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Visitors from afar
Dr. Sriyanie Miththapala unravels the mystery of migratory birds
''There are no robins in Sri Lanka,'' I snort derisively at my friend Janaki, who tells me on the phone that a robin-like bird has crashed into a window in her house in the heart of Colombo. She proceeds to describe the bird as a little bigger than a Mynah, but smaller than a Babbler, with a beautiful orange head and slate grey wings.

I sit up, startled. Surely, she can’t be describing an Orange-Headed Ground Thrush, an uncommon winter migrant? I direct her to look at the relevant page in my bird bible, Henry’s ‘Birds of Sri Lanka’. A pause while she looks up the plate. “Yes, I’m certain - that is the bird,” she says. I rush to her house. There it is, in all its splendour: a rare, winter migrant perched warily on a jasmine bush.

Migrant birds, who, like ubiquitous tourists swarm to the tropics during winter months, are a common feature of birdwatchers’ lives from about October to March each year. Intrepid birders are known to travel hours to Bundala National Park just to see a Red-necked Phalarope. Some migrants, such as the Orange-Headed Ground Thrush, are uncommon, while others such as the Blue-tailed Bee-eater are abundant. Some are not only common, but also so territorial that you can see them every year in the same place: one Indian Pitta is not only a regular visitor to Hotel Sigiriya, but also frequents the courtyard opposite Room 26.

Some migratory birds such as the Paradise Flycatcher make relatively short hops from Northern India to Sri Lanka, while the gold medallist of bird migration - the Arctic Tern - which breeds in the marshes and shorelines of the Arctic region and migrates annually to Patagonia and the Antarctic, travels a mighty 35,000 km.

Other species from large Grey Whales and small Monarch butterflies migrate about 20,110 km and 3200 kilometres respectively each year. Many other species such as emperor penguins and marine turtles are also migratory.
While travelling these immense distances, animals expend a great deal of energy and face countless dangers. An average bird crossing from Britain to Europe loses one fifth of its weight. They also face unfavourable weather conditions, dodge lofty mountains and evade unfamiliar predators.

Usually, species make these arduous and dangerous journeys from temperate to tropical climes when the weather turns cold, but come warmth, and they head off again, back to where they started. Back and forth they go, year in year out. Some of our migrant visitors turn up as early as August and some as late as December but in April and May, off they go again.

Given that they expend a great deal of energy at great cost to themselves, and that these journeys are gruelling and perilous, why do migratory species take on these annual voyages?

In the Polar and temperate regions of the world, seasonal weather changes transform a salubrious environment into an unliveable one and organisms in these regions are forced to make adjustments to their lifestyles. In these climes, humans don extra clothing and heat up their houses. Plants shed their leaves or lose their aboveground parts. Some animals hibernate; others migrate.

In biology, therefore, migration, is defined as a seasonal movement of a species to a region which continues to provide it with food and back to the same area where the species breeds. (When we refer to migration in relation to humans, we usually mean a one-time emigration from one country to another.) In the northern hemisphere, birds can trade bitterly cold winters of the north for warmth by flying thousands of miles south every year. In contrast, when they fly back during the summer, they exchange the heat and humidity of the south for the gentle warmth and long days of the northern summer.

Strangely, migration is less common in the southern hemisphere.
Species also migrate altitudinally (Dall Sheep, which live at high elevations in the mountains of Alaska and British Columbia, migrate down during winter, and up again in summer); from oceans to fresh water (salmon), vice versa (eels) or from dry to wetter habitats (wildebeest). Because Sri Lanka is in the northern hemisphere, and we have so many easily visible avian visitors, this article will confine itself to bird migration, focused on the northern hemisphere.

In addition to helping avoid harsh winters, migration also ensures that migratory species have a steady supply of food all year round. Birds can make the most of longer winter days in the south and lengthy summer days in the north. This shuttling back and forth ensures that year-round, they have long hours for eating.

When they head back north in the spring, their fancy, like Tennyson’s young man’s, ‘lightly turns to thoughts of love’. They court, they breed, they rear their young and, in the fall, they are ready to head back south. If it is obvious why birds need to migrate, the question of how they migrate has baffled biologists for many decades. Firstly, how do they know when to migrate? One biologist answered facetiously, “They’ve got rhythm, they’ve got rhythm; who could ask for anything more?''

Flippant though it was, his answer is correct: the internal body clocks of birds tell them it’s time to leave. External cues such as the length of day and temperature have provided stimuli over millions of years, and this has resulted in the fine-tuning of their internal body clocks to respond with precision to shorter days and cooler temperatures in the north that signal the onset of autumn. Birds then tank up on their flight fuel and gorge themselves, gaining as much as 50% of their total body weight in fat. Then, they fly away south.

Secondly, how do they know where to go? To migrate successfully, a bird must not only know where it is, but also know where it is going, and be able to maintain its course i.e., it must know how to navigate. Once it has reached its destination, it must know to stop. After the winter, it must do this in reverse. Every year, several million migrating birds fly thousands of kilometres back and forth, and do so with pinpoint precision to the same feeding ground and back to the same nesting site. So they not only have excellent navigation but also have highly developed homing abilities. Experiments have shown that birds released some 5100 km from the nesting sites to where they returned the previous year are able, unerringly, to find their way home.

