‘Reverse migration theory’ — wrong all along
Autumn at last. Most of the birds that spent summer in the Northern Hemisphere are in the throes of southbound journeys, destined for warmer climes where they can find food to survive the winter. The overall process is governed by genetics. That is a huge thought. Where is the proof? With the exception of large, family-orientated species such as wildfowl and cranes, most young first-time migrants set off alone. They are unaccompanied — even by peers. They follow nothing but a genetic impulse that navigates a rookie across continents to the same winter homeland used by its forebears. If you find it difficult to swallow the genetic argument, think about a young Cuckoo. Its migratory behaviour can only be determined genetically. Not only does the baby migrate long after its parents are gone, but it has not even been raised by them. Stripping away the lack of parental contact, you are left with an embryo that already knew a route to Africa’s tropics, where European Cuckoos spend the winter. There is no doubt that this principle applies to most other songbirds too. However, simply inheriting fore-knowledge of a compass bearing is not enough to deliver a new generation to safe havens thousands of kilometres away. Young migrants may be drawn from the infantry age-class but if nature does not prepare them properly then, long-term, their species is doomed.
Although genetic code determines migratory instinct, the trigger that activates preparations comes from day length. Across the planet, length of day varies with the seasons. This is a consistent pattern, year on year. It is easy to grasp the ‘awakening effect’ of longer days in spring. Plant growth is stimulated and much of the natural world bursts into life. This is obvious. But it is harder to apply the same argument to European migrants over-wintering in the tropics where day and night are closer to equal length. Yet they know when it is time to head back north. The reason they do, is because they were programmed at birth to respond to ‘photo period’ — a kind of subconscious timer that means they keep to a correct migratory schedule for life.
Not just birds, but most of the natural world runs to the same (circadian) rhythm. Why is there a need for such regulation? A big part of the answer is connected to reproduction. Breeding conditions for migrant birds only exist for part of the year and in certain parts of the range of a species. Travelling there in spring connects migration with an urge to breed; travelling away in autumn ensures winter survival. If genetic coding underwrites strategy, a bird’s season-to-season actions are controlled by photo period. Most importantly, photo period has a direct bearing on the growth and subsequent shrinkage of the sex organs — called gonads. So there you have it. Genes operate a ship whose timetable follows changing day length which, in turn, regulates breeding ability.
The credit for disentangling the ‘riddle of migration’ belongs to William Rowan. In 1931, he published his findings in a book of that name [1]. Why is he not as well known as other great scientists such as Darwin or Einstein? He was born in Switzerland, spent many years in England and had a lifetime interest in migration. In 1921 he became the sole member of a specially established Department of Zoology at the University of Alberta in Canada. At that time, leading theories that sought to explain migration suggested that food supply, temperature or barometric pressure were the cues that birds used to calibrate the seasonal changes in their biology, including migration behaviour. In a series of brilliant experiments using wild-caught migratory Slate-coloured Juncos that he kept in aviaries during winter, Rowan swept away all the flawed thinking. By manipulating light, he brought a set of ‘experimental’ birds into breeding condition in the middle of the Canadian winter. Was he greeted as a hero? Far from it. He was treated almost as badly as Galileo who, I’m sure you recall, narrowly escaped a death sentence because he proposed that Earth orbited the sun, rather than vice versa.
Let’s get back to the contents of the genetic hard drive located between the ears of a baby bird. The chick’s destiny cannot be helped, neither can it be stopped. To a large extent it is an automaton. During the course of its life — particularly its first momentous year when it will attempt a return journey between ‘home’ and ‘away’ — it will learn many things and turn new knowledge to its advantage, such as the whereabouts of good stopover habitat encountered between destinations. However, to begin with, all its travels are a slave to auto-pilot. Say it is a North American warbler, hatched in Canada, whose winter quarters lie in the tropics. Several species fall into that category. Some migrate mainly over land, others cross the sea. A few stake everything on an epic trans-oceanic flight which, in the course of two nights and three days, takes them in one hop from the northeast coast of North America to the north coast of South America.
Blackpoll Warbler is the chief exponent of this Olympian slingshot. Young Blackpolls cannot question the wisdom that dictates their fate; their predecessors have been making the journey for aeons. The latest recruit, less than two months old, cannot know that its life is being spared by migrating south before its food supply — insects — goes into a state of suspended animation. Or know why it is suddenly fattening and doubling weight for a flight during which it will change direction and reach South America in an arc that taps tailwinds blowing across the Lesser Antilles, the outermost islands of the West Indies, helping it make landfall between Venezuela and Brazil.
