By Asif Raja Khan*
Bird migration is a special type of behavioural adaptation found in some special groups of birds – migratory birds. They move from one place to another in search of better resource availability and better breeding ground on seasonal basis. They face different types of challenges during their journey; several natural calamities, predation, and habitat destruction cause high cost in mortality. According to Sekercioglu (2007) and Rolland J. et al (2014), among world’s 10000 bird species, 1800 are migratory in nature.
In the Indian subcontinent, more than 220 types of migratory birds can be found every year, most of them choose the Indian subcontinent as their wintering ground, though some of them are monsoon and summer visitors. They come here from different corners of the world – some of them come from Arctic tundra region; some of them come from Europe, Northern Asia, and even from Iceland. But not all migratory birds do long-distance migrations, some bird species are short-distance migrant in nature; they travel only some hundreds of mile in search of better food resources. In the Indian subcontinent some short-distance migrants of Himalaya change their altitude only, in response to the weather condition – this is called altitudinal migration.
Why they choose India?
Generally birds migrate from an area with decreasing resources to an area with high resources. We all know, because of the geographical condition, Indian subcontinent is always full of resources throughout the year. Indian subcontinent is geographically, as well as ecologically very variegated; different types of habitats, like large wetlands, green grasslands, a super long coastline, warm desert, evergreen forests of great Himalayas, Mangrove forests, dry deciduous forests of central India have adorned Indian subcontinent. A large number of winter migratory birds choose different types of habitats for wintering. Some summer migrants and monsoon migrants arrive in the Indian subcontinent every year.
Birds of northern hemisphere can’t tolerate the freezing temperature of Northern latitude during winter. Every year in winter, they try to escape from the deadly cold by migrating towards South and choose some areas with preferable weather condition and enough food resources for wintering. The mild cold weather of the Indian subcontinent during winter is very comfortable for them. Large number of migratory birds from the northern hemisphere terminate their migratory journey after reaching the Indian subcontinent.
How birds migrate?
There have been so many debates over a century about this topic – How birds migrate? Great researchers from all around the world proposed their own hypothesis about this mysterious behavioural adaptation of birds. Let’s try to list up some of the popular hypotheses about bird migration. We are not scientist, it’s very tough for us to understand those research articles; we will try to discuss in very simple language.
i. The control, timing, and response of migration are actually genetically controlled. Their navigation and orientation ability during migration journey is very complex phenomenon, including both learning and endogenous programs (Helm B. et al, 2006; Fries-soler et al, 2020).
ii. Variation in day length is the main triggering factor for bird migration. Before migration, hormonal activity becomes very high in birds. Johann Friedrich Naumann in 1795 for the very first time observed migratory restlessness (in German: Zugunruhe) in those birds just before their migration; those birds become very active in fat deposition.
iii. Some birds use the Sun as their compass, they have the ability to track out the magnetic field of earth, they use some visual landmarks, as well as chemical signals (olfactory cues) during their migratory journey (Walraffet et al, 2005).
iv. Wind drift is another important tool for migratory birds. According to Thorup et al (2003), older individuals of Osprey and Honey Buzzard make better detection of wind drift than young individuals during their journey. Actually experiences make the mental map of birds more accurate.
v. In most of the cases, birds migrate in flocks. This behavioural adaptation of migratory birds is to minimize energy cost of migratory journey, as well as the chance of predation (Weber et al, 2009)
vi. Some birds depend on their ‘nocturnal flight call’ to communicate with each other during their nocturnal migration. Nocturnal migration is a special behavioural adaptation of migratory birds to reduce the risk of predation and overheating (Lincoln et al, 1979).
My 10 favorite migratory birds of Indian subcontinent-
i. Taiga Flycatcher-
Taiga Flycatchers are tiny winter visitors of Indian subcontinent. Every time in winters, those cute guests arrive in Indian subcontinent from far off Taiga region of northern Eurasia (Eastern Russia to Mongolia).
ii. Northern Pintail-
Northern Pintails are migratory duck of Northern Europe and Northern America. Some populations of Northern Pintail migrate to the Indian subcontinent during winters.
iii. Indian Pitta-
Indian Pittas are one of the most colourful birds found in India. They are short-distance migrants, breed in Himalayan forests, Western ghat and central Indian hills, and migrate to different parts of Indian subcontinent during winter season.
Gadwalls are very common duck species in northern America and northern Europe. They are long-distance migrants and can be found in wetlands of the Indian subcontinent during winters.
v. Blue-fronted Redstart-
Blue-fronted Redstarts are Himalayan short-distance migrants (altitudinal migrants). They come down to lower Himalayas during winters to avoid the harsh weather and decreasing food resources of higher altitude.
Except for the White-browed Wagtails, all Wagtail species of the Indian subcontinent are winter migrants. Most of the Wagtails breed in northern countries, though some populations of White Wagtail, Grey Wagtails, and Citrine Wagtail breed in Himalayas.
Bluethroats are insectivorous winter migrants in Indian subcontinent. They mainly breed in bushy swamps and birch woods of Europe; they spend entire winters in the Indian subcontinent and in some parts of North Africa.
viii. Curlew Sandpiper-
Curlew Sandpipers are migratory sea-shore birds and mainly breed in Tundra region of Arctic Siberia. Maximum individuals of Curlew Sandpipers choose Africa as their wintering ground, though a small population migrates to Indian subcontinent every time during winters.
ix. Ruddy Shelduck-
Ruddy shelducks are one of the most beautiful migratory duck species found in Indian subcontinent. They are also known as Brahminy Duck in India. During winters, they migrate to the Indian subcontinent from central Asia and Europe.
x. Siberian Rubythroat-
Siberian Rubythroats are old world flycatchers, mainly found in undergrowths of Siberia. They choose the undergrowths of Indian subcontinent as their wintering ground.
“अतिथिदेवो भव “ [Atithi Devo Bhava]; Indians consider our guests as God. Then don’t know why, day by day these winged guests are rapidly vanishing from India. We are destroying their habitats, building obstacles in their migratory routes, growing unsustainable tourism in their wintering ground. On the other hand, illegal poaching and unawareness among people are other causes of headache. If we are not aware of how to protect these migratory birds, in future, they may not come to India and those ‘अतिथिदेवो भव’ lines become meaningless.
*Author is a passionate birdwatcher and wildlife blogger from Canning, a small town near Kolkata, India. He has completed his Masters in Life Sciences with Ecology specialization from the Presidency University, Kolkata.