People have always noticed the effects of migration even if they were unaware of the cause. Rock art with drawings of the birds have survived and there are paintings in Egyptian tombs depicting birds such as cranes (from the north) and white-breasted geese which nest in the Arctic. These species could only have been observed during periods of migration. Aristotle (384-322 BCE) thought that birds such as swallows and warblers magically changed species (transmuted) when they disappeared in the winter, and this idea persisted into the Middle Ages. Nomadic hunter-gatherers would have depended on seasonal migration for food, and rock art drawings of the animals could be indications of good hunting regions. Some birds have been known to return to the same twig as the year before. How do they do this? The answer may be in their genes.
Figure 1: Cranes from the Fifth Dyansty mastaba of Ti, Saqqara, with one Demoiselle Crane (left) and one Common Crane(right)
This article looks first at how new technology and developments in genetics have allowed new and more advanced studies to take place. It then provides an overview of research focusing on the role of genes in migration with one study examined in detail. An attempt is made to illustrate the link between genes and migration before identifying areas where progress has been made and those where work still needs to be done.
Development of Technology
Birds embark on migratory journeys along established routes between breeding and non-breeding areas. These journeys, which are crucial for their survival, are still not fully understood. Nonetheless, developments within genetics and technology are enabling scientists to further their understanding of all aspects of migratory behaviour.
In an article looking at new opportunities through advances in technology, Guildford et al. comment that “The genetic basis and heritability of migratory behaviour, and hence the underlying control mechanisms, is another exciting avenue where advancements in tracking technology can lead to new insights” 
The positions of migratory birds can now be relayed in real time to scientists on the ground. Light aircraft can track birds, and details such as height and speed of flight can be monitored by radar. Tracking devices (which have become smaller and lighter) are fitted to the birds and used in conjunction with satellites orbiting the earth to send information to computers.[1[ Advances in genetics mean that the data provided by this technology has an even greater impact now, as it can be examined in finer detail and applied to further research.
Figure 2: Warbler with tracking device
Development of Genetics
“Genetics is an astonishing science because it is the only science that traces itself to only one person, which is Mendel. All other sciences: Chemistry, Evolution, Physics, can trace themselves back through hundreds of years and thousands of people, but not Genetics. It was one man only”. (Steve Jones, Emeritus Professor of Human Genetics at University College London) 
Figure 3: Gregor Mendel
The study of genetics began in the 19th century with Mendel, but his work was largely ignored when it was first published. Mendel himself could not have had any idea of what a gene was, what it looked like or its arrangement on a chromosome. He did, however, understand that certain traits were more readily passed on, and he concluded this was due to recessive and dominant factors, which we now call genes. Jones says that Mendel was the first biologist to count. This was important, because counting rather just measuring results in quantitative data allows the comparison of discrete situations and subjects. It was not until the 20th century that his work was rediscovered, and not until midway through the century that Crick and Watson progressed with their work on DNA, giving scientists more knowledge of how genes worked. Once scientists understood that the phenotype of an organism was a result of the interaction between the genotype and the environment, then the possibilities of investigating and analysing behaviour such as migration were greatly increased. Researchers have cited migration as an opportunity to explore not only how one gene affects changes in behaviour but also how many genes interacting together can result in variations of phenotypes.
Genetics is a relatively new branch of biology, so scientists still have different views of the role of genes in migration. Some scientists suggest that birds have a navigational method as yet undiscovered, such as a gradient system involving a continual comparison of two changing features of the surface of the planet. An accepted idea, however, is that birds have a mental map coded for by their genes. If this is true, birds must be able to store huge quantities of information and access it repeatedly during migration, along with the use of visible and non-visible clues. Studies show that migratory birds have a larger hippocampus (the part of the brain concerned with functions such as memory, learning and spatial awareness), compared with closely related non-migratory species.