Little wonder that birds exhibit increased activity when they are in the grip of migration fever. A German word — Zugunruhe — was coined to cover the combination of weight gain and nocturnal restlessness (most migrants depart just after dusk). Both actions were noticed in wild birds kept as cage-birds as long ago as the eighteenth century. Zugunruhe subsides — times out — whenever a bird’s internal clock tells it that the impulse to travel is no longer required. Put simply, because the internal clock assumes that the bird has arrived at its predetermined destination, it reverts to a ‘business as usual’ schedule, bringing migration to a standstill.
By now, you may have inferred correctly that genetic control is tailored to suit individual species. Blackcaps breeding in Ireland and Britain head south in autumn and (mainly) occupy winter quarters extending from Iberia into northwest Africa. However, Lesser Whitethroats, a close relative breeding in Britain, travel southeast and head for the Middle East. There they change direction and cross the Red Sea to enter Africa in southern Egypt, Eritrea, eastern Sudan and northeast Ethiopia. From here the population tracks west and is believed to winter in semi-desert and ‘African bush’ habitats in Sudan, Chad, northern Nigeria and Mali. Comparing the route of Blackcap with Lesser Whitethroat reveals a stark difference: southwest for Blackcap versus southeast for Lesser Whitethroat. Moreover, Lesser Whitethroat’s journey is likely to be more than double that undertaken by Blackcap. Routes, therefore, are customised to species.
Although some migration routes may strike us as barmy, the principle ‘if it ain’t broke don’t fix it’ applies. Depending on what works, within the same species there are separate routes. Once again, Blackcap provides an example. Like Lesser Whitethroats, Blackcaps breeding in Eastern Europe — roughly from Germany eastwards — migrate southeast in autumn. Their winter quarters lie from the eastern Mediterranean down the African rift valley to Ethiopia. And, yes, that route is genetically encoded. How do we know that?
The most comprehensive attempt to address the question of whether migration was innate and under genetic control was made in the 1970s by Peter Berthold in Germany. He established a veritable Blackcap ‘breeding factory’ and cross-bred individuals from populations that went in different directions. Furthermore, he widened the scope of the research by including Blackcaps from the Canary Islands. Their inclusion was important because they are virtually resident. His results provided unequivocal proof of the overarching role of genetics in calibrating migratory behaviour. Cross-breeding results produced birds with intermediate ‘settings’. Such as offspring whose instinct was to migrate due south — the product of a parent that possessed a ‘southeast gene’ mated with another whose parent carried a ‘southwest gene’. Not only did this suggest that migratory instinct was inherited but also, because the behaviour was intermediate between that of the two parents, that it was controlled by several genes.
Berthold showed how populations of Blackcaps and, by inference, other migratory species, could evolve a novel pattern if individuals within a population exhibit a range of directions. Even when some directions are plain wrong! Well, not ‘wrong’ per se, but outside the normal choice of orientation encoded within a species’ genes. Such deviation is called a mutation. When it occurs — invariably in a youngster — the result is likely to be fatal. Setting off in a dud direction, the offspring will not reach its intended destination. It could fly north and perish in the cold; or it could fly out to sea and meet a watery grace. In a way, evolution would finish up the winner because faulty migratory genes would be eradicated. But everyone, including, it seems, nature, loves a trier. Maybe, in the scheme of things, mutations inadvertently serve as a stress test — weeding out poor navigators? Or they could be Mother Nature’s sweepstake, randomly visited upon the young, resulting in exploration with the potential for range expansion? Evolution does not take a fixed view and mutation is analogous to low-key mould-breaking — having a throw of the dice at no cost to the status quo of the species but an action that could come up trumps. And, once in a while, that is what happens. In a way, migrants that err are evolution’s fortune cookies.
An example that we can see with our own eyes in Ireland and Britain is the staggering increase in the number of wintering Blackcaps. Within the last forty years the species has become ubiquitous in back gardens, mainly after Christmas, when insects and berries become scarce, displacing Blackcaps into suburban habitats where they come to feeders. Having survived thus long, the birds can then tap ivy berries, which ripen in February and March. Over previous decades a few Blackcaps had been encountered in winter. But not on the scale of the newfound population, that reached critical mass in the 1980s. Peter Berthold investigated the phenomenon. By capturing wintering birds in southern Britain and breeding them in captivity in aviaries in Germany, he established that, once again, genetics had determined their migratory instincts — choice of direction, distance travelled and duration of Zugunruhe. Berthold’s conclusions are lucidly explained in a lecture that he delivered in 1993 for the British Trust for Ornithology. Here is the full transcript:
https://www.tandfonline.com/doi/pdf/10.1080/00063659509477155?needAccess=true
[To read about a seemingly deliberate attempt to emasculate Peter Berthold’s findings, see footnote.]
