Of wolves and ravens

the complex relationship between two animal nations

A major source of mortality for wolves, in areas without much human interference, is starvation. Because food-deprivation is a real risk, much of what wolves do is driven by the search for food. An adult wolf can gobble down prodigious amounts of meat in one day. For example, on Isle Royale in Lake Superior, where wolves have been studied for decades, adults averaged about six kilograms (over thirteen pounds) of meat (usually moose) per day.

The traditional explanation for why wolves often hunt in packs is that there is a greater chance of a successful hunt if more wolves participate. That supposition often may be true, for wolves hunting moose or deer or other ungulates. Even so, on Isle Royale, only about six percent of moose chases were successful, even for packs of ten to sixteen wolves.

However, more successful hunts by large packs don’t mean more meat for each wolf, because the meat is divided among all the members of the pack. It turns out, from studies on Isle Royale and in the Yukon, that when wolves are hunting moose or deer, a pack of only two wolves obtains the highest yield of meat per wolf.

But many wolf packs are larger than two, and some are much larger. In some cases, a family with offspring will hunt together while the young are learning their techniques. Many packs, however, are composed of unrelated adults, sometime ten or twenty or even more.

So the question is Why are wolf packs often so large, given that large packs do not result in the best-fed wolves?

This is where the ravens come into the story. Ravens are well-known to attend wolf kills (of ungulates). They often follow a hunting pack of wolves and may even notify nearby wolves of an available winter-killed carcass. But ravens cannot get at the meat, except for the eyeballs and perhaps the tongue, until the carcass is opened by the slicing teeth of the wolves. When the wolves are there, they open the carcass and ravens can feed.

A single raven may ‘steal’ as much as two kilograms (about four and a half pounds) of meat from a large carcass, eating some on the spot and caching the rest. And sometimes there are many ravens attending a single carcass; five or ten ravens at a carcass is not uncommon, and occasionally, over a hundred ravens have come to one carcass.

Although wolves and ravens sometimes feed placidly side by side, in many instances the wolves defend their carcass from ravens. Wolves don’t conceal their kills the way the grizzlies or pumas often do. Instead, they commonly rest near the carcass in between bouts of feeding and try to chase the ravens away. They eat very fast, too, which helps to reduce the loss of meat to scavengers (and to other wolves in the pack). Nevertheless, big packs are better at excluding the ravens than small packs are. So, where ravens are likely to ‘steal’ a sizable proportion of the meat, a big wolf pack will end up with more meat per wolf, despite having to share with each other.

Walking Gustavus beaches

predator leavings, big snails, and boring clams

A recent walk with two friends on some of the great sandy beaches of Gustavus provided several observations of interest. The four-footed friend probably had the advantage of us mere humans, because she could sniff many messages that were beyond our ken. Nevertheless, the curious-naturalist humans found much to see and discuss.

A line of wolf tracks followed the upper edge of the sand, steadily headed…somewhere. One huge wolf scat held remains of a murre, probably scavenged from a carcass, and another was made up mostly of clay, with a few feathers. Do wolves self-medicate with clay, as some birds do (to counter toxins in their food)?

There was evidence that predatory birds had feasted on murres, mallards, and a loon. Owls and eagles undoubtedly accounted for some of these avian remains. But also, perched on a log within distant binocular range was a slim, gray bird that we thought might be a peregrine falcon. Some owl pellets held the bones of voles, including skulls with teeth, which made identification of the prey relatively easy. A set of vole molars looks, on the grinding surface, like a row of tightly packed triangles; this is quite different from the cusped molars of deer mice, for example. Perhaps I needn’t have bothered to look closely, though: I was interested to learn, from a well-known naturalist in Gustavus, that deer mice are scarce over there, for reasons undetermined.

Scattered along the sand were several strongly ridged, giant snail shells, the biggest whelks I’ve ever seen. These specimens were four or five inches long, but they are said to reach a length of seven inches or so. They belong to the genus Neptunea, but the species name is still undetermined, thanks to some taxonomic confusions. They hang out in the sediments but emerge to travel, feed, and lay their eggs. Neptuneas make their living by drilling (with their file-like radulas) into the shells of other molluscs and slurping out the contents, eating polychaete worms, and by scavenging dead and dying critters. Females produce masses of egg capsules that are spread over rocks and in rocky crevices. Each capsule contains about two thousand eggs, but many of these are not fated to become juvenile snails, because they are eaten by their siblings. After developing inside the protective capsule for many months, well-fed young snails emerge.