Many decades of experimentation have revealed that birds rely on several cues to navigate. Visual landmarks, such as a coastline or mountain, provide easy reference points for birds that have flown a particular route in the previous year. Where the sun is at a given time also provides an easy reference compass to a migrating bird.

However, this begs the answers to two questions: a) how do birds migrate at night and b) on overcast days? An elegant experiment using a simulated map of celestial constellations revealed that caged birds oriented themselves according to the stars. When this map was inverted, they reversed their direction of movement. So at night, birds were using the stars, like the proverbial wise men did, to guide them on their way.

But again, one wonders how birds migrate on cloudy days, when visual cues are fuzzy? Blindfolded pigeons were able to wing their way home with startling accuracy. Pigeons deprived of all obvious directional information, transported and released thousands of kilometres away, unerringly found their way home. However, pigeons with magnets - with their magnetic fields reversed - strapped to their backs, were disoriented completely. This led biologists to infer that birds were using the earth’s magnetic field – geomagnetism – for orientation.

Startling experiments have shown that birds sense geomagnetism by actually seeing patterns of colour or light differences that reflect the earth’s magnetic lines. Decades of research have clarified that birds have a multitude of cues at their disposal and are able to use one backup after another when these cues don’t work.

Millions of years of evolution have enabled migrating species to move precisely each year from their breeding grounds in the north to their feeding grounds in the south. These set routes are often called migratory pathways or flyways.
In today’s world, however, these internal mechanisms that have been fine-tuned over millennia can be wrecked within a year by human activities. When developers fill marshes, cut town forests and alter habitats beyond recognition, the different winter and summer ‘homes’ of migrant birds are often lost.

We are all aware that the loss of tropical rainforests all over the world has been severe during the last century. Migrant birds from Northern climes reaching these tropical areas often find considerable changes in their winter feeding habitats: they lose areas in which to feed, and much more insidious, find once contiguous tracts of forests now fragmented into smaller patches and interspersed with inhospitable farmlands, homesteads and other developments.

Similarly, the disappearance of wetlands - often, prime targets for developers – means that migratory wetland birds have nowhere to go during the winter.
As detrimental to migrant species is the loss of summer breeding habitats. Again, fragmentation and degradation seriously affect these travellers.
Another human activity of intensively developing beach fronts impinges critically on the success of many migrants, who need these beaches to stop over, tank up on fuel and rest, before heading out over huge expanses of open ocean to reach their southern destinations.

A startling report from the US revealed that an estimated four to five million migrant birds of 230 species are killed annually at communication towers, because their navigation cues appear to have been disrupted by these tower lights.

In South and Southeast Asia, poaching of migrant birds is a serious problem and these birds are sold in open markets for food. These combined human-induced threats are acting separately and synergistically to threaten migratory species all over the globe.

In addition, migratory species pose a legal problem. Historically, countries have had sovereign rights over the biological resources found within their territories. Migratory species, flying or swimming from one country to another are at the mercy of the laws that exist at each of their stopovers. Take a hypothetical example of a migratory species that has breeding grounds in Country A, a stopover in Country B, and feeding grounds in Country C. Even if Country A has very stringent laws protecting this species, it is of no avail unless both Countries B and C have equally stringent laws for its protection.

This means that unless there is international co-operation, migratory species are doomed for extinction, as they must be managed as single units, irrespective of jurisdiction and despite disparate legislation. Mercifully, a few international legislative tools have been developed for the protection of migratory species. The Whaling Convention, ratified by 22% of the world's countries, was signed to safeguard whale populations in oceans worldwide. The strength of this convention is that the majority of signatories are not whaling nations, which has allowed for the establishment of many protective measures.

The Convention on the Conservation of Migratory Species of Wild Animals – commonly called the Bonn Convention - ratified by nearly half of the world's countries (Sri Lanka is one them) - obliges signatories to protect endangered migratory species and to develop agreements for the protection of other migratory species.

Siberian Cranes, seriously threatened by the loss of feeding grounds and hunting along their migratory routes, are now protected by a Memorandum of Understanding among twelve countries, formulated under the aegis of the Bonn Convention.

The Convention on Wetlands of International Importance, especially as Waterfowl Habitat - more commonly referred to as the Ramsar Convention - takes a different approach and obligates signatories to protect habitats that are of international significance to migratory birds. Because many migratory birds are wading birds that need wetlands for their habitats, the Ramsar Convention is particularly important. Currently the Ramsar Convention has three quarters of the world's nations as signatories.

In Sri Lanka, the establishment of three Ramsar sites -Bundala National Park in the south, the Maduganga estuary in the southwest and Anawilundawa Sanctuary in the north west – has meant a reprieve for our winter avian visitors. The northwestern coastal stretch, in particular, is exceedingly important for our visitors. However, the picture is not all rosy at these sites, each of which has problems of its own: Bundala is being overrun by Prickly Pear and Mesquite - both invasive alien species aggressively replacing natural wetland habitat; Anawilundawa is overcrowded with livestock and hemmed in by human habitation; among other threats, the mangroves of Maduganga are being clear-felled and the discharge of sewage and racing of high powered boats are increasing.

Clearly, international co-operation alone is insufficient. Each co-operating party must uphold the conditions upon which they agreed. However, each country does not exist in isolation and must operate in a larger context involving other nations. Hmmm. Perhaps the example of Siberian Cranes is a model for us all.

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