Berthold coined the term ‘micro evolution’ (henceforth microevolution) to describe how a species’ migration route can shift, based on genetic variation being exercised in choice of migratory direction. Fortunately you do not need to be a member of Mensa to get the point of this. Basically, if some members of a migratory population choose to go ‘off piste’ — directed to do so by their genes — but still manage to survive and return to the breeding homeland, then their tribe stands a chance of multiplying. However, one survivor coded for a novel direction is like one Swallow not making a summer. A contrary direction will not be replicated within the population unless the bearer of the genes manages to breed, preferably with another individual that happened to have made the same navigational ‘mistake’. When that occurs, the resultant offspring are hard-wired to follow in parental footsteps. Fortune clearly shone on the 1980s contingent of Blackcaps that wintered in our back gardens. After decades of failing to survive, thereby ensuring that their baton was not passed on, milder winters and a meteoric rise in garden feeding stations conspired to make wintering in Ireland and Britain (and the near-Continent) a viable option, not a death trap. Once like-minded migrants met on the breeding grounds, their offspring preferentially headed west in autumn.
You may have come across the term ‘reverse migration’. What does it mean? Could it, perhaps, describe the novel direction that new generations of Central European Blackcaps are following when they arrive here in autumn? On the one hand, a subpopulation of Central European Blackcaps has certainly developed a new migratory direction to the northwest; on the other hand, this is hardly a ‘reverse’ from southwest — the birds’ traditional route. Reverse migration is, instead, a term used to describe a migrant that heads off in the diametrically opposite direction; such as, in autumn, a Swallow heading north rather than south, or a migrant warbler breeding in Siberia and wintering in Southeast Asia departing west towards Europe rather than east towards Asia. By way of trying to explain why some migrants allegedly head off in the opposite direction, it has been argued that such individuals miss-read the seasons and, as it were, engage spring migration co-ordinates in autumn. The topic becomes engrossing because it could offer an explanation for the astonishing autumn arrival in Ireland and Britain of small numbers of rare migrants whose breeding grounds are in Eastern Europe and Siberia, yet whose winter quarters lie in the Oriental region.
One species in particular has been the focus of debate — the Yellow-browed Warbler. The first to be recorded was on the coast of Northumberland on 26th September 1838; in Ireland, the first individual was shot on the Tearaght, Co. Kerry, on 14th October 1890. Up to 1962 there had been around 310 records in total but around the turn of the millennium the annual average hit close to 300 records. You might say that an increase in birdwatchers probably accounts for the jump in sightings. Although that is certainly true and the bird had become a regular autumn stray rather than a major rarity, how do you explain a meteoric rise, in Britain alone, from an annual mean of 328 (during 1990–99) to 749 (during 2000–09) to 1,984 (during 2010–16)? [2] Then, in 2016, a tsunami of Yellow-browed Warblers poured south down the eastern seaboard of Scotland and England (with many also in Ireland) that amounted to ‘a minimum of 4,000, and perhaps as many as 5,000.’ [2] In many areas, Yellow-browed Warblers outnumbered regular European migrant warblers such as Chiffchaffs and Blackcaps.
You need to see the wood from the trees. Yellow-browed Warbler is a tiny warbler, close in size to a Goldcrest. Yes, a titch like that could, quite literally, be blown off-course. Sometimes that happens. Such as in Iceland on 7th October 2014 when, following in the wake of a southeast gale and rain that battered the east of the country, 34 Yellow-browed Warblers were discovered — eclipsing the previous cumulative total of nine for a single year [3]. The birds were not swept to Iceland from the nearest breeding grounds in northwest Siberia. Research findings [4] show that a fully-loaded Yellow-browed Warbler is capable of a maximum flight distance of 800km. Even allowing for displacement by strong winds, the source area for those that were drifted across the sea to Iceland was, most likely, the Norwegian coast. To reach Norway, the birds had come west (or northwest) from Russia and the reason they did that was because almost the entire lot were migrating based on genetic instinct.
