Migration
Living in England, a young and decidedly amateur ornithologist, I didn’t pay much attention to annual bird migration. Like many other watchers, I noted, for example, when the swallows arrived and left. I suspect I imagined there was a meeting, a convocation, one day in the late summer, perhaps at the Marble Arch end of Hyde Park, when instructions were issued (I did have to wonder how??) to the flock advising them of the departure arrangements in three days’ time. The assembled swallows would be reminded they had those three days to fatten up, young birds were warned the flight was long and dangerous, and then it was a set of simple instructions: ‘go from Hyde Park Corner to the White Cliffs of Dover, turn a little to the right, and then go straight on for Spain and North Africa’!
A little later in life, I migrated, not for the summer but permanently. To travel from the UK to Australia required permits, visas, and a flight itinerary: it was complex, in this case, because we decided to take three months and visit various places on the way. We didn’t fatten up for the trip, but our bags got bulkier with various gifts and mementos as we went along. By the time we arrived in Adelaide, we weighed a lot more than when we left (the exact opposite of those migrating swallows.
Arriving in Australia saw an end to anything more than casual birdwatching. The country was full of varieties I had never seen before, and I decided to abandon adding to my life-list of observations. However, with parrots and blue wrens in the garden, and black swans on the river, for the first year or so it was a birdwatcher’s dream. Soon enough I became adjusted to cockatoos and galahs (yes, they’re a bird variety as well as a name for people who make stupid mistakes!). However, it took another move, this time to the USA in 2011, to see my interest in ornithology returning to the fore, largely driven by Ruby Throated Hummingbirds, hovering just outside the patio door. In a previous blog I explored how these tiny birds flew north in the late Spring form South American, crossing the Gulf of Mexico, and returned in the middle of Autumn. Even smaller than swallows, that must have been a demanding journey, over water and without any nectar pods available!
Older, less driven by work demands, I have returned to an earlier question. How did birds manage to migrate, sometimes for days on end, in the daytime and at night, and yet find their way to their other home? Perhaps you are unaware of the extraordinary migration paths some birds have followed. I could start with the bar-headed goose. Not a particularly exciting bird to look at, but every year flocks fly from southern India to Mongolia. It’s a distance of around 1,000 miles, but to cover it they fly over the Himalayas, five miles up, where winds often blow at 100 miles an hour, temperatures drop down to thirty or forty degrees below zero and oxygen levels are one fifth of that at sea level. As one writer explains “The air at that altitude, in fact, is so thin that helicopters cannot fly because their rotors can’t get enough lift.”(from Jim Robbins in The Wonder of Birds). The bar-headed goose flies at 50 miles an hour (or faster with a tail wind) and crosses the Himalayan mountain range in one day!
The distance travelled by a bar-headed goose is only mildly impressive. As far as we know, the world champion for migration is the Arctic Tern, going from the Arctic to the Antarctic, and, incidentally, seeing more daylight per year than any other species. One Arctic tern caught and ringed as a youngster in the Farne Islands, off the British east coast, reached Melbourne in just three months from its fledging capture, a sea journey of over 22,000 kms.
Perhaps you’d prefer to hear about the exploits of the Manx Shearwater, It is a bird that looks a bit like a gull in flight, although it is from a quite different family of birds. Its wings are straight, black on top and while underneath. Its wingspan is around 80 cms, and its body length around 35 cms. Most the bird’s life is spent over the ocean, but they nest in the northern UK and parts of Scandinavia. However, in the winter, they go south, and end up in South Africa, Brazil, Argentina and Chile. Some even go to the Falkland Islands. They fly from the northern regions to the south in around two weeks, a journey of some 14,000 kilometres. As they are long-lived birds, some will have cover enormous distances during their lives; one record-breaking Manx shearwater is calculated to have flown 8 million kilometres during its over-50-year lifespan (according to a report on CNN in April 2002). These are serious travellers!
I could continue to describe various bird species and their extraordinary migration adventuress. Many species of birds travel enormous distances, in several cases remaining aloft for the whole journey, although I should mention that the Manx Shearwater does take breaks as it heads south (or north). However, what intrigues me most about these supreme athletes is how they know where to go. Many studies have shown that youngsters, birds that have never migrated before, set off from their nesting area and go south for the winter, knowing exactly how to get to their alternative homes. How do they do it?
