Bar-headed Goose (Anser indicus)
Photo: J.M. Garg (Wikimedia Commons)
Migration, in the broadest sense, is the regular movement that animals make between specific destinations. Many organisms migrate: Blue Wildebeest (Connochaetes taurinus) across the Serengeti, Hoary Bats (Lasiurus cinereus) between the Canadian boreal forest and Mexican deserts, Red Crabs (Gecarcoidea natalis) over Christmas Island, as fascinating as these migrations are, its birds that really inspire awe, because they are the most extreme migrants of all.
Birds can move, that's for sure. Some Wandering Albatross (Diomedia exulans) fly 267 000 km in their first year of life alone! Though not a migration in the strictest sense, the peregrinations of a yearling albatross are a kind of long-ranging foraging mission; conventional avian migrations are hardly less impressive. They range from short seasonal movements up and down mountain slopes to extraordinary flights spanning continents and crossing hemispheres. It's those long journeys, those incredible feats of endurance, that I find most fascinating.
I have the good fortune of living at one of the best locations in Canada for observing bird migration, Long Point, Ontario. Long Point reaches 30 km into the tumultuous waters of Lake Erie, funneling migratory birds into fantastic concentrations at its tip, before they make the flight across the open lake. Long Point has been the focus of bird migration studies for nearly 60 years and more than one million birds have been banded there, providing an unprecedented data set on migration patterns, population dynamics and general ecology. The discoveries made at Long Point and by researchers at other migration hotspots all over the world have helped us appreciate just how incredible the bird migrations can be.
Arctic Tern (Sterna paradisaea)
Photo: Andreas Trepte (Wikimedia Commons)
Perhaps the most famous long-distance migrant is the Arctic Tern (Sterna paradisaea), which can cover between 30 000 km and 50 000 km annually. Arctic Terns from eastern Canada and Greenland leave their tundra breeding grounds and proceed southeast across the Atlantic. Upon reaching the west African coast they turn south, ranging into the Southern Ocean. Nearing the bottom of the globe, they follow prevailing winds eastward, with many young-of-the-year birds joining Southern Giant Petrels (Macronectes giganteus) in completely circling Antarctica. Almost two years later, the young terns find themselves back in the southern Atlantic, from whence they proceed north for their first breeding season. Adult terns usually don't circle Antarctica, but do travel widely in the Southern Ocean before making the 40 day journey back to the arctic. Given that they may live for two decades, some Arctic Terns may cover an astounding one million kilometres in migration.
Another seabird, the Sooty Shearwater (Puffinus griseus) makes an annual migration that out paces even the Arctic Tern, taking it across virtually the entire Pacific Ocean. Sooty Shearwaters that breed in New Zealand winter in the north Pacific, getting there by tracing a 64 000 km figure eight across the ocean. Some birds will travel almost 200 km per day when riding favorable tailwinds. Other seabirds can travel even greater distances; a tagged Northern Royal Albatross (Diomedia epomophora) traveled an astonishing 1800 km in just
24 hours!
As a long-distance runner, I have to train; I just can't get up one day and run for 20 km. But birds, they have no problem setting off on a flight that would make a human endurance athlete wilt, even after months of training. Barnacle Geese (Branta leucopsis) often fly for only a few minutes per day in the weeks before migration. Perhaps they are resting up; likely they are trying to avoid any kind of unnecessary flight while laden with the massive fat deposits which they have acquired in preparation for migration. Yet one day, flocks of geese ascend en masse into the sky, traveling non-stop for up to 13 hours at a stretch. They'll stop only a handful of times on their 3000 km journey across Europe. There are no practice flights, no warm-up calisthenics. The only preparation most birds do is to become absurdly overweight, exactly the opposite of a human athlete.
Bar-tailed Godwit (Limosa lapponica)
Photo: J.J. Harrison (Wikimedia Commons)
For almost all birds massive weight gain is absolutely critical to a successful migration. This is particularly true for long-distance migrants that very seldom stop to rest or refuel, or that don't stop at all, like Bar-tailed Godwits (Limosa lapponica) which breed in eastern Siberia and Alaska. On a tailwind, they speed over the Pacific Ocean at 100 km/h, but even at such an impressive velocity Bar-tailed Godwits must fly for 175 uninterrupted hours, or more than seven days, to cover the 10 400 km between their northern breeding grounds and New Zealand, where they spend the nonbreeding season. Like other extreme migrants, godwits pile on fat before their flight, doubling their body weight, from 285 g before fuel-loading to almost 600 g when fattened up. Perhaps most astounding is that even after a week of non-stop flight, some birds still maintain enough body fat that they could theoretically continue flying on those fuel reserves for thousands of kilometres more. This extra fat probably acts as a reserve, allowing godwits to complete their migration even if they encounter severe headwinds which would significantly slow their progress. In order to make room for enough fat on board, godwits must reduce the size of their internal organs. Reproductive structures nearly disappear. Other organs become smaller and more compact, and fat fills all available space. The northward journey of Bar-tailed Godwits is impressive as well. They fly in two stages: leaving New Zealand, they fly up to 10 000 km non-stop to Korea, Japan and the Yellow Sea coast, where they refuel. The comparatively short second leg of their migration takes them on their Alaskan and eastern Siberian breeding grounds.
