Bird Migration and Navigation:
The super-pilots who also navigate by sun, moon and stars!
By Wolf Lange.
Annually, billions of birds (as many as 4 to 5 billion worldwide) migrate from north to south and back again. This involves as many as 200 species of which at least 80 are migrants from Europe and Asia to South Africa. Two main streams of birds find their way from Western Europe and Eastern Asia mainly across two "land bridges" to Africa in early spring. These bridges are in the west: across Spain, parts of Portugal over Gibraltar and down the West Coast of Africa; and in the east: across Turkey, Israel, Saudi Arabia and down the East Coast of Africa. The main reason for this overland migration taking place is that most birds - other than sea- and water birds as well as some raptors - do not like flying over large stretches of open water where they cannot rest and feed. Most migrants fly at night for very similar reasons airlines fly at night: Less turbulence, cooler at lower levels and higher fuel efficiency, but also because of less raptors who could attack them. So how then do these small super pilots find their way down here to South Africa and into some of our back gardens?
First lets look at why they migrate and who they are ..
The main reasons for bird migration are inter alia:
Land-based non-migrants tend to be mostly seed eaters and migrants insect eaters. Seabirds (not the inland seagulls that feed at our marketplaces and which we see around roadhouses, drive inns etc) fly enormous distances to get here or fly past us to the Antarctic Deep South. The arctic tern e.g. will fly twice a year as much as 12 000km from the arctic north coast to the Antarctic south coast! Here are some of the more well know migrants:
Each of these migrants, other than their normal senses of seeing, feeling, tasting and smelling have build into them two very important but largely invisible devices: a genetic clock and a physiological compass, both of which are essential for navigation and orientation!
So how do these small super-pilots navigate?
Their genetic clock works roughly on a 6 to 8 months cycle. It will trigger off the necessary initial activities that birds have to go through to prepare themselves for the long trek south or north. This genetic clock is not disturbed by any unseasonable winter weather such as frost in summer or a prolonged summer. It is best observed by watching swallows starting to gather in groups growing gradually grow bigger by the day. An unusually behavior of swallows is that these generally quiet birds, will twitter and chatter during this time almost like people going on a long holiday will encouraging each-other to get prepared. They will almost double their weight in a few weeks, as they eat to fuel up for the long flight of 8 000 to 9 000km.
The swallows will be one day here the next day gone, leaving overnight. Their navigation relies initially (especially for the young birds undertaking the first time the journey) on the physiological compass, which is attuned to the magnetic field of the earth. By this, the bird will know if it travels across or along the magnetic field. This was researched with homing pigeons who have a minute crystal bearing organ in their brains which is influenced by the earth' magnetic field.
The other navigational guides are the stars - or rather the movement of the stars across the sky - as they fly either north or south. It is still a mystery is - how they know how long to fly on a specific course before changing direction? It is generally easier for inexperienced swallows travelling the first time, to follow their experienced parents, but this rarely happens as often the inexperienced leave earlier. Tests undertaken in a Planetarium with migratory birds in a cage - open at the top and surrounded by carbon paper - shows that as the stars are moving so the birds align themselves in the cage either to the left or right of the celestial pole (depending whether they face north or south) this was indicated by the scratch marks on the carbon paper. It appears that not necessarily the star patterns themselves but the rather the movement around the celestial poles gives them clues on aligning their flight. Genetic imprinting enhances the complex navigation systems. This was proven by taking a storks egg from a nest in Holland and swapping it with a storks egg taken from a nest in Eastern Europe. Instinctively the western conceived stork now placed in the east will on migration fly west to find the route across Gibraltar down the west coast of Africa and vice versa. Amazing, but true.
It has also been proven that moths that migrate by night from Northern America down to New Mexico use the moon as a guide, because once their eyes were covered, they lost their way. Some birds flying in the day such as raptors, align themselves with the sun, in such a way (just like homing pigeons) that the sun is always at an angle on one side. The miracle is that they have the ability to reverse the route, aligning on the return flight so the sun is on the opposite side.
Once close to home for the returning birds, smell does also play a role, such as that of farmyards, open fields, forests and visual features such as mountains, streams, even buildings and large structures. But it is during the long flight by night when it is cooler and there are less raptors, when the occasional storm blows the migrants sometimes thousands of kilometers off course. So at times pelicans from Mozambique may land up inland in Gauteng, being blown there by storms, to settle in for a week or two on open water; a dam, a nature reserve or a lake, only to leave and find their way across unknown territory to the place where they should be. This is achieved through the magnetic compass and being able to instinctively navigate back on course, after feeding and building up energy and fuel, through looking at the position of the of sun and the movement of the stars.
Not a single navigational system in birds works in isolation. Magnetic compass, genetic clock, sight, smell even taste and instinct work together to create a navigational system which is as sophisticated as any of that of the latest Boeings with their autopilots etc. Much remains a mystery but whoever thought that calling someone a "birdbrain" could actually be a great compliment!