Research Studies: Overview
Studies of Eurasian cuckoos have highlighted the role of genetics in the birds’ migration. Eurasian cuckoos lay their eggs in nests belonging to birds of different species, who then raise the chicks unwittingly as their own. Despite being reared by foster parents, the young birds still set off at the end of their first summer, following the same migratory routes as their real parents, even though they have never met them. This suggests that they inherit a migratory map, timetable and route from their parents. Some scientists make the distinction between the importance of the role of genes in the migration of longer-lived and shorter-lived birds. Longer-lived birds such as geese seem to learn their journey routes directly from their parents, whereas shorter-lived birds make their first migration without their parents. They must therefore have an “Inbuilt genetically programmed directional and time-based clock and compass algorithms.” 
Figure 4: Eurasian Reed Warbler raising a common cuckoo
The willow warbler makes up one fifth of all migratory birds flying from Europe and Asia to Africa. These tiny birds undertake journeys of thousands of miles, but scientists are now worried about the effect of climate change on the timing of their journeys. Research shows that British willow warblers now lay their eggs a week earlier than in the past, and that they leave Britain later than they used to 40 years ago. The first year they migrate alone, meaning they cannot have learnt this journey from their parents. A tracking study in Sweden suggests that the willow warblers use an internal solar compass which works together with the internal body clock to take account of changing time zones during the migration. The study also proposes that the birds have a magnetic compass which they use for precise navigation. Discussing this aspect of migration, Tim Guildford of the University of Oxford, makes the point that this magnetic compass allows birds to distinguish between going toward the poles or toward the equator, but not between north and south.
The same study also found that two subspecies of willow warblers share the same wintering grounds in East Africa, and are genetically inseparable according to their nuclear and mitochondrial genome. The assumption has been made that these subspecies may have originally come from the same population of willow warblers, which gradually expanded across Asia or Europe. This expansion would not have required major genetic changes of their migratory programme, as the same mechanism would work from other longitudes and would explain why their genes are so similar. Genetics here enables the authors to understand much more about these birds:
“Based on these observations it is reasonable to assume that they comprised one common refuge population during the last glaciation. Our study area might have been colonized by willow warblers during the Holocene climatic optimum between 6000 to 8000 years BP”. 
This type of historical and geographical knowledge would have been inaccessible with any degree of accuracy before the advancements in genetics.
In the 1960s, studies with European robins provided evidence that magnetic fields might be used by migratory birds. European robins typically migrate from northern Europe to southern Europe or Africa. The robins were kept in secluded rooms with no environmental cues. However, they still experienced a period of migratory restlessness (zugunruhe), during which their heart rate increased as it would normally do before migratory journeys. They appeared to want to fly south, despite having no clues as to which direction south lay. However, when their cages were covered in electromagnetic coils the birds became confused, and changed the direction of their hopping and fluttering.
A more recent study developed these ideas using European blackcaps, with a focus on the genetics behind the behaviour. Jakob Mueller and his team at the Max Planck Institute for Ornithology in Starnberg captured blackcaps from fourteen different populations. These blackcaps are normally only active in the day, but fly during the night while migrating. The scientists collected data from the birds’ night-time restlessness, and obtained blood samples. They used these samples to identify genetic signatures which could explain the changes in the birds’ night-time activities. The researchers concentrated on four genes which are thought to be connected to changes in daily rhythms and so lead the birds to migrate at night. The gene ADCYAP1 was cited as fundamental to the shift in behaviour at night; the longer the allele, the greater the restlessness of the birds. However the research concluded that the gene, which codes for the protein PACAP, is responsible for more than general restlessness. It is also essential for controlling energy metabolism, melatonin secretion and feeding, all of which enable the bird to prepare for migration. “This is the first step to bringing research on avian migration down to the molecular level,” says Mueller.
Scientists have emphasised the importance of this study as opening the way into new investigations into the role of genes in migration. John Wingfield, of the University of California, has suggested that the next step would be to ‘silence the gene’ and observe the effect this has on migration.
