Bangladesh is said to have about 712 species of birds although the IUCN Red List of Bangladesh confirmed about 570 species.

Out of these 712 bird species, about 320 are migratory birds as the country falls on two major bird migratory flyways – the East-Asian Australasian Flyway and the Central Asian Flyway.

In Bangladesh, when we talk of migratory birds, we immediately conjure up images of hunting ducks, geese, waders, egrets, storks, and ibises. These are the birds known to us as migratory, and often wrongly branded as guest birds. Hunters and bird poachers usually prefer the species that are halal or edible to the majority Muslim population in the country. But that's no more than 50 species and we have nearly five times more migratory species than that.

By and large, most people do not know of most charismatic and tiny migratory birds such as wagtails (Khojon), pipits (Tulika), leaf warblers (PataFutki), Warblers (Futki), Buntings (Bagheri or Bhuruipakhi), shrikes (KoshaiPakhi), Swallows (Ababils) etc -- ranging from 10 to 20 cm in length and 8 to 25g in weight that come to the country in the tens of thousands. Their total number could be over several millions.

Meanwhile, larger species of ducks, geese and waders could be present numbering in the several hundred thousand only.

These tiny birds live in bushes anywhere in a private house or office compound, public parks, roadside hedges, shrubs and trees in a busy city to a remote hamlet in the countryside or forests and are usually not noticed by people.

This is mainly because these birds are small and drab coloured so that predators can't spot them easily. But these millions of tiny birds feed almost entirely on insects that are harmful to us or our precious crops. So, it's time we get to know of the tiny species of birds that come to our country as winter or summer migrants, and as autumn and spring passage migrants.

Why and how migration is important?

Considering that the world has roughly 10,000 species of birds, 40% of it or at least 4,000 species are said to be migratory.

As a rule of thumb, all living beings need food, shelter, and breeding opportunities.

In case of animals, most are mobile when majority of plants are immobile although their seeds or spores as well as eggs/larvae of some sessile animals can be mobile either on their own or are being carried from place to place by air/wind or water or are transported by human beings and other animals.

Birds, almost entire spectrum of their populations of some 10,000 species, are mobile and most move by flying when a few are walkers or swimmers that are unable to fly.

Birds are defined as feathered biped- that means two-legged animals whose bodies are covered with feathers. All animals with feathers are birds and no living animals can be considered as a bird without feathers.

Barring the non-flying birds, such as the ostrich, emu, cassowary, rhea, kiwi, kakapo, penguins, etc., other birds do fly. They do so to move from location "a to b", cover distances for getting food, shelter, mates, and nesting areas. As a part of these basic requirements birds prefer to change their location of occurrences from low food available areas to food rich areas, from harsher weather conditioned areas to cozier places and to avoid steep competition for suitable space in one locality or country and region.

Basically, too many birds breed in the northern hemisphere during late spring to summer when those areas get very rich in insects (and other smaller animals dependent on insects), most invertebrates and freshly sprouted vegetation cover. So, this rich food, lush green habitat suitable for supporting the highest density of life forms soon starts becoming unfavourable for bearing normal life as the temperature dives down to below zero. So many animals, mostly birds, are forced to move southward or from the upper hills/mountain tops to the lower reasons or foothills and nearby plains, where temperature is more favorable for supporting life. But during winter months, the temperature, humidity, daylight hours or sunshine are much more suitable in the tropics and subtropical areas in most continents.

Migration of birds, mammals, fishes, insects, and rarely few reptiles are based on the principle that migratory animals must move annually to a destination away from its breeding colony and must return to the same area/country by year-end or a particular season.

So, migration must be an annual event unless interrupted by extremes of weather. Most migration from the Northern Hemisphere in winter but many species could do so in spring and autumn as passage migrants. Some species may do the same in summer.

Usually, summer migrants are called summer migrant breeders as these birds breed in the country where they migrate. This is the opposite of winter migration. Also, a few species of birds do make south to north migration, although it's uncommon.

How do birds fly long distances?

It's easy to explain why birds migrate but quite complex to reason how they accomplish these marvellous fits. It is commonly accepted that birds use various means in unison or separately to reach their destination country, in to and fro movements.

Migrants can orient themselves during the day by following the sun's position and keep on flying.

They can follow the positions of stars at night and relate their flying paths to destination or the traditional routes they habitually follow.

Certain species of birds that usually fly at night are known to detect and follow polarized light for navigation.

They are also known to use geomagnetic fields to navigate their long-distance migration as scientists believe that migratory birds' brains have elements of magnetic material that help in navigation during day and night.

Do birds fly by day or night?

Birds may migrate by day or night or a combination of both. Usually coastal and wetland birds are known to follow the coastline when terrestrial birds follow routes that encompass countryside, mountains and river courses but avoid crossing oceans, and mountain tops.