Another population of Bar-tailed Godwits, those that breed in central and western Siberia, travel 10 000 km between there and their west African wintering grounds. For these godwits, the strategy is somewhat different from their Alaskan-eastern Siberian counterparts. The central-western Siberian godwits make the trip in a series of flights between suitable feeding areas. Birds leaving Guinea-Bissau at dusk can reach their first suitable stopping ground, the mud flats of Mauritania's Banc d'Arguin, by lunch time the next day. That first leg is a direct flight of 1000 km. Another 1000 km flight brings them to estuaries in Morocco, and 1000 km beyond that, the mudflats and marshes of the Loire and Gironde Rivers in France. A more modest 600 km leg brings the godwits as far north as the Netherlands and Denmark where they'll spend a significant amount of time refuelling, piling on the fat, along the shores of the Wadden Sea. Their Siberian breeding grounds await, for some as far as 4000 km away, a distance that most godwits will cover handily in one last non-stop flight.
Bar-tailed Godwits migrating between Guinea-Bissau and Siberia have to cross one of the most inhospitable regions on Earth, the Sahara Desert. They are not the only birds to do it, over 180 species make the crossing on their annual migrations. It's a 1500 km traverse that is dangerous in the extreme and certainly one of the most impressive overland journeys that birds make. Daytime temperatures throughout most of the Sahara during the migration seasons can exceed 38oC. The temperature of the sand itself can skyrocket to a horrendous 70oC. This is a challenging environment for any organism.
It would seem that a logical way to tackle a desert crossing as formidable as the Sahara is oasis hopping, but its a strategy that is used by only a small number of birds. One problem with oasis hopping today is that many of the once productive and verdant oases have been highly altered for growing Date Palm (Phoenix dactylifera) and other crops; most migrants would be unlikely to find suitable stopover habitat among the industrial-scale plantations. Garden Warblers (Sylvia borin) and Lesser Whitethroats (Sylvia curruca) will sometimes seek the shelter of oases or even rest in the shade of rocks in the open desert, but in general it would seem that most birds prefer to cross the desert in one long flight. Another potential route through the desert, The Nile River Valley with its riparian vegetation and planted urban landscapes, would seem to be a preferable route to crossing the barren wastes of the Sahara proper. It is however used by only a small proportion of trans-Saharan migrants.
Trans-Saharan migrants face different challenges in spring than in autumn. Luckily for spring migrants the Mediterranean coast of north Africa is green thanks to winter rains, allowing birds the opportunity to refuel after the long desert flight. In autumn the Sahal on the Sahara's southern fringe, is at the end of its wet season, a time when there are ponds brimming with water, green vegetation and insect prey. The wet Sahal can effectively reduce the flight over arid terrain by some 200 km in autumn. During the spring migration, the Sahal is reaching the end of its dry season, offering little in the way of refueling opportunities for most species. Instead, northbound birds must accumulate fat reserves before entering the Sahal. Prevailing winds also create different challenges for spring and autumn migrants. In spring migrants face headwinds that can require flying above two kilometres to avoid them. Autumn migrants have the benefit of tailwinds pushing them south across the desert.
Besides the risk of overheating, the other major physiological stress desert migrants face is dehydration. Burning of fat alone can lead to a moisture deficit, which some species seem to offset by also burning muscle tissue, which has a higher proportion of water. Eurasian Golden Orioles (Oriolus oriolus) use fat for about two thirds of their fuel and muscle for the remainder. In the process they gain only about three quarters the amount of energy as if using fat alone, but also gain a quarter more water, thus staving off dehydration.
Western Yellow Wagtail (Motacilla flava)
Photo: Frebeck (Wikimedia Commons)
Many species that cross the Sahara also cross the Mediterranean Sea, which itself requires another 1200 km of non-stop flying or island hopping. Some species like Sedge Warbler (Acrocephalus schoenobaenus), Western Yellow Wagtail (Motacilla flava) and Common Cuckoo (Cuculus canorus) cross both the desert and the sea in one non-stop flight of 2500 km. Sedge Warblers have the peculiar behaviour of establishing all of the fat reserves they require for their autumn desert-sea crossing in Britain and northern Europe. They probably hardly feed at all between northern Europe and the Sahal, south of the Sahara. The similarly-sized Willow Warbler (Phylloscopus trochilus) can take 44-74 hours of non-stop flight to complete the desert-sea journey, depending on tailwinds.
Sahara desert migrants are facing the prospect of an even longer journey in coming years. Human-induced desertification along the southern edge of the Sahara is increasing the distance migrants need to cover in order to reach suitable resting and refueling habitat at the end of their flight. British Barn Swallows (Hirundo rustica) in particular are thought to be declining in part because of the increased distances they have to travel during migration.
