This might seem like a story about a dead seal, but read on, birders. It’s really about the birds who scavenged it over a few days. While animals who eat dead stuff aren’t the most glamorous critters out there, scavenging is important. Not only do carcasses provide a wealth of good quality food, scavengers clean up what most of us would rather not have lying around to become a stinky health hazard.
A few weeks ago on my usual jaunt out to the beach I inadvertently spooked two bald eagles who lifted off from the shoreline down below the lookout bluff. The two turkey vultures also on the beach did not fly away and I could see why. There was a freshly dead seal pup, minus its head, partly immersed in the shallow water directly below me.
The vultures were clearly interested, but the tide was coming in, and it looked like they were reluctant to walk in the water. Nevertheless, the boon of meat would be out of their reach in no time, so one tried to salvage it from the rising water. It stepped from one rock to another, wading when unavoidable, and once it reached the carcass, tried to drag it to higher ground.
I took a brief video of its attempts. The vulture’s finesse snatching with its beak was impressive. I didn’t understand its last maneuver until days later.
But it was having no luck. Just too heavy. Even a partial pup carcass is more than a vulture can hold. Their beaks are very effective at tearing soft tissues but aren’t strong. In fact vultures usually have to wait until some other predator tears open the hide. The eagles who had decamped earlier could have sliced through the hide without much trouble. Eagle beaks are big, heavy and razor sharp (www.baldeagleinfo.com ). I couldn’t tell if they had carried away some portion of this pup, although they could easily have grasped a piece with their very strong talons. Just part though — even eagles aren’t able to lift a whole carcass that size. Newborn harbor seal pups weigh at least 25 pounds, and an eagle’s lifting limit is about 5 pounds.
The two vultures watched as the rising tide took their bonanza out of reach. Unlike eagles, vultures can’t carry food away...their feet are too weak. They must eat where the carcass lies. Beach scavengers surely understand how tides work, and I can only imagine what the vultures were thinking, knowing it would be many hours before they had another chance at this carcass, if ever.
It was very foggy the next morning when I went back down to the beach. This time I tried to approach quietly, but even so the two eagles there took off immediately. One settled onto a rock offshore and seemed to be working on something, maybe a piece of carcass.
After a brief flurry, half a dozen vultures returned to the carcass, which had been deposited by the next high tide a short distance down the beach.
At first, the gathering of vultures looked quite peaceable. There was plenty of food for all, and there were hours before the tide was due to cover it again. But as I watched over the next half hour I realized only one vulture was freely eating. The others stood around watching, occasionally reaching in to snatch a bite while that one wasn’t watching. Over and over, the feeding vulture lunged out at the others, snapping its beak and even hissing.
A short video sequence will give a sense of the dynamics among them:
Intrigued, I looked into vulture feeding dynamics. What I saw is typical: one vulture does the feeding, and of the others, some are more assertive. It turns out there’s a distinct hierarchy among them, with body size being the most important factor. Several studies described at Birds of North America (birdsna.org/… ) found that whoever got there first also plays a role, as does satiation. Interestingly, their faces become a brighter red when actively feeding, which is a signal to the others. Adults are dominant over juveniles, who have no red at all. All that lunging and flapping is mostly for show though. Rarely is contact made, and 99% of encounters see the dominant vulture prevail.
Down the beach I saw what looked like yet another vulture perched on driftwood. Checking my photos later I discovered it was actually a juvenile bald eagle. Why wasn’t it taking advantage of the food down the beach?
Studies such as this one in Peru (sora.unm.edu/… ) demonstrate clearly the importance of body mass in scavenger dominance at a carcass, not just within a species but between several kinds of scavengers. Comparing Andean condors, King Vultures, Black Vultures, Turkey Vultures and Crested Caracaras, the researchers were able to create a linear dominance hierarchy in that order. A group of a lower-ranked birds could dominate a single higher-ranked bird though. Here in the Pacific Northwest, a bald eagle weighs ~ 14 pounds as compared to a turkey vulture’s ~ 4 pounds. Might this juvenile eagle, who’d ordinarily dominate vultures, been deterred from feeding by the half dozen there? The study found that when multiple hungry birds are feeding at a large carcass, lower-ranked birds were pecked at and displaced, but this eagle may not have been hungry enough, or confident enough, to drive off the vultures.
