It's amazing how many different kinds of beings there are on Earth, each with it's own life and concerns, each with some sense of what they are and where they need to be. This time of year when I see birds, of many different kinds, heading off for their wintering grounds either individually or in formations I often wonder where they go; and even more I wonder how they know where to go and when to go. How did migratory paths evolve in the first place? Has it been a gradual process? And why did some birds evolve to migrate whereas others stick around? During the winter I sometimes see very small birds I think would die of exposure to the cold, but who stick around in the snow and survive nonetheless.
Migratory birds aren't just running away from the cold and snow, though it might be part of the reason (during winter, resources tend to be more scarce...spring and summer bring a certain abundance that sustains far more lives), as many migratory birds leave during good weather and oftentimes pass through very nice places with great weather on their way to some far-off, unseen destination.
Year-after-year many species of bird fly between their breeding grounds and wintering grounds along the same sky-ways their ancestors took. Some of the individuals, or even all of them, may never have migrated before and yet they will still find their way. Extraordinary examples of migrations (and way-finding) are the Bar-Tailed Godwit who flies 8 days non-stop on a 7,000 mile journey from New Zealand to Alaska.
|Bar-Tailed Godwit in Alaska|
|GPS track of Arctic Terns making journey to and fro the Antarctic and Arctic.|
If I was free to go where I wanted, I'd use a map or ask for directions or just follow the signs on the highways and byways. If I had to get to Africa from Utah, but without maps, signs or asking for directions I don't know how I would do it. Perhaps I would use the knowledge I had of the stars on the sun to set a course East and as I ran into different cultures, kinds of people and languages I might roughly know where I was and where I was going, but this is relying on my previous knowledge to find my way. The intriguing thing with some bird migrations is that some individuals might never have made the migration before and might have no knowledge of what the larger world is like to use as reference for orientation.
But despite all obstacles, most migratory birds find their way. So how do they do it? From doing a little digging into the scientific literature I distilled out a few tools and methods the birds are using on their epic journeys to find their way. Many interesting experiments had to be done to discover these.
The main known tools and methods by which birds find their way and the ranges over which they're probably most useful, in no particular order of importance:
-Sight (short-to-long range)
-Smell (short-to-medium range)
-Experience/Memory (short-to-long range)
-Magnetic Fields (long range)
-Astronomy (long range)
-Culture, Teaching and Learning (short-to-long range)
-Skylight polarization (long range)
-Internal Map and Compass (short-to-long range)
-Landscape/Atmospheric Patterns and Trends (short-to-long range)
-Circadian Rhythm/Internal Clock (?)
Visual way-finding seems straightforward enough, recognize a site and fly to it, but there are many levels of complexity therein we shouldn't overlook. When birds see their destination they can go to it, but depending on how it is approached it might take a minute for them to orient. Ever been driving in a place you normally know, but because you drove into it from a different angle you didn't even recognize where you were?
Seeing your destination seems a sure method of finding your way, but most migratory destinations are out of sight. Over long flights birds use their vision to follow landmarks like mountain ranges and shorelines to navigate.
|Major migratory flyways over North America. Notice the use of landmarks like the East and West coasts, the Rocky Mountains and the Mississippi and Missouri Rivers.|
Other things some birds are known to visually navigate by are patterns of wave direction and snow drift direction which is related to prevailing winds.
But how do birds navigate when there are no real landmarks to visually track, such as far from shore over a calm ocean? or what about when the days and nights are cloudy and the sun and stars can't be seen? When pigeons wearing frosted contact lenses, so that they can't see with any clarity, are released far from their roosts they still return within about .25 - 1.5 miles of their home, but can't get any closer, presumably because they can't see it. This means that birds can navigate without actually seeing where they are going. One tool the birds are believed to be using to accomplish this feat is olfaction, or the sense of smell.
A strange fact that supports olfaction as a tool for navigation in birds comes from several experiments and observations. Homing pigeons can be born in captivity and never allowed to fly around their roost to get a sense of the area and yet when taken tens or hundreds of miles and released from a foreign location still find their way back home. However, if the pigeons are not exposed to the open air while in their roost, they will lose their way. Furthermore, if they are in a controlled environment where the air stream is manipulated so that they aren't exposed to natural winds, they will also not be able to home. What's so special about open air and the wind? The wind carries information. If we had a better sense of smell we'd realize that a wind blowing from the North smells different than a wind blowing from the West. The only experience I can think of for around here is that when the wind blows from the North, we can smell the Great Salt Lake in Utah County. Each place has its own scent signature and surprisingly most birds have an incredibly sensitive sense of smell that seemingly can distinguish between the smells of different places for hundreds of miles around. When pigeons are in their home roost they take note of the different odors that accompany winds from different directions, forming an olfactory map of their local area. When they are taken to a location unknown to them, they seem to be able to recognize the scent of the place and to know what direction it was relative to their roost and they can home accordingly. This method works for nearly a thousand mile radius as this is the maximum limit homing pigeons have been able to find home. Thus, if we had better noses we might see a landscape of odors as clearly as we see the landscape with our eyes.
A cruel experiment that also supports that pigeons, starlings and other birds are using their sense of smell to home has to do with cutting the olfactory nerve so that their brains no longer receive information from their noses. Without the ability to smell, birds have an extremely difficult time of homing from any distance greater than that from which they can see their home. Other, less cruel experiments that involve anesthetizing the noses of birds provided similar results. The question remains about exactly which aromatic compounds the birds are using to home. Some seabirds have been known to use the smell of dead fish in dense fog at night to find fishing boats over 50 miles away. Whatever molecules bird use to navigate what is clear is that experience still plays a role because some pigeons who are experienced at homing and who have no sense of smell can still find their way back home from distances greater than within-sight. What's more, the sense of smell for practical purposes is mostly limited to a range of about 200-300 miles (which would be like living in SLC your whole life, getting dropped off in St. George and sniffing your way back to SLC!). For longer ranges other senses seem to be needed for birds to navigate.
