The Mechanics of Migration

Warning: This column is a bit technical but so very fascinating!

Migration - the birds go north in the spring and south in the autumn, in our neck of the woods. Pretty simple - right? Well, I’ve studied birds for over 60 years, and it’s way more complex than ever I imagined. It involves some combination of: innate knowledge, solar bodies, a built in GPS system, magnetic fields, polarization of light and olfactory clues which allow birds to migrate successfully to and from the breeding grounds – whoah! Consequently, the more I thought about it, the more questions arose in my mind. Do birds get lost in migration? How do young of the year birds, which are not accompanied by experienced adults, find out where, exactly, they should migrate to and how to get there? How do they know when they’re there, if they’ve never been there before? Do they have some sort of travel memory? If they use the sun and the stars to help them migrate, what do they do on cloudy days and when flying during the night?

Since, right now is exactly when our birds start their autumn migration, it’s the perfect time to explore this incredible annual migration. The first migration-assisting ‘tool’ in their arsenal is, perhaps, the most difficult one for us to relate to, since we can’t ‘see’ it. Polarization of light is a phenomenon which is critical to birds and many insects, in their migratory journeys. Simply stated, it involves the filtering of light waves, so they vibrate in only one direction, instead of randomly moving through the atmosphere in a haphazard fashion. Thus, an ‘invisible’ grid or road map is created in the sky, whereby, at dawn and dusk, their lines form bands perpendicular to the sun’s position. While we can’t see this ‘grid,’ birds and many insects can, since birds have special cells in their eyes which catch the angle of polarized light, aiding the birds in seeing these lines.

These polarized bands of light, always accurately aligned with the direction of the sun, are then utilized by the birds to help orient themselves in calibrating their internal compasses. The birds don’t move towards or away from these polarized bands, but they use them to pinpoint the sun’s location, then calculate the exact angle they should follow, in relation to the sun, to get to their desired migration destination. Wow!

Polarized light patterns remain detectable even under cloud cover, though, birds rely more on magnetic cues, when the sky is heavily overcast. A variety of redundant ‘tools,’ such as star patterns, experience and geomagnetic fields are also critical components of their strategy. Once they have established the location of the sun and determined the angle they must follow, relative to it, they then match this to the geomagnetic forces inherent in the Earth’s structure, to ensure their compass is calibrated correctly, at the start of the cycle. Birds likely sense magnetic fields through specialized proteins (cryptochromes) in their eyes, which respond to Earth’s magnetic field. See, I warned you this was technical, but bear with me.

Throughout the migration, constant adjustments must be made, to stay on track, as the season progresses and the sun changes position, relative to the poles, or the birds change course for various reasons, such as storms or finding food.

We’ve talked about the sun already, but what about the stars? Are they influential in navigation? Absolutely! Birds possess a migratory program which enables orientation soon after hatching, guided by inherited knowledge. While still in the nest, young birds utilize genetically encoded orientation instructions, to develop their local map, to aid in their first journey southbound and to develop their star-linked compass, by being exposed to the rotation of the night sky around Polaris (i.e. the North Star). Once out of the nest, they must rapidly align this knowledge with the earth’s magnetic lines, as mentioned above. Once they have developed this map, it lasts the rest of their lives, although, it is periodically tweaked, as necessity and experience dictate. Next time, I’ll talk about what can go wrong.

Geoff Carpentier is a published author, expedition guide and environmental consultant. Visit Geoff on-line on LinkedIn and Facebook.