We have no condors in the islands, nor any other strictly scavenging birds at all, so these kind of interactions aren’t something I’ll ever see. And in fact we don’t have any mammal scavengers either, who might work on carcasses at night. No coyotes or possums. We do have raccoons, but they tend to stick with more easily scavenged food sources like garbage cans. In fact raccoons are more likely to be the scavengee than the scavenger in these parts. This juvenile eagle (on left) was making a meal out of one on the beach a few years ago (chances are high the raccoon was shot and dumped in the water).
Who are our other big scavengers then? Eagles and ravens. I did see a couple of our local ravens nearby keeping an eye on the beach, but I didn’t see them partaking. Ravens are skittish about using carcasses, from what I’ve read (the Bernd Heinrich books) and wait until they are alone to work on a carcass. I’ve seen them at roadkill in ditches, but not with other scavengers.
What about the eagles, who are both larger and stronger than vultures in every way?
I went out to the beach again that afternoon. The sun had burned off the fog, which is typical for August here (Faugust heh). There was an eagle on the beach again, who — again — took off immediately.
One reason the eagles didn’t dominate this carcass (while I was watching anyway) might have been my presence. Vultures are tolerant of human proximity here in general. They can take flight more easily than eagles. But another reason I suspect is that eagles aren’t dependent on carrion. They will readily scavenge roadkill, but as predators they have other sources of food such as fish, ducks, gulls and rabbits. At this season, now the eaglets have fledged, most of our eagles are away at mainland rivers feeding on spawned out salmon. Scavenging elsewhere. Eagles are opportunistic feeders.
While the few vultures at the seal carcass did do some desultory bickering, I got the impression most of the meat was gone by then. It had been really stinky six hours earlier, not so much now. Amazing how fast the carcass was consumed. One vulture used the time on the beach to sunbathe, holding its wings outspread to warm up and drive away parasites. Another perched in a tree cleaning its beak.
The following morning was foggy again, and while a few vultures were down on the beach they flew off when a woman arrived at the overlook with her yappy little dog. They settled into a tree down the beach where an eagle was already perched, gazing out to sea as they do.
Seeing that the carcass was likely to be unattended for a little while I walked down onto the beach to see what remained after 36 hours of scavenging — whatever part of that time the tide and daylight allowed.
Very little it turned out.
The hide was fascinating to me in several ways. While every bit of flesh had been picked clean, the skin itself was left uneaten. Vultures can’t tear into it and evidently the eagles weren’t interested, even in a baby seal’s hide.
Speaking of baby, see how small it was! Born a month or two ago. Harbor seal pups are weaned at 4-5 weeks and then must find their own food and escape threats like orcas and sea lions. And power boats. This pup was recently on its own. Note how similar the seal pup’s “hand bones” are to ours, even though inside flippers that may not be so evident. While its “hands” are relatively much bigger than ours its limb bones are much shorter. Regardless, seeing the similarity in kinds of bones brings home to me how closely related we are.
The hide also shows how the vultures got access to the flesh inside: they peeled it inside out. The fur is now inside the tube of hide; we can see some of the grey fur at the tail end. That’s what I think the vulture was doing on the first day, attempting to expose enclosed flesh. It just didn’t have a chance, with the carcass already partly floating.
There was one last sign of remains on this day, the dark oily patch on cobbles you can see just past the kayaks (the other dark patch in the foreground is where kayakers had scraped cobbles away to drag their boats into the water over sand):
I didn’t have a chance to check out the carcass and scavengers after that for a while and the tide was up when I could get over there. A couple of weeks later I went down to the site. There was nothing at all remaining but a few rib bones, which were cleaned right down to the mineral, but intact. All residue of flesh was gone. The rest of the skeleton, the hide and even the oil on the rocks were gone too. That level of cleaning is done by other kinds of scavengers such as crabs, amphipods, fish such as sculpins, marine worms, all sorts of microscopic critters. Many creatures make a living by scavenging and nothing goes to waste.
Turkey vultures are most likely to find a carcass first, having a superior sense of smell and the ability to soar close to the ground. Eagles will watch for where they gather, and come in to take what they want. That can be good for the vultures, having eagles tear open a big carcass.
We have lots of vultures in this area. In summer I’ll see a dozen every day cruising the skies. Between the land and sea there’s evidently a good supply of carrion to support their population, as well as supplement the diet of eagles and ravens.
What scavengers do you see in your area? I’d be interested to hear about the dynamics of yours and what kinds of carrion they dispatch.
And as always, the Dawn Chorus is now open for you birdy reports of the week.