Long-distance migration

The Arctic terns that nest near our local glacier (and around the whole Arctic) are champion long-distance migrants; they are said to have the longest regular migration of any bird species. They fly from Arctic and subarctic summer nesting areas to the southern oceans around Antarctica to feed during the winter (southern-hemisphere summer), and then they fly back. They do this gigantic circuit every year. The average flight distances they cover are huge: one estimate is about twelve thousand miles (one-way) but another estimate found distances over twenty-two thousand miles in a zig-zag route over the Atlantic.

Short-tailed shearwaters do quite well too, flying up to the Bering and Chukchi seas (to feed during our summers) from their nesting areas around southern Australia. The distances covered are estimated at about nine thousand miles, one-way. Both the shearwaters and the Arctic terns feed in the open ocean and make stopovers to feed along the way, fueling up for the next leg of the journey.

Bar-tailed godwits are even more remarkable! These are large shorebirds that nest on the tundra in western Alaska and Eurasia. They feed in shallow waters in wetlands and along the coast. The Alaska nesters fly to New Zealand waters for the southern summer (our winter) and then come back in the spring. On the way back north, they often stop over in the Yellow Sea (between China and Korea)—a rapidly disappearing resource, because of shoreline development by China. One bird clocked over six thousand miles—non-stop– from New Zealand to the Yellow Sea remaining wetlands and then went another three thousand miles back to Alaska. Still more impressive was a godwit that made the southward migration directly over the open Pacific Ocean, a non-stop (!) trip of over nine thousand miles, taking about nine days without eating or drinking!

How is that possible??

We are used to the idea that migrating birds put on a lot of fat before migrating, sometimes doubling their body weight, and some (like the terns and shearwaters) can eat along the way. But godwits cannot feed in the open ocean. They put on some fat, of course, but that is not all they do: they also lose weight from various body parts: the digestive system, liver, and kidneys atrophy, shriveling up to a fraction of their former weights. Fat and protein from those organs are recycled and used as a source of energy. This also reduces the wing-loading or the ‘freight’ carried by the (non-feeding) migrant.

This weight-saving trick is used by some other fairly long-distance migrants as well, including some other shorebirds and songbirds. Furthermore, as migration continues, protein and fat from the muscles—including the flight muscles and heart!—are gradually metabolized and used as fuel for the journey.

The fascinating thing is that when these birds arrive at their destination, the atrophied digestive tract and associated organs are restored to their former functional size and condition! Lost muscle mass is restored too. The birds are able to “turn off” the internal organs and turn them back on again.

That kind of information has some medical researchers thinking about a human affliction called cachexia, which is a dramatic, potentially catastrophic, loss of muscle mass and fat that often occurs along with other afflictions, such as certain kinds of cancer, HIV, or multiple sclerosis. If research could figure out how migrating birds can turn off and then restore digestive tissues and rebuild muscle mass, they might figure out a means of mitigating cachexia in human patients. That’s a long way in the future, but it is interesting and significant that knowledge from avian migrations—seemingly quite far removed from cancer wards and hospitals—might yet contribute to human health.

Still an open question is how did such tremendously long-distance migrations evolve?

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Tracks in December

A warm, very wet spell in early December made the lichens and mosses all perky and colorful. Beavers left their distinctive foot marks in a thin dusting of snow and swam out around their winter caches of twigs, tail-slapping when we passed by. In a ‘real’ winter, they would be tucked up into their lodges, snoozing a lot, talking quietly with their offspring, and occasionally nibbling a twig from the cache. The kits of the year, however, would be chewing twigs all winter long, as they continue to grow. Bears were out and about too, mom and cub leaving their tracks near Dredge Lake, instead of entering into serious hibernation. That entails a profound reduction of metabolic rate, shutting down digestive processes, and very little activity inside the den, quite a contrast with beavers.

Then, in mid-December came a lovely and welcome snowfall, just a few inches at sea level. It wouldn’t last, of course, in this time of warming climate, so I dug up my snowshoes and headed to Eaglecrest. There the snow was maybe a foot or so deep and just right for poking around on a day when the lifts weren’t running. Snow was falling thick and fast, quickly covering any little tracks of mouse or shrew. But under the trees were prints of snowshoe hares. A small-footed canine creature had run across a wide open area, leaving a long, straight line of well-spaced prints. There was no evidence of any human anywhere nearby, so I guessed that a coyote had raced along. But very few critters made themselves visible—a porcupine that seemed to think that if it could not see me, then I could not see it; and one flying insect, probably a stonefly. Nary a bird to be heard or seen not even a hopeful, attendant raven.

A couple of days later, a nice little cold snap meant that even at sea level, there remained a few inches of snow cover. I went out the road to some meadows, where I plonked along on snowshoes—a convenient way to deal with snowy humps of frozen grass. Oddly, there were no shrew tunnels to be seen, nor any squirrel tracks, and again not a bird could be found.

