Soaring ever singest. Pondering bird migration
One of the most magical days of my life was spent with my friend Diane when we drove to western Nebraska to witness the sandhill crane migration. I quite literally nearly killed us with awe - for on the freeway all my attention was grabbed by the dark cloud on the horizon unbelievably resolving into an innumerable horde of birds. Luckily the truck driver behind me was on the ball and honked me back to consciousness. Migration is an extremely risky business.
Monarch butterflies migrate thousands of miles, each generation only travelling some portion of the distance. Along the way, the butterflies lay eggs that eventually morph into the next generation who continue the migration. Emperor penguins migrate inland by walking up to 100 miles to lay their eggs, and then taking multiple trips back and forth to the ocean to feed - all with their painstaking flipper footed plod. Some killer whale pods migrate 1000s of miles, which until recently we believed was motivated by feeding but may be to maintain healthy skin. Why do animals choose to undertake such strenuous and dangerous journeys, and do how they know when to leave and where to go? These questions are still not completely resolved.
Bird migration, in particular, is a tale that boggles the mind. About 4000 species of bird migrate every year, which is about 40% of all bird species. Many return to the same exact nesting location year after year. Some fly short distances. Some migrate as high as six miles up into the atmosphere. The animal with the longest migration is the Arctic tern, traveling up to 80,000 kilometers (50,000 miles) in their annual round trip migration from arctic to antarctic and back again. It is believed that Arctic terns experience more sunlight every year than any other animal. Over their long lifetime, as much as 30 years, arctic terns travel as far as the moon and back, multiple times. They spend most of their life aloft - where they can eat and sleep gliding along the winds. These birds mate for life and raise their young together - though dad provides more food for the nestlings. Arctic terns are known to be vicious in defense of their young and will strike the back and top of the heads of any who get too close. They’ve been known to repel humans and polar bears, leading to an extraordinary survival rate of 82%.
One of the key mysteries of bird migration is how arctic terns and other birds know when to get moving. Timing is key to get to the breeding sites early, to secure a good nesting location and to get eggs laid, but not so soon that there is no food available. One of the mechanisms for signaling that it is migration time are a special set of cells in bird brains, call photoreceptors, that register light directly. Rather than forming an image, these cells are more like a photon counter. As the days lengthen in the spring time, the photoreceptors send a “time to get ready message” to a suite of body systems. Worn feathers are shed and new ones grown. Eating and digestive patterns are altered so the birds put on more weight and build muscles. And eventually, the birds’ digestive tract secrets the hormone ghrelin which makes the birds restless. They get the urge to get moving. I wonder if we could inject teenage boys with this?
But not all birds follow these patterns. For example, there are birds that leave their winter roosts just south of the Equator, as the days there are shortening rather than lengthening, to travel into the northern hemisphere’s summer. Even within a species, different birds will migrate at different times - for instance a comfy winter home with abundant food may entice a bird to migrate later or even not at all. It’s all rather complex and we are just beginning to understand how birds know when to migrate. As to what triggers the equatorward journey - that is still pretty much of a mystery.
How migration evolved is even more unknown. It’s a very risky undertaking - with uncertainty about food, weather, and predators all threatening the birds as they venture into unfamiliar territory. The pay off must be large. Some scientists believe that migration is just an extension of local wanderings following food sources, perhaps amplified by glacial-interglacial cycles. Birds with a genetic predisposition to wander poleward in the spring may have encountered abundant food sources, fewer predators, and less competition. Subsequent generations, inheriting this predisposition for wandering, would have continued the migration, and these lines may have developed into their own species. A relatively new study has analyzed the evolutionary tree of 800 song birds and how their geographical distribution has changed since the last glacial maximum, about 18,000 years ago. The scientists found that most long distance migration evolved from species in the temperate or polar regions seeking food equatorward during the winter, rather than tropical birds migrating poleward in search of better breeding grounds. Understanding how migratory patterns evolved will help us to understand how birds will respond to human induced climate change.
But the biggest question for me about migration, is how do the migratory birds find their way? Some species migrate together as a group, so in this instance the experienced birds lead the way, as is the case for matriarchs leading pods of killer whales or herds of elephants. Arctic terns start out their journey together - gathering together until at some point the normally noisome flock falls silent in a behavior referred to as the “dread”. Then, the entire colony takes off together to start their epic journey to the other end of the Earth.
However, it is not just following the flock that tells a bird where to fly on its migration. For instance, some bird species inherit a directional impulse from their parents. If their parents have different wintering grounds, the chick will fly in the intermediate direction. It seems, also, that birds inherit a set of instructions such as “Fly south-west for three weeks, then south-south-east for 2”. In addition to these recipes for migration, birds also have the ability to successfully resume their migration if they are blown off course - and this is much more than simply following a set of instructions - it implies some kind of global map. Adult birds that are transported, in isolation chambers, 1000 km (600 miles) off course, can find their way back to their breeding or wintering grounds. Something other than recipe following is going on with bird navigation and it has to do with the many ways birds are able to determine where they are on the planet relative to their destination.
We’ve known for decades that birds use celestial cues to decide which direction to fly in three different ways. First, birds know about the apparent rotation of the night sky - stars appear to rotate about the north pole as the earth spins - and birds will fly away from the north pole as winter closes in. This was confirmed by an experiment in which birds were raised in a planetarium that was programmed such that the stars apparently rotated around a different star, Betelgeuse. When placed in the planetarium at migration time, these poor birds oriented themselves away from Betelgeuse, with no sense of proper south.
