Long Distance Migrations

Many animals make regular seasonal migrations. Some are quite short: I’m remembering the snakes that hibernated in deep crevices in Midwestern bluffs and came down to nearby swamps and floodplains for the summer, going back to the bluffs for the winter.

At the other end of the spectrum are some extremely long migration distances (note that the following distance estimates take little or no account of detours to follow shorelines or concentrations of prey, etc.). The Arctic tern is generally considered to be the champion, migrating from Arctic to Antarctic and back every year, an annual round-trip distance of perhaps as much as 25,000 miles.

But other species are also impressive: the bar-tailed godwit flies nonstop (!) from Alaska to winter in New Zealand, a distance of almost 8000 miles in eight days; on the way back, they take a longer route and stop over to fatten up in the Yellow Sea (between China and Korea) –in total, a round trip of over 18,000 miles. Aleutian terns go from Alaska to Indonesia and south-east Asia for the winter, covering perhaps 20,000 miles each year. The little shorebirds called sanderlings breed in the High Arctic; some of them fly to Tierra del Fuego for the winter, and back again in spring, a round-trip of close to 20,000 miles.

Some seabirds that nest in the southern oceans go north for their winter. Sooty shearwaters fly from their nesting areas in the Falklands and Tierra del Fuego to Arctic waters near Norway and back again, over 17,000 miles in the Atlantic (round trip); in the Pacific, they fly even farther, from near New Zealand to the Gulf of Alaska. Short-tailed shearwaters fly even farther: they breed near Tasmania and migrate to the North Pacific; some even go through the Bering Strait to the Arctic Ocean.

All of these critters are relatively long-lived, with lifespans measured in years or decades, and may make those journeys many times in their lifetime. And in most cases, they can feed along the way.

There are long-distance migrants among the insects too, but they are relatively short-lived, with lifespans (for active adults) measured in days or weeks, and they do it differently. A single individual does not make the round-trip journey; instead, they breed along the way and the next generations take over the route.

The best-known example in North America is the monarch butterfly. Monarchs that are born and raised in the eastern U.S. and southern Canada migrate to special forests in northern Mexico for the winter (occasionally some go to Florida or other places in southeastern U.S.). When spring comes, they start north again, but they stop to breed along the way. It takes two or three weeks for each generation of eggs and caterpillars to mature into the migrating adults. That new generation continues northward, and again they stop to breed. It may take three or four generations for monarchs to reach their northern-most range, where they produce the long-distance travelers to Mexico. There is also a smaller population of monarchs west of the Rockies; it migrates to California and northwestern Mexico. But monarchs are in deep trouble, due to habitat changes that reduced the availability of milkweed plants on which the larvae feed and to serious deforestation in their particular overwintering sites in Mexico.

Painted lady butterflies do something similar. From wintering areas in Mexico they migrate northward in multiple generations to the Canadian border; the European populations migrate from south of the Sahara Desert in Africa to northern Europe and even Iceland, making several breeding stops on the way. Red admiral butterflies winter in southern U. S. or southern Europe and commonly migrate north in spring, but shorter distances than the painted ladies.

Painted Lady. Photo by Bob Armstrong

Perhaps surprisingly, some dragonflies are good migrants too. In North America, the big, beautiful green darner had a complicated arrangement, with a roundtrip distance of nine hundred miles or so. It migrates from wintering areas in southeastern U. S., the Caribbean, and Mexico as far as southeastern Canada. One generation makes this great leap and breeds. Some of the resulting offspring stay there, overwintering as larvae in ponds. Others migrate south to the wintering areas, where they reproduce, and these adults are residents in the wintering area, but their offspring will be the next year’s northward migrants.

An even more far-traveling dragonfly is called the globe skimmer. It is widely distributed on many continents and apparently moves around a lot; they have been recorded flying over the Himalayas. But even better: some of them fly over the Indian Ocean from India to East Africa. Some of this over-four-thousand mile flight seems to be nonstop, although the dragonflies might stop to breed if they happen to find an island with suitable conditions. After breeding in Africa, they go back.

That’s not the end of amazing insect migrations. Two kinds of hoverflies, less than a centimeter long, migrate from the European continent to Britain. There they pollinate many kinds of flowers and their larvae gobble up aphids. There may be several generations in a summer. Then the last of the summer-produced generation flies back to the mainland. From there, one species heads to North Africa to spend the winter and make a new northbound generation.