Some species make nonstop flights from the beginning of the journey to the end of it. However, most birds prefer to follow routes that take them through forests, fields, cities, villages, rivers, and wetlands so that they can make stopovers to replenish their fuel source from eating as much food as possible. So, most of the species stop, eat, rest for few hours to a few days and then keep moving up to the time they reach their destination spots/countries.

To make nearly straight-line migration few birds are known even to cross the mountain peaks as the High Himalayas in Asia.

Nonstop fliers and world record holders

Almost all non-stop fliers are coastal or wetland birds that start journey from arctic tundra to coastal areas in New Zealand or Australia, southern tip of South Africa and South America are often the nonstop fliers. Several species of Asia, Europe, and North America are among those that hold the records.

Red Knot is a migratory bird that breeds in Siberia and migrates to west coast of Africa, some even reaching the tip of South Africa, covering 16,000 km each way, totalling 32,000 km.

Red Knot is a 'rare migratory bird restricted to the coastal areas in Bangladesh'.

An Arctic Tern is a record holder for making longest distance flying during its migratory journey, including number of stopovers, to 59,650 miles or 95997.37 kilometres. This tern had flown from the Farne Islands in the UK and returned to the same island after 285 days. The journey lasted from the 25 July 2015 and ended on 15 May 2016 as reported in the Guardian UK.

Birds Follow Flyways

Like airplanes in the sky and oceanic vessels in the seas, migratory birds are known to follow aerial paths termed as bird flyways for their migration from point A to B and then returning to the point A. Worldwide there are 8 major flyways with some sub-flyways as reported by the Birdlife International. Flyways have been proposed and used for over 2500 species of migratory wetland birds round the world.  Many terrestrial birds are known to follow the similar flyways too. The Flyways include three in the Americas- Pacific Americas, Atlantic Americas, and Central Americas Flyways. There are three flyways in Africa, Asia, and European continents- East Atlantic, Black Sea/Mediterranean, and East Asia/East Africa Flyways. North Asia, Continental Asia and East Asia plus Australia have just two flyways- Central Asia and East Asia/Australasia.

Bangladesh is situated within the two flyways- Central Asia and East Asia/Australasia. It has been estimated that world's largest number of birds, over 50 million use East Asia/Australasia Flyway on yearly basis as noted by the Bird Life.

Major migratory birds in Bangladesh

Most visible migrants that come and stay in Bangladesh or pass by it are the waders or wetland birds, oceanic or coastal species such as gulls, terns, ducks, geese, herons-egrets, birds of prey as the eagles, harriers, buzzards and falcons, storks, ibises, and spoonbill. The waders, wetland or shorebirds include 2 species of godwits, several species of stints with number of species of stint-like ones belonging to the same Calidris genus such as knots, sanderling, sandpipers, ruff, dunlin, and curlew sandpiper; snipes, phalaropes, plovers, turnstone, lapwings, oystercatcher, avocet, whimbrel and curlew. Gulls that we commonly see in our cities and waterbodies is the Brown-headed Gull that occurs in thousands. Then we have Black-headed, Lesser black-backed, Steppe, Pallas's Gull, etc. The terns include Little, Whiskered, White-winged, Gull-billed, Sandwich, Caspian, Lesser and Grater Crested, etc. 

Among ducks, most abundant migrants are the Mallards and Eurasian or Green-winged Teal. Some of the world's largest populations of Ferruginous Duck, Red-crested Pochard, Wigeon and Garganey are found in Bangladesh wetlands and coastal areas. Among other species, Tufted Duck, Common Pochard, Northern Shoveler, Northern Pintail, Gadwall, etc., are quite visible. Of the migratory ducks, two largest species are the Common Shelduck and Ruddy Shelduck. The geese species are represented by Greater and Lesser White-fronted Goose, Greylag Goose and Bare-headed Goose, etc. Largest migratory wetland or coastal bird is the Pelican, but it is a rare species.

Uncommon or rare migratory large birds include Greater Flamingo, Common crane, White Stork and Painted Stork, Ibis and spoonbill, etc. We get several species of migratory herons and cormorants. 

We have one of the largest migratory populations of the Indian Skimmer in the coastal areas in HatiyaUpzila. I found first tiny flock in 1994 at Mukhatria Khal near Jahajmara, where I noted nearly 1000 skimmers during January 2010.

Oceanic or pelagic birds include booby, petrel, tropic bird, gannet, and shearwater, parasitic jaeger or pirate bird, etc. But these are quite rare in our part of the Bay of Bengal.

However, the bird species that outnumber the total combined number of larger species mentioned above are the specimens of small, inconspicuous, drab coloured migratory warblers, leaf warblers, reed warblers, flycatchers, bluethroat, thrushes, stonechat, wagtails, buntings, pipits, larks, shrikes, swallows, martins, bee-eaters, finches, etc.