Clam shells were everywhere, mostly horse clams. But on one beach we found deeply arched clam shells, each with a pronounced internal projection, for muscle attachment, near the hinge. This beast was entirely new to me, so my learning curve took a jump. These clams are called piddocks (Zirfaea pilsbryi). Piddocks and some other bivalve molluscs burrow into the substrate using their shell as augers; piddocks make their tunnels in clay, sand, or even rock (!). The sharp, jagged teeth on the front part of the shell slowly rasps away, back and forth, as the piddock rotates, eventually making a full circle, only to start over on the next round. Their tunnels can be over a foot long, so their siphons (or the so-called neck: the paired tubes, one of which is used for breathing and drawing in food particles, and the other for excreting wastes) are substantial. If the piddock is eating well and grows as it slowly burrows, the first part of the tunnel becomes too small for the clam to back out, and it can only go forward.

piddock-kmh
Piddock (Zirfaea pilsbryi). Photo by K. Hocker

I am not a marine biologist of any sort, but I love finding out more about this unfamiliar world.

Hunting success

… you can’t win ’em all

One day this fall I watched a juvenile great blue heron that was fishing in Steep Creek. In typical heron fashion, it stood motionless for long minutes, then quickly jabbed its long bill down into the water after some hapless little fish that passed by. When one hunting spot petered out, the bird moved over to a new perch and waited again. Altogether, it made over a dozen tries to capture a fish and succeeded about one third of the time. A success rate of about thirty-three percent is not too bad, although an adult, with more experience, would likely have done better.

Those observations got me thinking about the success of predators in general. How often are they successful in prey capture? What proportion of capture attempts is successful?

Perhaps the best-studied wild predator in North America is the wolf, so that is a good starting place. Admired for their strength and intelligence, respected for their close family life, wolves are sometimes reviled as competitors to human hunters. Just how successful are wolves, when they go hunting? I focused on wolves hunting ungulates (such as moose, deer, sheep), because that interaction has been the most studied. Wolves also eat beavers, hares, mice, and fish, of course, but there are no data available on those interactions.

Hunting success of wolves obviously varies with many factors, including prey density, wolf pack size, physical condition of the prey, snow depth, escape routes for prey, and so on. Reviewing a number of research reports, I found that, for wolves hunting moose in winter, as many as 38% of hunts might be successful, but usually fewer than 10% of hunts are successful. And captured prey is sometimes lost to scavenging ravens or bears.

It is interesting to compare those figures with those (courtesy of ADFG) for human hunters of moose. The average recorded success rate over a ten-year period for much of Southeast was less than 25% (with the notable exception of one subunit in which humans were successful 63-100% of their hunts!). To take two examples from farther north: In Kenai and Talkeetna, 10-22% of moose-hunters were successful.

It is harder the find data on the frequency of moose kills by wolves, which also varies enormously. Although wolves are capable of killing two moose in one day when hunting is easy, far more commonly there are three or four days between kills (sometimes even eleven days or more). Records for human hunters show that it often takes two to four days for a successful hunter to get a moose.

Although statistical comparisons are not feasible, the data suggest to me that human hunters often have higher success rates than wolves, when hunting moose. In addition, there are at least three salient differences between wolves and humans as predators of moose. Wolves are not constrained by regulations; in the absence of regulation, the human success rate would probably be still higher. Each human hunter generally takes only one moose per season, but of course the wolves hunt repeatedly throughout the year. Hunts by humans tend to be heavily concentrated in areas that are easy of access (around settlements, or a short boat ride from town, for example), sometimes to the point that the prey population is quite depleted in those areas. In contrast, wolf hunts are typically widely spaced, often many miles apart, as each wolf pack ranges over its large territory.

Wolves hunting deer in winter are recorded to be successful on fewer than 20% of their hunts, although occasionally they may succeed up to 50% of the time. By comparison, humans in Southeast were successful 30-71% of their hunts (in various management units), on average. For Dall sheep, wolves caught one in fewer than 33% of their hunts, and humans averaged 28-46% success. Again, the numbers suggest that humans often may be somewhat more successful than wolves.

Bald eagles are another fairly well-studied wild predator, and data are available from all over North America. There are a few reports that eagles are more successful in capturing fish than waterfowl (for instance, 90% vs 20%, respectively), but most reports do not separate the two sorts of prey or the age of the eagles. Nevertheless, when fish comprise at least 90% of prey taken, the success rates tend to be quite high, ranging from 47% to 73%. In these cases, there was no information on what species of fish were caught, and I found no data on eagles catching salmon or herring, which would be very relevant here in Southeast.

