Think of yourself as pilot of a light aircraft confronted with the task of completing, in one or two months, a journey involving 50 to 200 hours of flight. Imagine also that winds on average blow about five times faster than normal, so that wind speed regularly amounts to a large fraction of the speed of your aircraft, and sometimes even exceeds it. The temperature is low. Fuel is precious and in variable supply at various prices along your flight route. Your challenge is to complete the journey with minimum costs, without exposing yourself to unnecessary hazards, and without getting delayed.

You would indeed have a lot to calculate before taking off: What is the most economical flight speed of your aircraft, and how does it vary with winds, flight altitude, and the extra weight of fuel reserves? Perhaps the gliding performance of your aircraft is good enough to permit you to travel some distances by soaring in up- draughts, with the engine off. It may even be economical to make detours in order to stay over regions with strong up-draughts. Would it be favourable to bring along large fuel reserves and fly non-stop for long distances without refuelling, or to travel by numerous short flights, saving transport costs for extra fuel? To solve that problem, you have to know about possible refuelling stations along your route, about petrol prices, landing fees, and the time delay you will face by landing at a particular site.

Which is the optimal route to your destination? Perhaps it pays to follow the Great Circle, or should you make a detour where you can benefit from favourable winds? Of course, you will fly only on days when the weather is as favourable as possible, but which is the best weather? And how can you know which weather condition is most favourable for you? What flight altitude will you prefer under different weather conditions? You must also consider whether and how you will compensate for wind drift, when to fly low along coast-lines and other leading-lines to gain protection from the wind and avoid drift, and how to exploit the winds at high altitudes. And you must also take into account many unpredictable situations that you will face. There are lots of questions and you would spend a long time looking up facts and making calculations before you could arrive at a reasonable strategy for your flight.

Your situation is analogous to that of migrating birds. The migratory strategy of birds, a harmonious mixture of rigid and flexible behaviour to achieve a safe and economical journey which can be affected by a bewildering number of factors, is an astonishing feat - where do they get the skills and capabilities to accomplish it?

There are many animals, not only birds, which migrate on a regular basis. Some examples: Bull elephant seals travel 21,000 km (over 12,000 miles) each year, from California to the Gulf of Alaska, the longest migration of any mammal. Millions of monarch butterflies fly 2,000-3,000 km from Canada to the Gulf of Mexico. Grey whales swim 10,000 km from the Arctic Ocean to California and Mexico. In Bracken Cave in Texas 20 million free-tailed bats assemble every summer. All are female. They leave their mates 1500 km to the south in Mexico to come here to give birth to their young. Herring migrate annually and cover over 3,000 km. The bison, the wildebeest, the sockeye salmon, the eel are also known to migrate long distances. But here we will consider the migration of birds - one of the most impressive feats in animal behaviour- across thousands of kilometres of ecological barriers, like oceans, mountains and deserts.

Arctic terns are transglobal travellers on a scale unequalled by any other migrant. Some annually commute between the Northern and Southern pack ice, crossing the equator twice. The round trip must be at least 40,000 km ( 25,000 miles ).

Birds store fat for use as fuel during their migratory flights. They start to do so well before migration starts. Stored fat is the most economical type of fuel in terms of high oxidizing energy per unit mass. Once the animal is in the right condition to migrate, it may need some further environmental cues to initiate the actual migratory movement. A bird may wait for the right weather conditions or at least the disappearance of the wrong ones, such as fog or very strong winds.

In order to locate its goal at a different position on the earth’s surface, an animal must possess certain sensory systems and have certain decision-making programmes in its behaviour. The ability to take up a particular direction with respect to some feature or property of the environment is called orientation’. The orienting animal is rather like a man who possesses a compass and an instruction to proceed in some particular direction. Animals may orient with respect to objects on the earth’s surface, to the sun, to the stars, to the earth’s magnetic field, to the directions of current flow and so on. Further, they may use more than one of these environmental sources of information at any one time, and may use different ones at different times.

