A Bird's Eye View

Bird Migration – Navigation

Coming into spring, with the first Willow Warbler songs being heard around Avon recently, this seems like an opportune moment to discuss how birds navigate in order to migrate. Whilst the feat of a tiny Willow Warbler travelling 12,000 km (in the case of Phylloscopus trochilus yakutensis) is humbling and physiologically incredible in its own right, migration becomes even more fascinating when you consider how the birds navigate, without the luxury of GPS, Google Maps and all the other tools humans need to find their way around.

It is thought that some birds use the Sun to navigate whilst there is also evidence that points to Homing Pigeons using olfactory cues to navigate, building up a local “map” based on odours at each location. However, we can’t explain navigation without considering the cognitive abilities of birds and their ability to build mental maps.

Perhaps the most remarkable way this is done is using the Earth’s magnetic field. Juvenile birds of some species on their first migration may have to make the journey alone; clearly there is an innate process at play that means the birds “know” where they are going without ever having made the journey.

Wheatears use magnetoreception to find their way to the UK and beyond

A chemical process called a radical pair mechanism allows them to do this. Birds have photosensitive pigments (a type of cryptochrome) in their eye that are chemically changed by short wavelengths of light, including some blue light; the way and extent to which this occurs is affected by the Earth’s magnetic field. Although this only works in daylight (light is needed for the pigments to be affected), this process is purely biochemical – it isn’t influenced by birds using the Sun to navigate. As a bird becomes familiar with its migratory journey, it will then use the magnetic field in a different way: to create a mental map. Part of the nervous system in birds (the trigeminal nerve) contains magnetites, a ferrimagnetic mineral that can essentially tell the bird the strength of the magnetic field, which changes as the it travels north to south or vice-versa. These magnetic cues, used alongside landmarks and the radical pair mechanism, allows birds to migrate with increasing precision as they age as more accurate and detailed mental maps are built up.

Bewick's Swans tend to migrate in family groups

It’s impossible for us as humans to understand exactly how a bird interprets this information; we don’t have a sense that comes close to letting us experience how the birds interact with the magnetic field. Crucially, magnetic sensations, unlike sound or light, are able to pass through body tissues so birds can detect the magnetic field via chemical reactions inside tissues and cells. That said, our behaviour can be influenced in a similar way: blue light affects the structure of melanopsin, which then affects the hormone melatonin (which induces sleep), which leads to blue light disrupting sleep in some people. However, there is the quite remarkable theory that birds can “see” the magnetic field, as there is a neural connection between the eye and the section of the brain that receives and interprets the magnetic field information.

Perhaps most excitingly, there is scope for more research into this field. Not only have there been improvements in tracking birds using satellite tags (and the new MOTUS technology) but we can look forward to studies that are more sophisticated than putting a Redwing in a magnetic field chamber so we can finally understand a mystery of nature than has puzzled humans for millennia: migration.

Something to Spot: Zugunruhe
With spring and summer on the way, look out for restless behaviour (perhaps in or flying over your gardens) in the last few Redwing and Fieldfare as they prepare to migrate, with birds being more active than normal at dusk and having changed feeding patterns.

References: [1] Wiltschko, W., Munro, U., Ford, H. and Wiltschko, R., 2006. Bird navigation: what type of information does the magnetite-based receptor provide?. Proceedings of the Royal Society B: Biological Sciences, 273(1603), pp.2815-2820. [2] Birkhead, T., 2013. Bird Sense. London: Bloomsbury. [3] Thorup, K., Alerstam, T., Hake, M. and Kjellén, N., 2003. Bird orientation: compensation for wind drift in migrating raptors is age dependent. Proceedings of the Royal Society of London. Series B: Biological Sciences, 270. 4. Hoyo, J., Elliott, A., Sargatal, J. and Baptista, L., 1997. [4] Handbook Of The Birds Of The World. Barcelona: Lynx Edicions, p.649.

George Rabin