After a caterpillar undergoes metamorphosis, a mature Monarch butterfly lives for approximately six weeks under normal conditions. But as we will see, there is one exception to this rule.

These butterflies, with a wing span of 8-10 centimeters, live in Canada and the Northern USA and they migrate to South America during their migratory season. They increase their body weight by consuming flower nectar before starting the journey, to prepare for the 5000 km long route ahead of them. During this journey, they change direction many times because of rain storms and extreme weather, but they are never lost.

Towards the end of August, Monarch butterflies begin their journey from North America to the Oyamel forests in Mexico. They fly around 80 km a day, reaching speeds up to 20 km/hour. Though most generations only live for six weeks, these butterflies live for months. After an arduous journey of 8 to 10 weeks, they arrive at the 3000 meter high mountains in November and December. The Mexican mountains make an ideal place for these animals. Butterflies hibernate in this region for four months, from December to March, and survive on their fat reserves.

While this north to south journey is completed by one generation, the return journey takes four generations to complete. With the onset of spring, Monarch butterflies start feasting on honey nectar to get the energy to head back north. Around mid-March, they start to migrate northward, headed back to their birthplace. Meanwhile, having reached maturity, they reproduce. Each member of this generation lays around 700 eggs in suitable places between March and April, and then dies near mid May.

Female butterflies lay their eggs near the leaves of poisonous plants. Hatching larvae feed on these leaves. These neurotoxic plant leaves serve as a protection against predators. This poison is found to remain in the tissues of the insect even after it becomes a mature butterfly and is not harmful to it. The protection is necessary, for monarch butterflies have charming colors that attract many other animals. Thus some birds try to prey on them, only to fail each time. Because of the neurotoxin, these birds become stunned as soon as they touch their beaks to the butterfly.

It takes about a month for an egg to mature to an adult butterfly. Thus, the first generation comes to existence on the journey home. Like a programmed machine, this generation continues their parents’ journey north. They leave their eggs, for the second generation, during April and May, and then die in mid-June.

The second generation also continues the northward journey of their parents. They lay their eggs during May and June, and then die around mid-July.

Third generation Monarchs, however, arrive in Canada around June or July. The females of this generation lay their final eggs in mid-July and die during August. As a result, the fourth generation reaches maturity in a month, thus completing the annual tour. The fourth generation begins migrating south, towards the mountains of Mexico, at the end of the same month and hibernates there. This final generation lives six months more than the other generations, long enough to lay their eggs during spring. Without this character, the final generation Monarch butterflies would have died before spring, and would thus be unable to reach adulthood to lay eggs.

The migration to Mexico of a butterfly with such a delicate muscle structure confounded scientists for a long time. It is a fascinating, extraordinary event for millions of butterflies that, unlike migratory birds, lack a complicated nervous system. Yet despite this, they are able to pinpoint, and meet, in an area of around 1000 square kilometers.

An ultimate version of the global positioning (GPS) system as invented by the scientists is blessed as a birth-right to Monarch butterflies. Biologist Steven Reppert and his team from Massachusetts University have uncovered very interesting facts while studying butterfly antennae. When butterflies were placed in a flight simulator type of device with their antennae cut, they changed their direction after flying a while. Migrating butterflies with their antennae continued to move Southwest, however butterflies without their antennae flew in different directions. Not only did butterflies lose their navigational skills without their antennae but also had problems with their daily, time-dependent navigation.

Is it sufficient to explain this phenomenon by instinct only?

Pinar Celik has a PhD in medicine. She is a freelance writer from Turkey.

References

• Zhan S, Merlin C, Boore JL, Reppert SM. “The Monarch Butterfly Genome Yields Insights into Long-Distance Migration”. Cell 147 (5): 1171–85. 2011
• Davis, A.K. “Wing color of monarch butterflies (Danaus plexippus) in eastern North America across life stages: migrants are ‘redder’ than breeding and overwintering stages”. Psyche: 1–5. 2009
• Merlin, C; Gegear, RJ; Reppert, SM. “Antennal circadian clocks coordinate sun compass orientation in migratory monarch butterflies”. Science 325 (5948): 1700–1704. 2009
• Kyriacou, CP. “Unraveling Traveling”. Science 325 (5948): 1629–1630. 2009

How beautiful it is to stop somewhere every day!

How nice it is to flow without freezing and getting muddy!

What word that belongs to yesterday,

Is gone, my loved one, with yesterday,

Now is the time to say new things.

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.

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.