Migration
Migration of birds
Many animals migrate, but land animals are limited to just that – the land, stopped by large bodies of water. