Prof B L Kaul
Bird migration is an interesting subject. In the words of legendary bird watcher of Jammu & Kashmir Master Samsar Chand Koul, “In May our souls are elevated by the celestial song of Thrushes, Orioles and Ringdoves, while in winter our swamps and lakes are full of Wildfowl. Where do these birds come from”? Obviously they migrate. Bird migration is a universal phenomenon and many studies have been made about it from time to time.
Migration is defined as a regular seasonal movement of a population of organisms. Birds are probably the best known of these but it is also a way of life for an amazing variety of other animals – from wildbeasts to zebras. Usually, though not always, migration takes place between breeding and wintering areas: so usually, though not always, it involves two journeys each year: one outward, and one return.
Birds migrate in order to find food and somewhere to breed, neither of which is available in one place all year round. Take a typical insect eating migrant such as the Golden Oriole . After breeding in north western India during summer, it flies up to 8000km to southern Africa in order to avoid the cold, foodless northern winter. There it spends the winter (the southern African summer) in a warm, sunny environment with plenty of insect food , before returning in spring when conditions are once again suitable for breeding. Although undertaking such a long journey may seem a risky strategy, the dangers of staying put for the winter, in a cold climate with little or no insect food , outweigh the dangers encountered on migration. Migration gives birds the best of both worlds. Infact studies suggest that birds that migrate actually suffer lower mortality rates over winter than birds that stay put.
A few mainly insect- eating species do remain in northern and western India all year round. They survive by exploiting hidden food sources, such as tiny insects hiding beneath the bark of trees. It is lack of food – not cold weather itself – which kills birds in winter; so even in Arctic Norway, Gold crests are able to survive in temperatures of minus 30o Celsius , because they can find plenty of food deep in the heart of coniferous forests.
Most songbird migrants from Europe and North America spend the winter in sub-Saharan Africa and South or Central America respectively. But the abundant food supplies they find there do not last all year round. If they were to remain through to the breeding season, when extra energy is required to produce eggs and feed young, it might become harder to find enough – especially when there is so much competition from local, resident species Birds that return to temperate areas tend to have larger clutches (and sometimes more broods) than birds in tropical regions.
The world’s longest migratory journey is undertaken by the Arctic Tern, from its sub-Arctic breeding grounds to and from the Antarctic – an annual roundtrip of up to 35,000km (approx. 22,000 miles). During its lifetime, a single bird may travel more than 900,000km (half a million miles) – more than any other species. As a result, the Arctic Tern experiences more daylight than any other living creature. One individual, ringed as a chick near Murmansk in Russia was recaptured alive a year later near Fremantle in Western Australia – more than 22,400km (14,000 miles) away. The longest journeys of land birds include those made by the Barn Swallow, which migrates in the Old World between Norway and south Africa, and in the New World from Alaska to Argentina – both journeys of roughly 9600km (6,000 miles), But this is dwarfed by the journey of the White-throated Needle tail, a species of swift, which undertakes a twice-yearly trip between Siberia and Tasmania of roughly 12,000km (7,500 miles) each way.
The smallest intercontinental migrant is the Ruby throated Hummingbird of north America , twice a year this tiny bird puts astonishing fat reserves, doubling its weight from 3.2 to over 6 grams (0.12 to 0.25oz) so that it can migrate almost 1000km (600 miles) across the gulf of Mexico, to and from its winter quarters in central America. It is often said that migrating birds head south for the winter – but this puts the cart before the horse. In fact it is thought that many migratory birds evolved in equatorial regions, and-first headed north to avoid competition with other species there. By doing so, they could also take advantage of the tong daylight hours and plentiful food supply of the northern summer. But they still had to return south again every winter, when the weather closed in. Thus migration was the key that opened up new lands for birds. It soon became a viable way of life-for many species.
The notion that birds somehow know when to migrate in time, but birds don’t follow calendar at least not in the sense that we understand it. Instead they respond automatically to certain natural stimuli. The main impulse for departure both from winter-quarters and breeding grounds is tiny changes in day-length, which affect the bird’s brain. The brain triggers the bird’s endocrine system to produce hormones that stimulate it to prepare for the tong journey ahead – for example, by feeding hard in order to build up fat reserves. Even cage birds may show signs of restlessness during spring and autumn, when migration is in the air.
Weather conditions can play a big part in the timing of the birds’ departure south. While the birds may be physically ready to leave, they need to use the best weather conditions for migration, so may stay put if the weather is bad. Thus songbirds will usually wait for the passing of a cold front with clear skies and following winds that help them on their way.
One of the greatest of all natural mysteries is how do migrating birds find their way. There is no one, single method used by birds to find their way; instead, most pick and choose from a number of orientation tools. The most important are the earth’s magnetic field, and visual compasses such as the sun (for daytime migrants), and the moon and stars (for night-time ones). Other useful tools include the ability to perceive polarized light [especially when clouds obscure the sun), and ‘vector navigation’ – similar o the ‘point-and- compass method used by early sailors.
Birds migrate by night as well as day. In fact more species- and many more individuals – are night flyers, and for several reasons. first, the air is cooler at night, which is especially important as the bird goes farther south, cool air allows a bird to fly faster, with less dehydration and loss of energy, second, there are fewer predators around, most of which are day time migrants, finally migrating by night and stopping by day allows birds to feed and rest during day light hours. Nocturnal migrants include most song birds, wildfowl and waders.
Some larger birds, notably swans, geese and cranes, often travel in a V-formation. This is an energy efficient way for them to cover large distance , as the leading bird creates uplift and reduces wind friction for the others (just like racing cyclists riding in a pack). The lead bird is usually an experienced adult, and will take turns with others to avoid becoming exhausted.
In recent years, global climate change – and its impact on-weather patterns and temperatures – means that some birds appear to be arriving earlier than usual in spring and ‘staying later in autumn. It is too early to say yet whether or not this will permanently change the timing of bird migrations. Climate change is also causing birds problems by reducing their habitat: for example, widespread drought in the Sahel Zone of western Africa has led to the desertification of large areas, depriving migrants such as Sand Martins of the vital green areas that sustain them on their long haul journeys. The climatic phenomenon known as ‘El Nino’ is also having a major effect on both breeding and migrating birds, by dramatically changing the world’s weather patterns, especially in the tropics.
(The author is a naturalist and environmentalist)
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