Birds That Love Mammals

Back in June I was leading a packrafting trip in the Arctic National Wildlife Refuge. The float portion of our trip descended the Aichilik River where we hoped to encounter the Porcupine Caribou Herd as we paddled across the coastal plain. We found some caribou, but only small scattered bands, rather than congregations of thousands. We’d missed the big groups by just a few days, sadly, but so goes the game of wildlife watching.

During our first evening on the coastal plain, I spotted some caribou as the appeared and disappeared out of the shimmering heat-haze. I figured they were a mile or two down the river from camp. Since this was the first band we’d seen, I wanted to get my clients (and I) closer. We grabbed cameras and binoculars and headed down the easy-walking tundra bench.

The first one raised its head to eye us when we were 250m away. We continued to ease forward, in stops an starts, allowing the caribou to grow comfortable with our presence before we moved in further. Eventually, we settled in about 50m from where the caribou were grazing. We sat on the ground, binoculars and cameras in hand, and watched.

There were about 30 bou’ in the group and they were moving no particular direction. Some were grazing and some had chosen to lay down and doze in the evening sun. Now don’t get me wro

ng, I really like seeing caribou (particularly when there are hundreds or thousands at at time), but they don’t excite me the way birds do. So I was more than pleased when a passing shadow caused me to look up.

I was more than pleased when a passing shadow caused me to look up.

Just a few feet over our heads a pair of Pomerine Jaegers cruised by. Then, to my surprise, they settled onto the tundra just 10m away between the caribou and us. As they closed their long black wings, I was hit by a memory.

A couple of years back I was wrapping up a June float on the Kokolik River in Alaska’s western Arctic. It was the morning of our pickup and my clients and I were hanging around our packed bags and scanning for wildlife on the flat, tussocky tundra surrounding our camp. I spotted a Long-tailed Jaeger cruising over a low bluff just across the river from camp, then another, and another, and another. A pair of Golden Eagles appeared and a Rough-legged Hawk before we saw the first caribou. From over the tundra, thousands and thousands appeared, while eagles, hawks, two species of falcons, and jaegers patrolled overhead.

Back on the Aichilik, I watched the Jaegers. Jaegers are gull-like, but more dashing, dressed in blacks, white and pale yellow. There are three species, the Long-tailed Jaeger, the smallest of the three has a pair of long thin central tail feathers that trail in an elegant train as they fly. The Pomerine is the largest of the three species, and while it also has protruding central tail feathers, those of the Pomerine are shaped almost paddle-like. The final Jaeger is the Parasitic. This species splits the difference in size and their central tail feathers poke from the rest of the tail in sharp, aggressive points (reflecting their fearless attitude).

All three species are predators. These are not beach-scavenging gulls, or eaters of small fish. No, the Jaegers are birds to be reckoned with, and they eat everything from small mammals and birds, to eggs, and even larger animals that they will hunt cooperatively. Too, they are keptoparasites, stealing food from raptors, gulls, and other predatory or scavenging birds.

Caribou, as far as I know, are not on their food list.

Why then do caribou seem to lure in Jaegers and other predatory birds? The Golden Eagles I saw following the herd along the Kokolik were probably hunting the caribou themselves, looking for weak or unattended calves. But the Jaegers and hawks had no such ambition. So what were they doing?

Well, I’ve got a hunch, and I’m pretty sure I’m right, but in an hour or so of searching the scientific literature I couldn’t find anything to back me up. Based on some my field observations and what is known about birds elsewhere in the world I’ve got a theory: the caribou are flushers.

Ever seen a bird-shooting scene from a BBC period drama? One in which a dozen beaters walk through the woods forcing pheasants toward the waiting guns of tweedy gentlemen? Think of the caribou as the unwitting beaters for the avian predators.

Think of the caribou as the unwitting beaters for the avian predators.

All those hundreds of stomping hooves and grazing snouts flush lemmings, voles, and small birds from their hiding places in the tundra. Those critters scuttle out of the way and right into the waiting beaks of the jaegers, and talons of the hawks. When hunting alone, the predators have to rely on the carelessness or poorly timed movement of their prey. When hunting without the aid of caribou, Jaegers cruise 1-3m over the tundra, searching for tell-tale motion in the grass, or the flash of fur or feather. Really, The caribou save the birds a lot of fruitless searching.

Though I couldn’t find any information directly related to this behavior in the arctic. It’s well documented elsewhere in the world:  African Vultures follow migrating herds of Wildebeest, and in the New World Tropics, there are well many example where birds follow raiding swarms of army-ants, wandering peccary herds, and even groups of foraging monkeys. It’s not a big leap to assume that arctic raptors and jaegers are doing the same thing.

Along the Aichilik, my clients and I watched for an hour, until eventually the caribou made the group decision to move off. Napping animals stood, wanderers returned to the group, they formed a loose line and marched west across the tundra. Our part complete, we stood, stretched and started the two mile walk back to camp.

Part way back I stopped and glassed. I could just make out the band of caribou, still moving slowly west, little more than antlered specks on the horizon. But fluttering above, just meters over their backs, were the unmistakable forms of the two Jaegers.

P.S. If you happen to know of any resources that document this caribou-following behavior in predatory birds, I’d love to hear about it. The natural history information on the Jaegers and others species noted here was drawn from the Birds of North America accounts.