A study by Peter Berthold and his colleagues also investigated the migration of blackcaps, but with a focus on the genetic basis for distance and direction. They cross-bred blackcaps from two distinct populations, one migratory and one sedentary. The offspring from this study displayed “intermediate migratory restlessness” , indicating a genetic component. Other experiments confirmed these results by using selective breeding on migratory and non-migratory blackcaps. Two hundred and sixty-seven blackcaps from a population raised in aviaries in France were used in the study. Three-quarters of the birds exhibited migratory restlessness. The researchers used selective breeding from either migratory or non-migratory parents. They found that within three generations, blackcaps that were 100% migratory were produced, and within six generations blackcaps that were 100% non-migratory were produced. This confirmed the role of genetics in this behaviour.
The same researchers demonstrated the genetic contribution to the evolution of new behaviours. During the last 40 years, the number of blackcaps spending the winter in the UK and Ireland has increased. Because these locations are 150 km north of their traditional wintering grounds in the Mediterranean, scientists at first believed the blackcaps to be British breeding birds which had remained in the UK due to the warmer weather. However, after tracking the birds, it was discovered that they were from central Europe, so the team decided to look more closely at the migration behaviour. They kept the birds in aviaries, and found that they had developed a migration route which had moved 70 degrees from the original route. In addition, they discovered that the birds’ offspring went on to inherit this new migratory route. The researchers suggest that the development of this new route, which entails a shorter distance to winter destinations and an earlier arrival at the European breeding grounds in spring, is possible because of milder winter weather and a greater availability of food in Britain. There is a genetic contribution to this new behaviour: “The different arrival times on the breeding grounds also lead to assortative mating by wintering area (males wintering in Britain tend to pair with females wintering in Britain) and hence restricted gene flow, which has likely contributed to the rapid evolution of the new migration behaviour” (Bearhop et al., 2005).
Research into seabirds has also used genetics to explore their behaviour. In the summer of 1990, a team of scientists caught eighty-five Brünnich’s guillemots. These guillemots nested on a set of small shelves a couple of hundred yards apart on a cliff over the Barents Sea. The team took blood samples from the birds and analysed the mitochondrial DNA. Adam Nicolson, exploring the migration of seabirds, comments that “the results confirmed the scientists’ hypothesis: each shelf was the home for a different sub-family of guillemots, sharing high proportions of maternal genes between them.” 
Figure 5: Brünnich’s guillemots
The mechanism of flight has also been investigated. Studies have found that birds have smaller genomes and fewer base pairs compared to other amniotes (animals that lay their eggs on the land). This may be due to a large amount of deletion events, in which DNA has been erased over time. All of this may allow birds to control their genes more quickly to meet the necessities of flight.
This study from 2016 led by Darren Irwin of the University of British Columbia is discussed in detail, as it demonstrates how both technology and an understanding of genetics were needed in the methodology and the interpretation of the results. It also shows how studies like this may leave scientists with not only answers, but questions as well.
The research looked at the role of genes in the migration pattern of Swainson’s thrush. The team were interested in seeing if the migration paths were learnt by flying with other birds or if the route was encoded in the birds’ genes. They chose the Swainson’s thrush because it allowed them to look at two groups of the same species that make the same migration but take different paths. Flying south from British Columbia, one group go over the coast of California towards Mexico, while the other flies over Alabama on the way to Colombia. Both groups return to Canada in the spring, and sometimes interbreed to produce a hybrid.
A tracking device was attached to the birds before they left, and retrieved when they returned to Canada. The data showed that the hybrid birds followed a path that was roughly in the middle of the flyways used by the two parental groups, and that some of the hybrids changed their routes (see Figure 6, below). Results confirmed the researchers’ initial thoughts: the hybrids could not have been taught this route by their parents, so their behaviour must be due to inherited genetic instructions from both parents. Researchers also think that different genes determine discrete routes for spring and autumn.
Figure 6: Swainson’s Thrush flyways
The next stage of the study was to find out which genes were involved. The DNA of the hybrid offspring and the parents were compared, and a gene known as the CLOCK gene was focussed on. CLOCK genes are involved in the control of the circadian cycle. (Circadian rhythms control gene expression, allowing the production of individual proteins to reach a peak during a 24-hour period when they are most needed for a specific physiological process).