There is surely a lot more to be learned about insect migrations! For instance, how do they navigate? There are likely to be more of such interesting migrations, yet to be discovered.

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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?

The end of August

A rather dismal August finally dripped to a finish. The sodden ground could hold no more water, so the streams were raging torrents and trails were squishy. The lovely long days of summer were just a memory, as day-lengths shortened rapidly. The fall season in Juneau can be pretty gloomy, but instead of pouting and whining (well, mostly instead), I found some cheering things to see.

Out near the glacier, a very late brood of barn swallows lined the edge of their nest with five widely gaping beaks every time a parent bird arrived. Each time, one lucky nestling would get a bug or two from a busy adult; feeding five big chicks took a lot of work. But all five chicks fledged a day later and were lined up on a fence railing, awaiting food deliveries.

On my home pond, there was another late brood, this one of mallards. A female appeared, trailed by two large offspring that were getting their real feathers. All that was left of the babyish down was a small poof on the rump. These two, about half the size of the female, were the remainder of a brood of five or six ducklings, but both of them, with mama, appeared for many days and finally looked just like her in both size and plumage.

The star of the show near the Visitor Center was a young porcupine, recently abandoned by its mother. That’s normal for this time in the porcupine year. Junior could be seen by numerous enchanted visitors, as it steadily gobbled alder leaves right next to the trail. For variety, it nibbled on some cottonwood leaves or climbed up a willow to demolish more leaves. Its right front leg seemed to be sore and was seldom used, but that didn’t deter the little guy from climbing trees and roaming around the area in search of green delicacies.

A whale-watching tour near Shelter Island found several humpbacks, and soon we were surrounded by them. The adults were placidly diving and coasting along, while a calf was showing off. It breached many time, it lunged repeatedly, and it rolled again and again. Its youthful exuberance entertained us well.

Mixed flocks of migrant songbirds flitted through the shrubbery in several places. Yellow-rumped warblers, ruby-crowned kinglets, Wilson’s warblers, and probably others searched for insects in the foliage. A friend watched an orange-crowned warbler probing both ends of a rolled-up cottonwood leaf in hopes of extracting the caterpillar within. Those masters of fast, erratic flight, the dragonflies, were no match for the wily olive-sided flycatchers, which perched in dead treetops and nabbed the big ‘darner’ dragonflies as they hunted small insects over a creek.

Those of us who hang out near Steep Creek and the glacier were pleased to see an old ‘friend’ appear, strolling on the beach (or what beach there was, given the high waters). This was Na Tláa, the Clan Mother, a.k.a. the grandma bear, who is about twenty-three years old and has not had cubs for several years. She was not very plump and, long after other local bears had their new coats, she was still molting; her back was covered with long, bleached-out, reddish fur, while the rest of her showed shiny black new fur. This old bear foraged on this and that in the vegetation, caught a fish and ate it, looked at more fish in the ex-beaver pond, and eventually wandered on down the lakeshore.

High on the list of fun stuff was an encounter on the Perseverance Trail on the very last day of August. Two friends and I were coming down the trail, just below the Horn (where two benches provide a view of Snowslide Gulch). Some distance ahead of us there appeared a large black lump, followed by two smaller black lumps, moving slowly up the trail. Ooooops! What now?! Steep cliff up on our right, steep cliff down on our left, and nowhere to go but back. So we quietly backed up a hundred yards or so to the Horn, intercepted two other down-hikers, and waited. And there they came, mom and two cubs.

I suggested that we all go up on the little rubble slope on the inside of the curve, to allow the bruins plenty of room between us and the railing. Bad idea! Mom took one look at us and turned around, heading back down the trail. But she hesitated and looked back, as if she really wanted to continue upward. So we all scuttled into a corner of the fence behind the benches. Ah! Much better! The family turned back uphill and sauntered past us—Mom completely calm and owning the trail, the kids a bit skittish. So on they marched, right up the trail. There had been clear signs that some bears had used the trail above, bears that had been eating loads of stink currants. Nevertheless, we hoped they’d find a good place to leave the trail, so as not to be bothered by other hikers.

She was presumably a Town Bear, because she had an ear tag. This female bear was very well-behaved, from our perspective. And we had respected her space. A good encounter!

Canada geese

Most folks love to hear flocks of Canada geese flying overhead, especially in spring when the northward migrations pass over Juneau. Sometimes the flocks land in local wetlands to feed, fueling the next leg of the journey.