Tow species of unusual winter migrants are the Kuro/Bowol Pallas's Fish Eagle and Painted Snipe, RongilaChaga. Both are winter migrants to Bangladesh. Also, they breed here during their sojourn in Bangladesh. Kuro prefers visiting TanguarHaor and a few other haors un the District of Sunamganj when RangilaChaga is a shy and sneaky birds preferring wetlands bordered by vegetation of reeds or other plants. In this species, male is less colourful, small and he incubates the eggs when female is brilliantly coloured and larger in size.

Notable migratory species and long distance migrants

1. Bangladesh has an interesting migratory bird that comes from Eastern Siberia in Russia to our coastal areas and rarely in haor belts. It is Dora-lejJourali/Bar-tailed Godwit Limosalapponica. This godwit has 5 subspecies. Our subspecies is likely to be  Limosalapponicamenzbieri Portenko, 1936. However, without analysing the DNA sample from the godwits in Bangladesh, it will be difficult to pinpoint the subspecies. As per the Wikipedia, this subspecies breeds in northeastern Asia from the Anabar River east to the Kolyma River delta in Russia. It winters in southeast Asia and northwest Australia. Godwits are long-tailed and long-billed wetland birds, preferring Bangladesh coastl areas and freshwater wetlands like TanguarHaor.

2. Black-tailed Godwit/KalolejJourali Limosalimosa migrates to our coastal areas and haors like the TanguarHaor in appreciable numbers. A Jagannath University assistant professor Delip Das Bisharga is studying it by fixing satellite transmitter so that the godwit with such tags can show which route it is following, how many days it is spending in which location/country and when it starts return journey from the breeding ground in Mongolia and East Asia. His study is part of his doctoral thesis under the University of Groningen, the Netherlands where he is investigating why Black-tailed Godwit choose to spend winter in different ecosystems of Bangladesh. Godwits have four recognised subspecies, of which Limosalimosamelanuroides – Gould, 1846- the Asian black-tailed godwit breeds in Mongolia, northern China, Siberia and Far Eastern Russia. Its plumage is similar to islandica subspecies, but the bird is distinctly smaller (Wikipedia). Bisharga fixed 20 satellite transmitters on Godwits netted in TanguarHaor and the coastal areas during last winter lasting up to March, this year. 

Bisharga has also fixed transmitters on 4 Whimbrels/ChhotoGulinda Numeniusphaeopus, is another long-distance migrant to our wetlands and coastal areas. Out of courtesy, he named one of the tagged godwits as Reza Khan. This bird has travelled from NijhumDweep in Noakhali to a part of China where the satellite transmitter has stopped sending messages. So, he is guessing that either the bird has problem, or its transmitter gone down that might become active when it is sufficiently charged by sunlight.

3. Ruddy Turstone/PathorghuraniBatan Arenariainterpres has two subspecies. These are- 

●       A. i. interpres (Linnaeus, 1758) – breeds in northeast Canada, Greenland, north Europe to northeast Siberia and west Alaska; winters in west, south Europe, Africa, south, east Asia, Australasia, Pacific islands, west USA and west Mexico.

●       A. i. morinella (Linnaeus, 1766) – breeds in northeast Alaska and north Canada; winters in south USA to South America vide Wikipedia. 

Bangladesh subspecies is the A. i. interpres that is known to migrate from Siberia to Bangladesh coastal areas during winter. In size, it is smaller than our common bird like a Myna.

4. Sanderling/ BalubelarChapakhi Calidrisalba. This coastal and shorebird is of the size of a bulbul and has the lightest plumage of all the small waders we have along our coastline. It breeds in the Arctic in Eurasia and North America. After the breeding season is over in summer almost all sanderlings migrate southward reaching almost every part of the world's coastal areas.

5. Our commonest and the smallest Siberian migrant is the Little Stint/ BamonChapakhi Calidrisminuta is just 13 cm. It occurs in large numbers in our coastal areas, estuaries, freshwater rivers, and wetlands as well as in wet paddy fields.

6. Critically endangered and at the same time one of longest distance migrants from the Bering Strait is the Spoon-billed Sandpiper/Kodal-thonti or Chamoch-thontiChapakhi Calidrispygmea. Bangladesh is one of the most important migratory destinations for this species having a world population of less than 500 birds. It is found just in a handful of islands in the Cox's Bazar and Noakhali Districts.

From the world point of view of importance as migratory destination of important species of birds, Bangladesh plays a pivotal role, so far as above five species of wetlands species and some of the duck populations, that of Indian Skimmer and Pallas's Fish Eagle are concerned.

Dr Reza Khan is the principal wildlife specialist at Dubai Safari Park.