Sangeetha Pulapaka

Firstly bird migration is a worldwide phenomenon that happens for many varied reasons. Temperature changes, seasonal fluctuations in food supplies, breeding needs, and territorial considerations all play a role in  why they migrate.

Secondly, how they are able to know which way to go is something so fascinating. We humans get lost on an unknown route (without any guidance system) easily.

Routes vary from species to species, as well as from bird to bird. How routes are found and followed is a fascinating aspect of this yearly pilgrimage. This is because they have own guidance system. Some may use magnetic field detection, or smells and sound. Some may use stars. 

We assume that birds migrate because of cold. But, not all birds migrate. Chickadees, jays, woodpeckers, redpolls, and nuthatches are among the many species that generally stay put through the winters of North America. Some of these birds are quite small— surely if the cold were the sole reason for leaving, these tiny birds would leave too.

Most migratory birds that die during migration probably do it on their first migration, and thereafter have the advantage of knowing the territory a bit better. So, migration is one way to handle winter. Note, however, that there are birds that live in the Arctic or sub-Arctic that migrate south to regions where it is still winter. (Some of these are referred to as snowbirds because they show up during the snowy season).

Coming back to your question, some birds like the artic fern, fly between its breeding grounds in Greenland in the north and the Weddell Sea on the shores of Antarctica in the far south, in a lifetime spent in perpetual summer. Arctic terns feed from the water while on the wing and can live up to 34 years. When the scientists added up the total distance each bird flew during its lifetime they found it equalled three round trips to the Moon – or more than 1.25 million miles.

When the birds migrate they feed on the worms and insects that are in excess, thereby helping control their menace. It has been discovered that out of some 10,000 species of birds found around the world, about 1800 migrate. Robins are not creatures of habit, so their patterns of migration are hard to follow and understand. . So robins tend to fly to all points south, but not in any particular migratory path. While other birds follow the same migratory path year after year, robins flock together and travel south looking for trees and bushes bearing fruit and berries. When they spot them, they swoop in for a feast, then stick around looking for other food sources.

Water birds such as Shearwater migrate also. Eighteen million of these arrive in Tasmania each year. There are known to be at least 167 colonies in Tasmania and an estimated 11.4 million burrows. The largest colony is on Babel Island which has three million burrows. Their migratory path is difficult to define because they don't come to shore during the months of the migration. Exhausted and starved birds are often washed up on beaches of Japan, the Aleution Islands, North America and Australia. Originally this led scientists to believe that the birds flew a figure of eight course across the Pacific Ocean and depend on the small fish etc. to eat while migration because they hardly come on the shore.

In general, according to University of Rhode Island researcher, physiological ecologist Scott McWilliams, the digestive systems of birds adjust to meet the changing energy demands of migration. The birds’ bellies increase in size and the cells get larger so they can eat more and store energy for their long flights. The digestive systems of migratory birds essentially shut down during migration so most of their energy can be used in flight. When they stop to eat along their routes, they eat less, until finally their systems re-adjust when they arrive at their destinations where food is plentiful again.

This reminds me of  Robert frost's poem "Stopping by woods on a snowy evening"

" The woods are lovely, dark and deep,  

  But I have promises to keep,   

And miles to go before I sleep,   

  And miles to go before I sleep"

Which is so true to migratory birds 'cos they have miles to go before they eat!

Sangeetha Pulapaka
cute tag for the question you asked - gonebirdygone. But I am going to add the tag ornithology anyway.
Arturo Torres

They migrate over the ocean by flying over it but then they capture the water that they know that they can drink.


I think that the way birds migrate over without a food supply is that they can indeed catch fish or I think they regurgitate food to their young.