Do we know where those that hit Iceland finished up? Although we don’t, I do not subscribe to the view that they flew on, like crazed Lemmings fixated on continuing in a standard direction, and met a watery grave. Maybe in the past that was the fate of lone migrants flying west, primed to do so because their genetic roadmap contained a mutation? That argument is consistent with a few youngsters in every population of migratory birds being foredoomed as ‘fortune cookies’. For Yellow-browed Warbler, those days seem to be over. Using Blackcap as a model, it seems that Yellow-browed Warbler is another species whose changed migratory behaviour is a result of microevolution. Other evidence supports this view. Increasing numbers are being found in winter on Lanzarote, Tenerife and Gran Canaria and, in autumn, unprecedented numbers of southbound migrants have been noted passing through islands within the Mediterranean.
Is the conclusion that, in travelling roughly west — instead of east — Yellow-browed Warbler began reaching our shores as a reverse migrant and that, after spending the winter in northwest Africa (?), the survivors returned to breed in Siberia — bestowing their offspring with a new migration route and wintering grounds? Thus, while millions still migrate east, hundreds — if not thousands — are booming out west. Overall, I believe that this is the case. Except that one term jars with reality. That term is ‘reverse migration.’ I agree with far more knowledgeable others [5, 6] who consider that the phenomenom is a myth. While most young migrants emulate parental flight-paths and ensure that tried and tested routes continue to operate, others disperse in random directions. Juvenile dispersal is a fact of life. In autumn, some juveniles travel hundreds of kilometres on what seem to be ‘scouting trips’ before they knuckle down and head off to ancestral wintering grounds. But others embark upon, not minor explorations, but what amount to boom-or-bust exploratory migration flights, decreed by their genes.
References
1 Rowan, W. 1931. The Riddle of Migration. Williams and Wilkins. Baltimore.
2 British Birds 111: September 2018, 519–542. Report on scarce migrant birds in Britain in 2016.
3 Icelandic Birding Pages (online resource)
4 Martha Maria Sander, Jana A. Eccard & Wieland Heim (2017) Flight range estimation of migrant Yellow-browed Warblers Phylloscopus inornatus on the East Asian Flyway, Bird Study, 64:4, 569–572, DOI: 10.1080/00063657.2017.1409696
5 Gilroy, J. & Lees, A. 2003. Vagrancy theories: are autumn vagrants really reverse migrants? British Birds 96: (September 2003) 427–438.
6 Phillips, J. 2000. Autumn vagrancy: ‘reverse migration’ and migratory orientation. Ringing & Migration 20: 35–38.
Footnote
I had the good fortune, while researching my book about migration — To the Ends of the Earth — to be able to borrow a copy of the British Trust for Ornithology’s massive and expensive The Migration Atlas, published in 2002. I am glad that I did not spend money on the book. Although the maps, because they contain raw data, are highly instructive, the contents that I read (a selection of species about which I was specifically interested) were poor. I found errors, interpretation that was wrong or superficial and a general impression of sloppy standards throughout. It was as though the perceived status quo about a species’ migration was cobbled together without bothering to investigate the data contained in the maps. Sloppier still were the line drawings that headed up each account. Most are mediocre but many are dreadful or contain incorrect field characters for some species: just appalling. Untalented illustrators can be forgiven. At least they made an honest attempt. However, when it comes to the information in the account for Blackcap, the author, Derek Langslow, ignores current understanding of the factors that led to the change in the bird’s wintering behaviour in Britain and Ireland. Perhaps this was because D Langslow’s earlier published prognostications on the topic were disproved by Peter Berthold’s pioneering research? In any event, given the painstaking research published widely by P Berthold and his co-workers (from the late 1980s onwards), it is inexcusable for D Langslow to write, in 2003: ‘Why a new west-northwest migration direction should have originated in the last 30 years remains unanswered, although Berthold (1995) explores the possible mechanisms.’ In fact, the 1995 reference cited by D Langslow is one of the last papers by P Berthold on the subject. The BTO, because they were aware of the importance of P Berthold’s work on microevolution in Blackcaps, invited him to give the prestigious ‘Witherby Lecture’ in 1993 — linked above. All of which raises the question: why did D Langslow ignore the detailed conclusions presented by P Berthold in 1993 in the BTO’s own publication published a decade later? Moreover, what does this say about the overall standard of editing in the BTO Migration Atlas?
Acknowledgement
Many thanks to Richella Duggan for skillful editing.
