Wednesday, November 11, 2015

A little about Bird Migration

Geese gathering at Alrø bird reserve

The migratory birds move thousands of kilometres each spring and autumn, in spring to the north to their breeding places in Scandinavia in order to nest and have kids. Here is a lot of space and food in summer. In autum the migratory birds fly south to their winter-quarter in the warm countries, where they can rest and find food while snow and frost cover the earth in Scandinavia.

Denmark is a crossroad for both water birds and land birds Birdwatchers estimate that about half a a billion birds pass the southern Scandinavia each autumn.

The land birds prefer to fly over land as far as possible before the flight across sea - therefore they follow the coastline until they are forced to cross the sea.
The water birds prefer to fly across water as far as possible -  therefore they also follow the coast line  until they are forced to fly over land.

An extreme example of bird migration is the Arctic tern which breeds far north near Arctis and overwinters at Antarctis. The species fly usually a yearly migration of ab. 34.000 kilometer. Some individuals can even traverse over 80.000 kilometer yearly, which is the double of the circumference of the earth.

The birds' orientation, the ability to find their way, is genetically determined. How the birds orient themselves is not quite clarified, but it is known that birds use several methods like the Terrestrial magnetic field, constellations, the sun and moon position, polarized light and visual Memory.

Some examinations have shown that the birds not only feed on their fat-reserves but that some of their organs grow smaller during the migration - so the birds exploit the bound energy from fx muscles, stomach, liver and other inner organs. In this way the birds get both energy and a lower weight, which makes the flight easier.

During the migration the birds feed on metabolic water, which is released as a biproduct during the metabolism when the stored fat is burned. The biggest loss of fluid during the flight happens via the exhaled air,  but the birds lose only little fluid through their droppings.

Most of the big birds, like swans and geese, must be taught by their parents in order to find the right migratory route between the winter quarter and the breeding place.

Several migratory birds can fly at least a week in a row without landing.

Some species, like geese and swans, fly in formation during the migration like the V-formation (the wedge formation). A common misunderstanding is that a formation like this has a leading goose in front,  but the birds change place alternately.

It is relatively easy to imagine that a big bird of prey can hover all the way to southern Europe or Africa on spread wings, but it is difficult to imagine a little bird on such a long trip. A bird weighing 10-15 gram has to bring fat in order to be able to survive the tour over long stretches, where it might have no possibility to seek food, water or shadow.

And what I have found here above is just a small part about this fantastic bird migration, the source is wikipedia, the photos; grethe bachmann . 

The bird migration is a wonderful miracle, isn't it? 


    Gerry Snape said...
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    Thyra said...
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