Perhaps I should explain one thing. Today’s understanding of migration paths has been achieved by playing some mean tricks on birds. They are loaded with equipment, including geolocators and transmitters, they are taken in darkened aviaries to new locations, often many hundreds of miles from their original home location before being released, and, in some amazing studies, even fitted with eyeglasses that distort their vision. There might be a large number of Manx Shearwaters, at least half-a-million, but some have done it tough!
Well, it might be you don’t find their travels a mystery. For many years, it wasn’t seen as anything that was that particularly curious. However, in more recent years, and certainly since the nineteenth century, it has been an enduring puzzle. They have to use a kind of ‘spatial cognition’, allowing them to traverse forests, deserts and oceans and keep going in the right direction. In case you might think it is just a matter of landmarks, many bird species can navigate on dark, cloudy and rainy nights when no physical cues are visible. This also means that the old sailors’ approach, navigation by the stars, won’t work (nor by the moon, let alone by the sun in daylight hours). We can dispense with maps: no evidence has been found of mud maps drawn on the ground near a nesting territory!
Actually, this is proving to be an extremely complex field, and blanket statements, like those I have just made, are often wrong. Those Manx Shearwaters mentioned earlier do find the sun useful in orienting themselves. Various studies of warblers have shown they used the night sky to set direction for themselves. Of course, in both these cases, that means the birds had to be conversant with the impact of time: they seemed to be demonstrating they possess some kind of inbuilt sextant and chronometer system to set direction, just as a sailor might do.
It might be worth changing focus for a moment, and ask how do we find our way to distant places? I think the answer might be that we use a variety of ways to do this. If I use myself as an example, usually the first thing I do is study a map. I look at where I am, plan what roads would be the best to use, and note the cities I’ll go through or bypass. Then I set off. However, without thinking about it, I orient myself by the sun, in order to make certain I am going in the right general direction. Without thinking? Well, I know when the route I have in my mind is interrupted, by major roadworks for example, I only place some reliance of various ‘Deviation’ signs, but I also keep in mind the direction I believe is correct, using the sun and a sense of how many shifts have taken place in one direction or another. I’m aware that this is an imperfect approach, and I can, to my chagrin, recall times when I have found myself going in the wrong direction altogether when sent off the main road, and taking what I thought would be the correctly aligned turnings! I also use visual information, of course. If I know a city, a town or even a major hill or volcanic plug is coming up, I look for that.
It’s challenging to remember what it was like the first time you drove in a new part of the world. Once we’re settled, we ‘know’ where to go, and there must be some kind of map in our heads we use. However, when I moved to the US several years ago, I was without a mental map and, to my surprise, there were significant parts of trips where there were few built up areas. As long as I stayed on a freeway, it was easy, but once I was on back roads, it was back to a sense of where I should be heading, using whatever I could to ensure I was going in the ‘right’ direction (not always easy when roads start to wander), and looking for clues (the names of unfamiliar hamlets are little help). Of course, my partner never had a problem: “Stop and ask someone”. She was right, of course, but there were times when I didn’t want to admit defeat.
How does this relate to avian migration. Well, I think part of the challenge is that different species, and birds at different ages, use a variety of cues. There is evidence that in species where the birds are long lived, then the older birds lead the way, and the younger birds learn. This has been shown in the case of White Storks. Other research has shown that history plays a role. The Northern Wheatear now travels from many areas in the northern hemisphere, even if they all end up in sub-Saharan Africa, a function of the expansion of their breeding grounds. Many species do not fly direct, but follow paths that allow them to take breaks, and several species of water birds follow paths that take them over wetlands where they can stop and recuperate, even though this can extend the overall migration by hundreds and even thousands of miles. Others fatten up, and just keep going, like the Great Snipe, which migrates with non-stop flights of between 4,000–7,000 kms, flying for 60–90 hours, during which time they change their average cruising heights from 2,000 m (above sea level) at night to around 4,000 m during daytime. I guess they are the marathon runners of the bird world.