But otters had been quite busy. They had fossicked along a tiny rivulet, trampling some spots quite flat; there were more than one of them, apparently, so perhaps a family of mom and well-grown pups. I lost their trail when it went under the trees where there was no snow. However, a few minutes later, I encountered their characteristic slide marks where they had crossed a snowy, open area, pushing off strongly with the hind legs and gliding smoothly even over flat ground. This is probably more fun than stomping around on snowshoes! A bit farther on, otters had come up out of a tiny stream and snuffled all around the nearly buried ends of several low, trailing spruce branches. What was going on there, I wonder.

Some days later, I looked for tracks in another meadow out the road, but there had been little recent activity. A couple of squirrels had explored the meadow edges, out of the trees and back again, diving under humps of bent-over grasses. Before the last little snowfall, porcupines had trundled over the meadow in several places, on their usual meanderings. They seem to travel quite extensively, perhaps in search of just the right twig to nibble (?). Along a small creek, some critter had burrowed into the bank in several spots—possibly an otter.

Surprisingly, there were no little shrew-size grooves on the surface of the snow, no tiny holes where a shrew dove under the white blanket. Yet this was a meadow that, in previous years, had been laced with trackways of shrews. One shrew had even taken a dive off a vertical mudbank and gone skittering over a gravel bar in a creek. But where are all those shrews now?

A fluttering on the creek-bank caught my eye and eventually turned into a dipper. This bird was foraging along the water’s edge but apparently found little of interest, because it soon took off, upstream. That was the only living animal to be seen, except for one red squirrel crossing the creek on a broken-branch bridge.

Later that day, on another stream, I checked a long-occupied beaver lodge. There were no signs of recent beaver activity here, although the lodge may be currently occupied. However, other woodland folks were interested in the place: porcupines and mink had visited on more than one occasion in recent days. Was this perhaps a multi-species condo? It wouldn’t be the first time that happened.

The slanting light of midwinter that stabs one blindingly in the eye at certain times of day on Egan Drive, did some beautiful things out by the meadows. Some conifer-clad hilltops were brilliantly lit, contrasting with darker slopes below. Light mists collected in the valleys caught the light rays and turned golden. Overhead, some dark clouds gathered amid some white fluffy ones, but bright rays came through the many unclouded areas, where blue sky was a cheery sight.

Mink

Right after a little (belated) snowfall in early December, I chanced to be prowling around some ponds in the Mendenhall Glacier Rec Area. Mink feet had been there before me, leaving crisply defined footprints in the trails. That mink mostly kept to the foot paths rather than humping over and under the frozen grasses, but made occasional forays to the edges of the almost-frozen ponds. Mink –and deer, bear, and porcupines—often use ‘our’ trails, where there is easy going; snowshoe hares don’t seem to do so very often.

Mink can climb very well and have a rotatable ankle joint that lets them come down a tree headfirst (like a squirrel). But they usually hunt on the ground and in shallow water, both salt and fresh. They swim well, with partially webbed toes, and can dive several meters deep. Their fur is water-repellent. They live all over Alaska, except for some islands and the very far north, reaching high densities in Southeast (except where heavily trapped).

Photo by Bob Armstrong

Dens are usually near water—in hollow logs or burrows, under tree roots, often in an abandoned den of some other animal, such as a beaver or marmot. The video cam at the visitor center sometimes catches a mink exploring even the occupied beaver lodge in Steep Creek. Mink aren’t likely to use a burrow that belongs to an otter, however, because relationships between mink and otter are generally hostile. They share many of the same eating habits, and otters sometimes kill and eat mink.

Mink are opportunistic foragers for meat of all sorts—everything from bugs and earthworms to fish, small mammals, and birds. When foraging in the intertidal zone, they take crabs, clams, little fish, and snails. Mink also gobble up bird eggs and carrion, including salmon carcasses. Cannibalism sometimes occurs. A big male mink sometimes may take down a hare or muskrat or a sitting bird twice its own size.

Photo by Bob Armstrong

Mink are fierce enough to tackle prey that is bigger than themselves. Years ago, however, my old cat who was an experienced hunter, observed a mink travelling on the other side of my home pond and got wildly excited. She could hardly sit still at the window, bumping into the glass, whining, champing her teeth, twitching all over. Little did she know that she would become mink lunchmeat, had she been outdoors and free to engage with this so-attractive creature.

Mating, for mink, occurs in early spring and young are generally born in June. There may be as many as ten of them in a litter, but four or five would be more usual. Both male and female mate promiscuously, so litter mates may have different fathers. Mating often begins with a rough and no doubt boisterous fight that may leave the female with some wounds. The male then grabs the female by the back of the neck and they copulate, often several times. Copulation is a prolonged process, sometimes lasting as hour.

Eggs are fertilized over a period of several days but do not begin to develop immediately. Mink, along with other members of the weasel family, delay the implantation of the fertilized egg in the wall of the uterus. That egg may float around for several weeks before attaching to the uterine wall, getting a blood supply (via the placenta) from the mother, and starting to develop. From implantation to birth takes only about a month but, as a result of delayed implantation, there can be as many as three months between copulation and birthing.