The second celestial clue birds use to determine south is the position of the sun in the sky, a so called solar compass. Above 23 degrees of latitude in the northern hemisphere, the sun is always due south at mid day, and of course it is east of due south in the morning and west in the afternoon. So to use a solar compass means that birds have an internal clock that allows them to compare the time of day with the position of the sun, and thus calculate where south is. Scientists confirmed this with experiments in which pigeons were raised in a closed room with an altered day / night pattern, and, upon release, flew in a predictably incorrect direction. Their internal clock was skewed and thus their solar compass was off. Further study has shown that birds that only see the sun in the morning never learn to use the solar compass in the afternoon.
And the third way that birds use information from the sky to find their way is the polarization of sunlight. As sunlight enters the earth’s atmosphere it is unpolarized - meaning that the electromagnetic field lines of the light are, on average, uniformly distributed in a circle perpendicular to the direction the sunlight is traveling. Kind of like the guard on a foil, if the guard were flat. But as the sunlight hits particles in the atmosphere, it is scattered and the direction of the light’s electromagnetic field lines is no longer random - the light is polarized - so more of the electromagentic field lines are pointing in a particular direction perhaps like the cross guards on a sword. The effect is greatest at 90 degrees away from the sun. So, if the sun is overhead, polarization is strongest at horizon. Birds and bees detect this polarization to know where to fly. This is very handy if you can’t see the sun because it is cloudy, the sun is below the horizon, or you are foraging deep within a plant and can only see a sliver of the sky.
But none of these celestial tools explain how a bird can find its way back to a specific place if it has been blown off course. This is because the rotating night sky and the position of the sun only tell you relative directions (which way is N,S,E, and W), not absolute directions (from here I need to travel SSE). Conceivably birds could determine their latitude from the celestial cues - if they knew the date. But I’ve not seen this conjectured in the literature. It turns out, birds have a further set of compasses. They can detect the earth’s magnetic field.
Birds use three aspects of the earth’s magnetic field in their navigation and indeed need all three to find their way back to their destination if displaced. First, birds can detect the strength of the magnetic field, which is greatest at the poles and lowest at the equator giving an indication of latitude to the birds. Second, the angle that the magnetic field makes relative to the ground (called the inclination) changes with latitude. At the magnetic north pole the magnetic field lines are perpendicular to the ground, while at the equator they are completely horizontal - similar to the lines on a pumpkin. Thus the inclination of the magnetic field lines give a second clue to latitude.
But it is the third aspect of the magnetic field that birds use to determine their longitude - i.e. whether they need to travel east or west to get back home. And this game changer is that birds can detect the difference between the direction of the magnetic pole of the planet and the rotational pole of the planet - which they know from the stars. The difference in the direction between these poles varies from zero degrees (along a line of longitude that goes over the western Pacific and the completion of that circle over the eastern Atlantic) to 4 degrees (along a longitudinal line that goes over the southern Rockies and through Afganastan). Birds detect the difference between these two poles to determine whether they need to travel east or west to return to their destination. Are you kidding me? I can barely find my way out of Ikea.
So birds seem to have quite a few ways to determine which way to fly - from inherited directions, to following the flock, to using landscape cues, to 3 celestial compasses, all the way to 3 magnetic field characteristics. By charting the flights of thousands upon thousands of birds, scientists have shown that 73% of birds’ migratory paths follow magnetic field quantities. In contrast, celestial compasses (sun, stars, and polarization) can explain at most 50% of routes. None of these navigational techniques are fail safe - for instance the earth’s pumpkin line magnetic field lines get tangled when near local magnetic anomalies or when a magnetic surge from the sun hits the earth’s atmosphere. Navitional systems will also develop at different times depending on the habits of the bird - birds born in the polar regions won’t know about star rotation or be able to use the solar compass because they grow up in constant daylight. But the young birds develop these solar compass skills when they head south. And here is where we learn the greatest lesson - we believe birds are continuously checking all their systems for navigation, learning how to use the different compasses, and calibrating them against one another. How magnificent.
Sadly bird numbers have declined by about 30% since 1970. But we can easily help. Habitat reduction, predation by cats, and bird strikes on windows are the three largest human driven causes of unnatural bird deaths. We can reduce our contributions to habitat destruction by eating less meat, drinking shade grown coffee, and practicing wildlife friendly gardening. To reduce cat kills we can keep our cats indoors, obviously, and promote the sterilization of feral cats. And to reduce window strikes, we can bird proof our windows with decals, turning our lights off, or drawing the curtain. Finally, there are some really fantistic citizen science projects monitoring birds that we can contribute to.
I’ve always marvelled at birds. But now I am in awe. Shelley says it better than I ever could:
Hail to thee, blithe Spirit!
Bird thou never wert,
That from Heaven, or near it,
Pourest thy full heart
In profuse strains of unpremeditated art.
Higher still and higher
From the earth thou springest
Like a cloud of fire;
The blue deep thou wingest,
And singing still dost soar, and soaring ever singest …
I'm glad you enjoyed the cranes. Come back to Nebraska later in the spring and see the Greater Prairie Chickens and the Sharp-tailed Grouse do their dawn mating dance on their leks!