Birds may use continuous flapping flight or soaring flight. Migration of soaring birds attracts much attention among bird-watchers because of spectacular concentrations of these migrants at passages with favourable soaring conditions. The migrants make long detours to avoid having to use flapping flight over the sea, and the sites most famous for soaring bird migration in Europe, Falsterbo, Gibraltar and the Bosphorus, are situated at minimal sea crossings. Migrants’ coasting behaviour probably is part of their ‘strategy’ to exploit winds and conserve energy at the same time. Coastal migration occurs mainly under opposed and cross-winds, while migrants usually fly across the coast and depart over the open sea with following winds. Due to differences in friction, winds generally are stronger over the sea than over land. Migrants minimize the headwind force by following the coast, where they can use local topography and vegetation to gain additional protection from the wind. Furthermore, over the sea they will be exposed to wind drift, and under certain cross-winds it is beneficial to follow a coastline, some distance in the direction of their goal, rather than to take a direct route over the sea.

Some birds travel by numerous short flights, each of about three to ten hours duration. Many species depart either by day or by night, whenever weather becomes favourable. Flying with a small load of fat is advantageous, since the labour costs for carrying the extra fat can be high. However, there are also drawbacks involved in migrating by numerous short flights: the birds have to refuel often and find suitable resting sites on their route.

Some species undertake enormous non-stop flights. A regular migration route from North to South America directly over the Western Atlantic Ocean, a distance between 3,000-4,000 km, is used by many species. Just to mention some examples of long, nonstop flights over oceans other than the Atlantic: there are some geese that fly almost 4,000 km across the Pacific Ocean from the Alaska Peninsula to the South Californian coast; two species of New Zealand cuckoos fly about 3,500 km to the Solomon and Samoa islands; passerines, bee-eaters and Amur falcons travel almost 3,000 km of the Indian Ocean between India and East Africa.

Many migrants fly between Europe and Africa across the Mediterranean Sea and the Sahara desert in one single non-stop flight, lasting at least 40 hours in the autumn, when winds are generally favourable, and 60 hours in spring when they are less favourable.

Honey buzzards migrate by cross-country soaring over a distance of 7000 km from Europe to tropical Africa without refuelling. Common buzzards cover a distance of 10000 km between East Europe/West Siberia and South Africa by soaring migration. Many young Manx shear waders, in their autumn flight from Britain to Brazil, cover almost 0,000 km without refuelling. Various water species wintering in South Africa cover the total distance to their high Arctic Eurasian breeding grounds, about 13,000 km, in four or so long flights.

Arctic terns are transglobal travellers on a scale unequalled by any other migrant. Some annually commute between the Northern and Southern pack ice, crossing the equator twice. The round trip must be at least 40,000 km (25,000 miles).

Low temperatures seem not to prevent birds from migrating at high altitudes, The highest migrants over Puerto Rico experience temperatures of about --12 C, and over Switzerland migration is perfectly regular at altitudes with temperatures around -IC to -15 C. In fact, the birds’ capacity to fly under conditions of low temperature and low oxygen pressure is so great as to be hardly credible: On 9 December 1967, a radar controller in Northern Ireland reported an echo at high altitude moving south over the Hebrides. The radar height finder indicated an altitude between 8,000 and 8,500 m. The pilot of an aircraft in the vicinity was asked by radio to make a course deviation to pass near the position of the echo. In doing so, the pilot reported a flock of about 30 swans at just over 8,200 m. The observation probably refers to whopper swans, which are known to migrate, sometimes in the middle of the winter; from Iceland to Britain. At 8,200 m, the temperature was

-48 C and very strong northerly tailwinds blew at this altitude.

Although, according to some radar studies, probably 90% of birds travel below 2,000 m, two small flocks close behind each other of curlew-like birds were observed at I 0,000 m above sea level.

These observations, indeed, are extraordinary. One wonders, are such enormous flight altitudes merely exceptional or do some birds regularly migrate that high?

We tend to think of events which happen regularly, as ‘normal’ or ‘ordinary’. But are they really? Very often, we do not realize the dimensions of the events. Hundreds of millions of birds migrate from one place to another. Tens of thousands of birds start to fly together. They fly very long distances together and land and feed together. Many of them have never flown that long, nor been to those places. But still they are able to fight against all the odds and finish their journey How do they know when to start? How do they recognize the landscape? How do they decide which route to take? How do they find their way to their destination? How do they know where they can find food? How come so many birds can act together? How and when did they learn to use the sun and other stars, or the earth’s magnetic field, for navigation? How did they acquire the necessary means in the first place?

There are numerous such questions to ask and very often, it is not easy to find the right answers. When the other migrating animals are taken into account, the questions get more complicated and more challenging Hundreds of millions of animals migrate twice a year covering thousands of kilometres. We must ask questions which start with how and why and not take ‘ordinary’ events for granted.

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