Photo by Bob Armstrong

On a mid-February hike near the scout camp, a small group of geese foraged in the meadow, and we managed to circle around them without sending them into an alarmed flight. Another small group flew in to join them, talking constantly with each other.

Several of the hikers remarked that it seemed rather early for the migrating flocks of geese to be here. Indeed it was! The geese we saw belong to a distinct sub-species, known as Vancouver Canada geese, that occupies Southeast Alaska and British Columbia year-round (although a few may migrate). In winter, we often see these residents on wetlands and in estuaries along Juneau shores. For instance, Echo Cove, the Cowee Creek estuary, Eagle Beach, the Mendenhall wetlands, and the Lemon Creek wetlands are often good places to see them.

Vancouver Canada geese are unusual in several ways (in addition to being here all year). They are larger than other subspecies of Canada geese; adults weigh an average of six to ten pounds in fall but even more in spring. And they nest in wooded areas, not in open areas such as marshes and tundra—habitats that are more typical of other subspecies; in short, Vancouvers have adapted to the commonly available habitats here in Southeast.

Nests are usually placed at the base of a pine or a group of pines in muskegs or at the base of a spruce or hemlock in denser forest. But sometimes, the nest is on a snag or even in a live tree. Nest sites can be far from tidal waters and are not usually adjacent to freshwater ponds; the nearest open water is likely to be a small, shallow forest pool. Whereas other Canadas escape to open water when disturbed, Vancouvers flee to cover in the forest. During the incubation period, the male may stand guard while perched high in a nearby tree; those great webbed feet somehow manage to let a big goose perch on a branch!

Other than residency, habitat, and body size, Vancouvers are much like the rest of the species. Males and females have similar plumage, but males are slightly larger. They reach breeding age in two or three years after hatching. They form long-lasting pair bonds; but if one member of a pair dies, the widow(er) may find a new mate. Each pair sets up a nesting territory, excluding other pairs.

Females do the job of incubating the eggs, for about four weeks, while the male keeps watch. There are usually four to six eggs in each clutch, but the average clutch size is smaller in nests that are started later in the season. Not much is known about nesting success, in part because the birds are so secretive and nests are hard to find. Even with radio-tracking, finding nests takes considerable effort. So it seems that only two studies of nesting success have been done, both on Admiralty Island, and the sample sizes are small (fewer than twenty-five nests in each study). A study in the 1970s found that eggs successfully hatched in fifty-six percent of monitored nests, and a later study found that about eighty percent of nests survived to hatching time. These estimates lie within the range reported for other populations. Canada geese whose nest is destroyed can sometimes renest in the same season, but the clutch size is smaller and obviously hatching time would be delayed. So juveniles would not be as well-developed when fall comes, but the consequences of such a delay have apparently not been studied.

After the eggs hatch, the goslings are able to walk, swim, and forage within a day’s time. They are guarded by their parents, which call out alarms if disturbed and shoo the young ones into cover. Sometimes, several broods of goslings are gathered together in what is called a crèche, and all the parents attend them. Goslings are able to fly after about eight or ten weeks, but they stay with their parents for a year.

After the nesting season, the geese molt and become flightless for several weeks while new flight feathers grow in. At least in some cases, birds seem to have favorite molting sites, probably not terribly far from where they nested, where they gather in flocks. I’ve paddled into Wachusett Inlet in Glacier Bay, for instance, and found the water surface littered with goose feathers (I soon retreated, so as not to disturb any still-flightless birds).

Canada geese are herbivores, eating a great variety of vegetation. Grasses and sedges are a mainstay in summer. In Southeast, a favored summer food is skunk cabbage leaves, but they also eat blueberry leaves and fruits and other things. On local wetlands, we have found evidence that they dig up the underground parts of silverweed, and in winter, we see them grubbing up the underground parts of sedges. We have sometimes found scats filled with poorly digested moss, which seemed unusual. The digestive system of these geese is not highly efficient, despite a good gizzard and a substantial microbial flora in the caecum and intestines, so they have to eat a lot.

Based on aerial surveys, the population of Vancouvers in Southeast is estimated to be about twenty thousand or a little more. It appears to be fairly stable, perhaps partly because it is not subject to excessive hunting pressure; historically, other populations in Alaska have crashed because they have been overharvested (in the Yukon-Kuskokwim area) or devastated by introduced foxes (on the Aleutians).

Thanks to Debbie Groves, US Fish and Wildlife Service, who provided several useful references.