standing on the low dike bordering the marsh. They were cold, for a couple of hoodies were about as much as they could imagine. They huddled in two groups, slightly separated, but tight within the groups, sheltering from the wind. All 11 pairs of binoculars were pointed skyward. Most of them had never seen a wild eagle, let alone one that might catch a duck. I hoped I was witnessing an important event in their lives, one that would be told tonight back in their Rice University colleges, Wiess, Sid, Baker, or Jones, and again in other years, perhaps whenever they visited a marsh, or sat around a campfire.

This is one of the joys of teaching, being there when the spark begins. Eagles hunting ducks cannot be scripted the way the quiz we were about to give can be. I suppose we need the quizzes, for we need to generate a little focus, but I’ll take the hunting eagle any day.

I’ve seen many eagles over the years. I’ve seen them sitting in the trees along the Yampa River near Steamboat Springs. I’ve seen them flying low over Pilant lake at Brazos Bend State Park. Two weeks ago we saw two bald eagles in trees overlooking the Mississippi river, near Alton, on the Illinois side. Last week, in February 2012, three years after the Brazoria eagle, I saw 4 eagles, all second year, from the Missouri River Runner Amtrak train bringing me home to St. Louis from Kansas City, where I had been to give a talk at Avila University. But enough about rivers and their motionless eagles. What is known about hunting eagles?

Bald eagles, scientifically known as Haliaeetus leucocephalus, in the Accipitridae, are meat eaters who are happy with carrion, but also catch their prey. Joseph Buchanan and James Watson (2010) watched bald eagles in Oregon and Washington. They found that eagles could catch their prey on the wing. They also found that eagles could predict exactly where a duck would surface, and swoop down to catch it just in that moment of interface between water and air. They saw western grebes and buffleheads meet their end in this way at Eld Inlet, off Puget sound, in Washington. They also reported a clumsy kind of cooperative free-for-all as mostly immature eagles hunted herring gulls. I can’t imagine gull would be a meat of choice, but they were abundant at the study site. The description is detailed. Here is a piece of it: “At 08:31 the 2 Bald Eagles abruptly discontinued their pursuit of the gull and began to attack another Herring Gull. Two other sub-adult Bald Eagles joined the attack almost immediately. The chase began at 30 to 50 m above water, the height at which most of the gulls were soaring. The eagles made at least 40 approaches of the gull until 08:34 when the gull dropped down to fly within about 10 m of the water, where the eagles made at least 40 more approaches.” The story goes on, ending with the eagles giving up, though other attacks were apparently successful.

Adult gulls flee hunting eagles successfully, but in some places the eagles find gulls that cannot leave so easily. Glaucous-winged gulls nest in groups in Barkley Sound, British Columbia. There, White, Heath, and Gisborne found indirect evidence that they preyed on chicks, and forced the gulls to put more time into vigilance, time taken away from foraging for the chicks. Indirect evidence means they did not see an eagle taking a chick or egg. What they did see was eagles hanging around the colony, something that peaked in late June, with about 4 eagles seen per visit to the 45 gull colony. The gulls were more likely to take flight when eagles were around and more of the gulls were vigilant, looking around, presumably for eagles and other predators. The investigators found that 89% of the gull nests were empty by 6 July 2005, though they could not rule out the impact of a storm. This study is highly suggestive that eagles are bad news for nesting gulls, but it cannot rule out other causes of the gull behavior. The eagles peaked when the chicks were young, so the increased vigilance behavior could be because of the eagles or it could be a correlate of something else. This happens commonly in science, and we must be careful to understand exactly what we know and what we do not know. This study indicates the researchers are onto something that warrants further study, not something completely understood.

Eagles eat a lot of other things, from fish to carrion. They migrate, making it likely their food varies by location. There are a lot of other interesting things they do, at the nest and away. But I’ll leave those for another time. For now, I’ll just remember the Texas eagle chasing a duck and the Missouri eagle high in a tree over the river.

Buchanan, JB & Watson, JW 2010. Group hunting by immature bald eagles directed at gulls. Northwestern Naturalist 91:222-225.
White, AF, Heath, JP, & Gisborne, B. 2006. Seasonal timing of bald eagle attendance and influence on activity budgets of glaucous-winged gulls in Barkley Sound, British Columbia. Waterbirds 29:497-500.

CC 2.0 license, Eric Frommer: Haliaeetus_leucocephalus_-Skagit_valley-8.jpg

About Joan E. Strassmann

Evolutionary biologist, studies social behavior in insects & microbes, interested in education, travel, birds, tropics, nature, food; biology professor at Washington University in St. Louis
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