Kits are born blind, deaf, thinly furred, and toothless. They get their milk teeth after about sixteen days, and their permanent teeth begin to erupt after about six weeks. Their eyes open at a little over three weeks and weaning occurs at about five weeks. Kits start hunting, along with the mother, at about eight weeks of age, but become independent after another month and disperse to find their own home ranges. They mature by the next spring and can breed then.

American mink were introduced to Europe decades ago and now occur across much of northern Eurasia. They compete with the smaller, native Eurasian mink, whose populations have declined dramatically from that competition and many other factors. Mink were also introduced, more recently, to southern South America, which previously lacked any similar predator—no doubt the expanding mink populations cause consternation and carnage among the native riparian and shoreline birds there.

Eagle eyes

We say that a sharp-eyed person has ‘the eyes of an eagle’. Eagles are reported to be able to spot a rabbit from a distance of two miles. Eagles, hawks, and many other birds are well-known for their visual acuity. Acuity is defined as the ability to discriminate two points as separate entities from a distance; more particularly, it refers to the ability to discern shapes.

For that ability to be brought into play, however, the eye must be able to focus. In humans and other mammals, visual focus is achieved by changing the shape of the lens, bending the incoming light rays so they hit the retina. But in most birds, both the lens and the cornea can change shape; that potentially gives such birds an especially good range of focus (diving birds are an exception—the cornea has the same refractive index as water, so changing its shape would have no effect on focus, and only the lens is used in these birds).

How does visual acuity work?

Visual acuity depends on the function of three interconnected but distinct levels. The first level is the retina, where millions of photosensitive cells line the back of the eye. In the second level, those cells are connected by short nerve fibers to nerve cells that are sometimes clustered together in ganglia, where they are often connected to each other. And in the third level, those ganglia are connected, via more nerve fibers, to the optic centers of the brain via the optic nerve, essentially a cable of millions of long nerve fibers. Here is a brief rundown about those three functional levels (structurally, the peculiar arrangements in vertebrate eyes are more complex and better omitted for present purposes).

Level 1. There are two kinds of photosensitive cells in the retina. Rods (so-called for their shape) that are very sensitive and work well in dim light; the eyes of nocturnal birds and many mammals have mostly rods. Cones (spindle-shaped cells with a cone-shaped point where the photosensitive pigments are located) work best in relatively bright light and provide sharp resolution and color vision. Birds typically have four kinds of cone cells, sensitive to red, green, blue, and UV, while mammals have only three kinds (no UV). Human eyes have millions of cone cells; avian eyes can have more than that, the numbers depending on both the size of the eye and the density of cones.

Cone cells are densest in an area of the retina called the fovea, which shows up as a small dip in the retinal surface. The eyes of humans and other primates have one fovea, as do most birds. Some mammals and birds (e.g., chickens and quail) have no fovea at all. Terns, swallows, swifts, hummingbirds, hawks and eagles, and kingfishers are all reported to have two foveas in each eye. One fovea deals with binocular vision aimed at a target, as when a hawk, kingfisher, or a swallow is pursuing its prey; the other deals with sideways monocular vision for spotting prey at a distance.

Level 2. The number of cone cells connected to a ganglion varies greatly. In the fovea, there may be just one cone per ganglion, but in the rest of the retina, the ratio might be a hundred cones to one ganglion (for example). Clearly, the more cones connect to a single ganglion, the less precise the visual resolution is likely to be. However, within a ganglion, the nerve cells sometimes inhibit each other or perhaps interact in other ways, so the cone/ganglion ratio is not the only factor.

Level 3. The long fibers of the ganglia transmit signals to the brain, which obviously must be wired to somehow interpret and make use of the incoming information. On the way to the interpretive centers of the brain, the optic nerves of vertebrates partially cross, such that some of the nerve signals from the left eye are delivered to the right side of the brain, and some of the signals from the right eye are delivered to the left side of the brain. The proportion of nerves that cross varies among species. This may be related to, among other things, the ability to coordinate movements of the limbs. Many birds are reported to be able to sleep on one side of the brain, while keeping awake and alert (with the eye frequently open) on the other side. Some individuals exhibit a preference for one side or the other. Siamese cats, noted for their usually crossed eyes, are said to have a defect in the optic-nerve crossover that makes the eyes try to compensate for the faulty signals.

Sharpness of vision may be assisted by other adaptations of the avian (but not the mammalian) eye. For instance, many day-active birds have oil droplets in the cones of the eye. The droplets are red, yellow, or clear; different kinds of birds and even birds in different populations of the same species may have different oil droplets. The droplets are light filters that can change the light rays that actually reach the retina. For example, they can narrow the range of wavelengths registered by the cones, increasing the color contrast. Sharpening the color contrast might improve discrimination of shapes in some circumstances.

Visual acuity is a matter of spatial resolution. But many birds also have excellent temporal resolution; they can process temporal changes in light very quickly. For comparison, humans cannot detect separate light pulses that flicker faster than about fifty times per second—faster flickers are just a blur to us. But birds can process fast flickers and rapid movements more quickly than we can. This may allow them to register wing beats of a prey insect, for example, or detect the location of twigs and branches as they fly through the tree canopy.