Deserts aren't the only barriers land migrants cross. Mountain ranges pose another challenge. The most famous mountain migrants are Bar-headed Geese (Anser indicus), which nest on the Tibetan Plateau and spend the nonbreeding season in India, on the other side of the Himalayas. The trip between Tibet and India requires Bar-headed Geese to fly directly over the highest part of the Himalayas, ascending to heights of 8 km above sea level. At this altitude, the air is extremely cold, sometimes as low as -50oC and oxygen is scarce, less than a third of what it is at sea level. This is the so-called death zone, the dreaded altitude at which human mountain climbers suffer extreme physical stress. Yet Bar-headed Geese can make the journey over the Himalayas with virtually no time to acclimatize. Heading north in spring, geese leaving the lowlands of India may gain over five kilometres of elevation in their first day of flying. Demoiselle Cranes make a similarly high crossing of the Hindu Kush on their migrations between Asia and Africa.
Another goose, the Brant (Branta bernicla) makes an impressive journey that requires it to travel east-west across some 5000 km of the most inhospitable terrain and ocean on Earth. Brant fly from their breeding grounds in eastern Canada to wintering areas in Ireland. Their route transverses part of the Arctic Ocean, the Greenland icecap, and much of the north Atlantic Ocean. As Brant climb in altitude some 2500 m from sea level to the top of the Greenland icecap, they do so at a very slow rate, probably out of necessity, because they are extremely fat at this point in their migration. They may even stop while climbing, resting on the icecap itself. By comparison, their descent down the far side of the icecap is swift and without rest stops.
Though most bird migrations are made under the power of flight, some birds like Emu (Dromaius novaehollandiae) walk, while penguins and auks swim. Among the auks, it was the now extinct Great Auk (Pinquinus impennis) that undertook the longest purely swimming migration. Great Auks breeding in the northwestern Atlantic wintered as far south as Florida, while those breeding in the northeastern Atlantic wintered as far south as Spain. Some Great Auks might have swum up to 3000 km round trip during their annual migration. Many of the smaller (and still extant) auks swim up to 40 km per day on their migrations which are a mix of swimming and flight.
Amur Falcon (Falco amurensis)
Photo: ChanduBandi (Wikimedia Commons)
While seabirds cruise around the oceans with impunity, resting on the water when necessary, land birds who undertake open water crossings don't have the same luxury, making their travels even more impressive. Amur Falcons (Falco amurensis) travel 4000 km over the Indian Ocean between India and southern Africa, the longest over-water crossing of any raptor on Earth. The falcons may stop on islands when they come across them, but for many of the birds that journey is completed in one fell swoop. Among the most impressive songbirds in this regard is the Blackpoll Warbler (Setophaga striata). Blackpoll Warblers fitted with light-detecting geolocators have been found to fly up to 2770 km non-stop between the Atlantic coasts of Canada and the United States and the Greater Antilles. From there the Blackpoll Warblers make their way south to overwintering grounds in northern South America. It is possible that some Blackpoll Warblers could fly even farther, with the fastest birds theoretically having enough fuel on board to travel 3800 km without refueling, a flight that could require 80-90 hours of continuous over water flight. Blackpolls are joined on their epic route by other trans-Atlantic migrants including American Golden-Plover (Pluvialis dominica), Hudsonian Godwit (Limosa haemastica) and White-rumped Sandpiper (Calidris fuscicollis); their larger size means that these shorebirds likely complete the journey in half the time of a Blackpoll Warbler.
Northern Wheatears (Oenanthe oenanthe) are also incredible travelers, in fact they likely hold the records for the longest trans-oceanic flight and the longest absolute migration distance of any songbird. They are the only songbirds in North America that regularly winter in Africa. For eastern North American birds, getting there entails covering 3400 km between their breeding grounds in the Canadian arctic and the United Kingdom, much if not all of that distance entirely over the raging waters of the north Atlantic Ocean. Next, they join European breeding wheatears and fly south, crossing the Mediterranean and the Sahara to overwinter in central and southern Africa. It's these wheatears that probably hold the record for the longest overwater migration of any songbird, eclipsing even the herculean efforts of the Blackpoll Warbler. For Northern Wheatears that breed in Alaska, their journey involves crossing the whole of Asia before reaching their east African non-breeding sites 14,600 km away. They take about three months to make the journey in autumn and two months in spring, meaning that they are migrating for almost half of the year. These Alaskan breeders hold the record for longest migration by a songbird.
Songbirds are small, but hummingbirds are smaller still. Tiny as they are some species do cover vast distances on migration. The most highly migratory of them, the Ruby-throated Hummingbird (Archilochus colubris), makes an impressive 850 km flight across the Gulf of Mexico on its way to and from its nonbreeding grounds in Central America. The myth that hummingbirds migrate tucked within the feathers of a goose still persists today; in fact I was asked about its veracity just last night by a visitor to my banding station. Nature is full of weird and wonderful things, but hummingbirds migrating on the backs of geese is simply apocryphal. To me, its even more amazing that hummingbirds undertake such a long migration on their own. Indeed, all of the examples I've given are almost unbelievable feats of endurance and navigation and they're being undertaken by the very creatures we see every day.