CLOCK genes interact with other genes by switching the next gene off or on. This sets off a chain of events with each outcome determining whether or not the following gene is expressed. They are also involved in physiological functions such as metabolism, body temperature and obesity, which are cited as important factors in migration.
The results of this research will provide scientists with new knowledge concerning evolution. For example, the hybrids’ flyway is over land where there is less food than there is on the journeys made by the parents. This could mean that the hybrid is more likely to starve during its migration than the other two subspecies. If the hybrid were unsuccessful, the two subspecies could start to diverge and become increasingly different, until they became two distinct species. This would provide evidence of “genes that control behaviour contributing to the origins of species”, something long speculated but seldom observed.
Storks do not transform into humans as Alexander of Myndus suggested, and that Charles Morton’s theory that swallows migrate to the Moon is also incorrect. Olaus Magnus’ sixteenth-century theory, that when swallows disappear in the winter they are hibernating in riverbeds, It is no longer believed. Thanks to many studies, including those that have been mentioned in this article, our knowledge of what birds do, where they migrate to and why, has improved exponentially.
Figure 7: a woodcut of Fishermen pulling up a net-load of hibernating swallows from Olaus Magnus’ History and Nature of the Northern Peoples.
This knowledge can be directed at protecting and conserving the birds themselves. Migratory species are particularly at risk from the changing climate. The journeys that they take have become more dangerous over time, as habitats change, and less food becomes available on the way. Arriving at their destination at the wrong time can be catastrophic for the birds. Changing migratory patterns and routes can be an early warning signal of environmental changes and sometimes of the increasing fragility of a species.
The methodologies and analyses involved in the studies above have used genetics to broaden our understanding of migration: they could not have been undertaken without new technology, and the conclusions reached would have been impossible without the deep understanding that we now have of genetics. There is much that we do not yet fully comprehend, however. For example, most scientists agree that only birds with experience of migration can truly navigate, whereas birds on their first migration depend on a system of distance and direction (vector navigation or a clock-and-compass method) inherited from their parents, to reach their destination.
Scientists still do not understand in detail which genes code for migration, how many are involved and what affect factors such as climate change and habitat loss might have on the migration of birds. Some scientists suggest that all birds have genes needed for migration, but they are only switched on in migratory birds. We have more knowledge now about the rate of travel of migration, and this helps scientists to begin to understand the energy cost to the birds. Scientific exploration of how and why birds migrate could ultimately help scientists to understand how the evolution of behaviour is driven by selection pressure.
“The methods used by birds for navigation are becoming clearer, but the physiology of the senses and sensory processing needed to take advantage of natural cues is not well understood. Birds are a good study model, but migration and navigation are part of the life of many animals, and research on bird navigation will have a much wider impact and command a wider interest in biological science.” 