However, research in recent years has shown navigation is based on a surprising variety of senses. Many birds have been shown to use a sun compass (perhaps that’s true for Shelducks!). Using the Sun for direction involves the need for making compensation based on the time. Quite apart from the sun, and the time of day, stars, and visual landmarks, and more, satellite tracking of birds of prey, including ospreys and honey buzzards, has shown that older individuals are better at making corrections for wind drift. However, the most exciting findings concern the role of ‘electromagnetic tools’.
In the case of some varieties of bird, it seems a young bird on its first migration flies in the correct direction according to the Earth’s magnetic field, but it does not know how far the journey will be. Research suggests that this is a consequence of chemical reactions in special photo-sensitive pigments in the bird’s eye, which are sensitive to those short wavelengths which are affected by the earth’s magnetic field. This only works during daylight but it does not use the position of the Sun in any way. With experience, the bird learns various landmarks, ‘mapping’ using magnetites in the bird’s trigeminal system, a key cranial nerve. As birds migrate between northern and southern regions, the magnetic field strengths vary at different latitudes, and this system appears to let it know when it has reached its destination. Further, other research shows there is a neural connection between the eye and the part of the forebrain that is active during migrational orientation, suggesting that some birds may actually be able to see the magnetic field of the Earth (reported by Heyers, D.; et al, in PLOS ONE in September 2007). More on this in a moment.
Mind boggling? There’s more. Considerable research has gone to what is known as ‘path integration’ in animals, usingcues from different sensory sources within the body, without relying on visual or other external landmarks, to estimate position relative to a known starting point or return point, doing so continuously and while travelling on a path that may wander considerably from a straight line. Obviously, path integration is important in animal navigation, and has been studied since Darwin first wrote on animal instincts. When vision (and hence the use of landmarks) is not available, path integration must rely on clues generated by the animal’s body. Path integration in mammals has been shown to make use of their sensory organs. However, information from other senses such as echolocation and magnetoreception may also be integrated in certain animals. That would be like me going off track, realising that I had strayed from my way home, and automatically compensating. Wow, now wouldn’t that be useful. It sounds like some kind of internal Global Positioning System! However, it seems path integration isn’t so obvious in human beings, or perhaps we just ignore the signals our brain is receiving.
All this modern work suggests that there are many systems used by birds that hadn’t even been imagined fifty years ago. Several could certainly play a role in getting a bird back to where it had wintered a year before. That leaves us with the puzzle of getting there the first time. All this is the subject of a lot of current research. Geolocator studies of our old friend the Manx Shearwater confirmed what we knew, those birds travel a very long way, and now there is data to suggest they do down south along the coast of Argentina. However, a real surprise was to find that they don’t fly straight down, but stop off regularly, to rest and refuel – and will stay in some of those rest points for as much as a couple of weeks! All this is revealed in Tim Birkhead’s wonderful 2022 book, Bird Sense, with the very appropriate subtitle ‘What It’s Like to Be a Bird’.
As for magnetoreception, recent research has identified cryptochrome in special cells devoted to vision in bird eyes. When photons (tiny individual light particles) strike the cryptochrome, it seems its electrons are briefly (that means for one hundredth of a microsecond or less) in a state of quantum entanglement with the earth’s magnetic grid. The grid lines identified “allow the bird to see and be guided by them” (in Jim Robbins book, The Wonder of Birds, page 209). Yes, this is mind boggling. Continuing research shows that some bird species use this identification of grid lines to compensate for relocation: these findings are based on those ‘friendly’ studies when birds are taken in light-proof boxes to sites hundreds of miles from the nesting site, and released to see if they can migrate successfully
There is still much to be understood in bird migration. For once, that great line (from the 1800s) ‘there’s more to this than meets the eye’ is both true and untrue! There’s even evidence that that gravity anomalies could play a role in homing pigeon navigation. Some bird species complete truly amazing feats of long-distance navigation. There are many factors that appear to play a role in their ability to find direction, even if blown off course. Perhaps, like Peter Sheldrake, they use a variety of methods, relying on one some of the time, and another when circumstances change. One thing is clear, reading studies based on recent research has reawakened my interest in ornithology, but now I mix my observations of the birds around me with armchair studies of migration.