Such a candidate seems to be found in the magnetic sense. As far as is known, humans have no way of detecting the Earth's magnetic field with their bodies, but birds can. In many bird beaks are cells with deposits of magnetite, a mineral sensitive to the Earth's magnetic poles, used in compasses, that can detect magnetic north and south.
The magnetic field of Earth is created by convection currents in the core where there is an abundance of heavy metals such as iron. The magnetic field of Earth isn't static, it changes quite rapidly over the long-term and short term, but it seems to be a reliable enough source of information that birds and another animals have evolved to be able to detect it in order to orient themselves. Experiments such as attaching magnets to the heads of honeybees and sea turtles have found that interfering with an organism's ability to detect the Earth's magnetic field, if they can detect it, can cause them to become disoriented. Experiments with birds have shown similar results. In one such experiment, birds were conditioned in an artificial environment where there was a very strong magnetic field that was oriented differently than the Earth's magnetic field. When released to home, the birds headed off in a direction offset by the angle from truth North that they had been conditioned to in their artificially created magnetic field. Also, homing pigeon's have a harder time homing during events that affect the Earth's magnetic field such as solar flares and solar winds. What's even stranger are experiences where the eyes of birds were covered to see if it affected their sense of Earth's magnetic field. Oddly enough, it did have an affect. With their right eye covered birds can no longer detect the magnetic field, but with their right eye open and their left eye closed they can sense the magnetic field again and what's more when the birds have their left eye closed and their right eye has a frosted contact lens over it they can't sense the magnetic field either. This implies that it isn't light, but the clarity with which that they can see that helps birds detect the magnetic field of Earth. Therefore, there might be something about the horizon which betrays the magnetic field. No one yet knows. These examples provides evidence that birds along with their many other senses and strategies are using the Earth's magnetic field to navigate too.
Besides sight, smell, astronomy, magnetic fields, and landscape/wavescape patterns birds probably use their experience and memory, skylight polarization, culture, teaching, learning, an internal map and compass, perhaps their genetics, time and some other as of yet unknown tools and methods to navigate on their migrations and daily trips.
Do birds ask for directions as they travel along? It might seem unlikely, but so did the idea that birds have culture until it was proven that songbirds of the same species have different dialects of song in different areas much like people in the USA might speak with a twang down South or with more slang in the Bronx. It is also known that many species of birds teach their generations their flight routes, but other species seem to have no such passage of information between generations. How do these birds find their way? Can birds tell each other about their travel experiences? Can they communicate across species about what birds, other beings and places are like in other parts of the world? It is known that a crow that has a run in with a bad-tempered human can communicate to other crows for tens of miles around about this trouble person and the crows will respond by cawing angrily at the human where ever they go. It is a mystery how crows can tell each other about the appearance of a human without actually having to see them and how they can recognize them, but somehow they can. Perhaps they have a different kind of expression than language that they can convey information through. We shouldn't be surprised considering that birds have a magnetic sense and an incredible sense of smell that humans can't even fathom. Additionally, many birds can see into the ultraviolet spectrum of light. Something else I like to consider is the circadian rhythms. Is time a compass? Can time be used to navigate through space? Do birds keep track of how long they've been flying in order to give them an idea of how far they've flown?
It is truly remarkable that these feathered descendents of the dinosaurs find their way all over this Earth on some of the most epic journeys life is known to undertake. Somehow, between their cells and molecules that make a whole organism there is a mechanism by which some, like the Arctic Tern, can literally find their way anywhere on Earth.
It seems likely to me that migratory birds use all of their senses to find their way and that there are probably senses and tools better for different ranges. For instance, the magnetic sense is probably most useful for long range navigation while smell is best suited for medium ranges and sight probably most effective at short range navigation and orientation.
What role does genetics play in bird migration and orientation? Is there an internal map of the world contained within their genomes somehow? Are there other means by which birds obtain information in order to find their way in this world? Probably and they probably have to use every means possible in coordination with thought, memory and comparisons with previous experiences to orient. I say that there must be quite a bit of complex thought going on in their bird brains or wherever thought goes on because having tools says nothing about how you use them. Just because you have a saw, hammer, wood and nails doesn't mean the house is going to build itself i.e. just because birds have so many senses and methods for finding their way doesn't mean they will find their way. Birds do get lost and can't find their way back home, thus it seems that having tools and instincts still isn't enough to survive; instinct and tools still require conscious modification in order to serve a purpose and to be effective. So when you see the birds taking flight on their migrations take a moment to remember just how amazing those birds are and what they are doing really is.
1) Michael Walker, Todd Dennis, Joseph Kirschvink, 2002. The magnetic sense and its use in long-distance navigation by animals. Current Opinion in Neurobiology, vol. 12, pg 735-744.
2) Maria Dias, Jose Granadeiro, Paulo Catry, 2013. Individual variability in the migratory path and stopovers of a long-distance pelagic migrant. Animal Behavior, vol. 86, pg 359-364
4) Hans Wallraff, 2003. Avian olfactory navigation: its empirical foundation and conceptual state. Animal Behavior, vol. 67, pg 189-204.