1. Ben Hoare, Animal Migration. London: The Natural History Museum, 2009
2. In Our Time, “Bird Migration” Produced by Simon Tillotson, Presented by Melvyn Bragg with Barbara Helm, Tim Guilford and Richard Holland, aired July 6, 2017, on BBC Radio 4. https://www.bbc.co.uk/programmes/b08wmk5j
3. Tim Guilford, Susanne Åkesson, Anna Gagliardo, Richard A. Holland, Henrik Mouritsen, Rachel Muheim, Roswitha Wiltschko, Wolfgang Wiltschko, Verner P. Bingman, “Migratory navigation in birds: new opportunities in an era of fast-developing tracking technology.” Journal of Experimental Biology 214, (2011): 3705-3712. Accessed July 9,2020 https://jeb.biologists.org/content/214/22/3705
4. In Our Time, “Genetics” Presented by Melvyn Bragg with Steve Jones, Richard Dawkins, Charles Simonyi and Linda Partridge, aired December 13, 2001, on BBC Radio https://www.bbc.co.uk/programmes/p00547md
5. Gregor Mendel, Wikipedia. Accessed July 10, 2020 https://en.wikipedia.org/wiki/Gregor_Mendel
6. Nicholas B. Davies, John R. Krebs and Stuart A. West, An Introduction to Behavioural Ecology (4th Edition). Oxford: Wiley-Blackwell, 2012
7. Carly Cassella, “These Teeny-Tiny Birds Fly an Incredible 13,000 Kilometres One Way to Migrate.” Science Alert, November 24, 2018. Accessed July 12, 2020 https://www.sciencealert.com/these-tiny-birds-fly-an-incredible-13-000-kilometres-one-way-to-migrate
8. Giuseppe Bianco, Mikkel Willemoes, Diana Solovyeva, Staffan Bensch & Susanne Åkesson, “Ten grams and 13,000 km on the wing – route choice in willow warblers Phylloscopus trochilus yakutensis migrating from Far East Russia to East Africa.” Journal of Experimental Biology 6, no. 20 (October 2018): Accessed July 9, 2020 https://doi.org/10.1186/s40462-018-0138-0
9. Jennifer Ackerman, The Genius of Birds. London: Corsair, 2017
10. Janelle Weaver, “Urge to migrate found in bird genes.” NewScientist, February 16, 2011. Accessed July 11, 2020 https://www.newscientist.com/article/dn20132-urge-to-migrate-found-in-bird-genes/
11. Adam Nicolson, The Seabird’s Cry. London: William Collins, 2017
12. Abigail Tucker, “Migratory Birds May Come Programmed With a Genetic Google Maps.” Smithsonian Magazine, October 2016. Accessed July 13, 2020 https://www.smithsonianmag.com/science-nature/migratory-birds-programmed-genetic-google-maps-180960442/
13. Gene Cards. Accessed July 13, 2020 https://www.genecards.org/cgi-bin/carddisp.pl?gene=CLOCK
14. Urs Albrecht and Jürgen A. Ripperger, “Clock Genes.” Encyclopedia of Neuroscience, 2009. Accessed July 13, 2020 https://link.springer.com/referenceworkentry/10.1007/978-3-540-29678-2_1080
15. Lucy Cooke, The Unexpected Truth About Animals. London: Transworld Publishers, 2016
16. Mandy Dyson and David Robinson, “Migration.” OpenLearn, Open University. Accessed July 14, 2020 https://www.open.edu/openlearn/science-maths-technology/migration/content-section-6
Fig 1. Unknown Artist, Tomb painting, V Dynasty, Mastaba of Ti. Accessed July 16, 2020. https://www.egypttoday.com/Article/15/28791/Out-of-the-Blue
Fig 2. Photo of Warbler with Tracking Device, date and author unknown. Accessed July 16, 2020. https://www.birdlife.org/americas/news/tiny-transmitters-tracking-birds-north-south-america
Fig 3. Unknown Artist, Gregor Mendel, BBC Radio 4, In Our Time, Genetics, Illustration, date and author unknown. Accessed July 16, 2020. https://www.bbc.co.uk/programmes/p00547md
Fig 4. Per Harald Olsen, Reed warbler cuckoo, April 3, 2007. Accessed July 16, 2020. https://commons.wikimedia.org/wiki/File:Reed_warbler_cuckoo.jpg
Fig 5. Photo of Brünnich Guillemots, date and author unknown. Accessed July 16, 2020. https://oceanwide-expeditions.com/to-do/wildlife/brnnichs-guillemot
Fig 6. Charles Floyd, Hybrid routes, date unknown. Accessed July 16, 2020. https://www.smithsonianmag.com/science-nature/migratory-birds-programmed-genetic-google-maps-180960442/
Fig 7. Olaus Magnus, woodcut from History and Nature of the Northern Peoples, 1555. Accessed July 16, 2020. https://blogs.northampton.ac.uk/mikeredwood/2013/10/28/swallows-hibernate-in-lakes-and-mud/