For fun: here are a few other serendipitous bits of data: Coyotes hunting snowshoe hare succeeded 28-69% of the time, compared to 20-40% for lynx hunting hare in the same area. Orcas hunting minke whales off the shores of British Columbia and Southeast Alaska were successful in four of nine observed hunts (45%); orcas hunting humpback whales in Argentina were successful 21% of the time in open water and 34% of attacks on beached whales. Great blue herons in Nova Scotia were successful in 29-100% of their strikes on fish prey.

One striking feature of such observations is that hunting success for any species varies enormously, which must have huge consequences for the predators. I did not find information on how much energy is expended on a hunt (and perhaps how much energy is spent defending the catch from competitors) compared to how much each predator gains by eating the captured prey. Some times or places might make it easy to obtain the energy needed for daily maintenance and for reproduction. But many predators must sometimes be close to starvation, and thus be faced with the hard choice of whether to hunt harder or to rest in order to conserve energy. The critical importance of getting enough food is one reason that juvenile animals commonly have a high mortality rate, before they learn to become proficient hunters. Some predators, including orcas and wolves, often use sophisticated strategies and complex tactics in capturing prey, and in such cases, the learning period for juveniles is extended to several years.

A quick visit to Gustavus

…a change of place

When the ferries are running, it’s an easy ride to Gustavus: about four and a half hours, usually, with chance of seeing Dall’s porpoises and other marine critters. The ferry often has a Monday-Wednesday schedule, which makes a quick two-night visit quite possible. The great, wide sandy beaches over there are a big draw; they offer a very different habitat from anything here in Juneau and therefore the possibility of seeing different animals, and it’s easy walking, too.

I made a visit there in mid-January. My naturalist friend had set a trail camera at a place where moose habitually cross a wet ditch, carving deep, narrow trails in the banks. The camera captured plenty of moose images, including mamas with calves. One image showed a very odd thing down in one corner, and for a long time we couldn’t figure it out. Then my clever friend got it: ‘twas the rear end of a duck, dabbling in the ditch in the dark of the night. All we could see was an end-on view of the tail with crossed wingtips above. Very odd-looking, indeed.

Out on the grassy flats where spruces have begun to colonize, we found owl pellets, probably of a short-eared owl, containing tiny mammal bones and a shiny beetle. We noticed that clumps of young spruces often seemed to grow on low mounds, where drainage might be better than in the swales. But do they need to grow on these slightly elevated places? Apparently not, because we found a number of very small spruces getting started in the low spots. So maybe a clump of little trees makes its own mound when needles and twigs are shed, or the branches intercept wind-driven dust and silt??

On the sandy beach, we enjoyed following the tracks of a raven fossicking in the tidal wrack and digging up some treasure from the wet sand. There weren’t many mollusk shells left below the high tide line, but I did find one nice piddock shell; just one, though, a contrast with last summer when there were many. Piddocks are burrowing clams, with a jagged edge on the shell for scraping a way into wood or packed sand or even soft rock. Other shells were scarce too: a few whelks in good condition, and some cockles and ordinary clams.

We made a brief foray into one of those long meadows that eventually drain out onto the beaches. Moose tracks going every which way, of course; moose are really common over there. Wolves had gone single-file as they entered the meadow and then fanned out, leaving their big paw prints on the way to the beach.

A rivulet that meandered through the meadow had some open water, despite the recent low temperatures. Peering down into the openings of the ice, we could see amphipods, caddisfly larvae, a diving beetle, and a couple of very small juvenile salmonids that quickly dashed for cover. The water temperature couldn’t have been much above freezing, yet all these critters were active.

We returned to the car on a game trail through the woods. Many critters used this trail, at least in places: moose, wolf, coyote, and best of all, a wolverine that had gone from the trail to the meadow, leaving nice clear footprints. Later, we went back to set the trail camera in this area and hope for some good videos. Along the road, we chanced upon a flock of pine grosbeaks, busily foraging on seeds (more on this next week).

swans-and-cygnets-hocker
Photo by Cheryl Cook

Back at the house, looking out on a bend of the Salmon River, we were treated to a small parade of trumpeter swans: a pair of adults, then another pair with a handsome gray cygnet. They pulled out on a gravel bar a little way downstream and so gave us a good look at them. An uncommon sight here in winter, although much of the Alaska-nesting population winters along the coast in various places. The swans have the interesting habit of incubating their eggs on their enormous feet, rather than in a featherless incubation patch on the adult’s belly, and both male and female can incubate. Cygnets keep some of their gray juvenile plumage into their second year, becoming fully white-feathered by the next year. Although occasionally they may pair up and start nesting when they are two or three years old, this does not unusually happen until they are about four years old.