a heron in the forest, a frozen feast, and raven excavations

The first halfway decent snowfall in mid November drew me out to look for animal tracks and anything else of interest. I went with a friend to the forested banks of the lower reaches of Eagle and Herbert rivers. Deer, both big and small, had wandered extensively throughout the area. Mink had a regular route along the top of one river bank. Porcupines had been out before the snow stopped falling, but squirrels left very fresh prints. Just as we were commenting on the lack of bird tracks, we happened upon some clear prints left by a heron strolling through the forest.

Then we heard a ruckus made by some squabbling ravens, over on a sandbar across the river. We approached quietly, with several trees (and the river) between us and the gang of ravens, but they spotted us immediately and took off. A number of magpies then moved in. The big attraction was the bony torso (spine and rib cage) of a deer, already well picked-over but still clearly worth serious attention. We settled down among the trees to watch.

We counted at least nine magpies; the precise number was not readily determined, because they were constantly flying to and fro: pecking and tugging briefly, then departing for a few minutes, and returning to grab another morsel. Were they caching these little bits of meat or just going off to eat each bit in peace? All those magpies seemed to be able to forage together without altercations (unlike the ravens); there apparently was room around and even inside the rib cage for them all.

A juvenile eagle arrived, briefly scattering the magpies, but they soon moved in again—on the side of the carcass away from the eagle. This was not very profitable feeding for the big bird, however, and it soon departed. Meanwhile, one or two ravens cruised by, or perched up in the spruces, occasionally hopping over the sand toward the bones but nervously taking off without feeding there again. Maybe nine magpies were too much for them, but I think they knew we were still there and did not like being watched.

A few days later, we had wonderful snow and lots of it. Spruces bore thick white blankets on drooping branches and alders bent almost to the ground under the heavy load. Rather than do the various tasks I was ‘supposed’ to do, I took off out the road to do a little exploring on snowshoes. ‘Twas the first time on ‘shoes this season, and it showed (sadly). Tracking was good, however: fresh deer trails, old otter slides leading from one patch of open water to another, not-so-old porcupine trails, deeper than the otters’ marks, a few squirrels, and a mink.

Two ravens were assiduously digging in the snow, in selected spots, tossing snow aside with their bills. Sometimes they dug down several inches, apparently getting very small, unidentifiable items. What could they be finding, and how did they know where to dig? I shared a few crumbs with them.

That lovely snow didn’t last, here near sea level. But I sure liked how it brightened up our short days!


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.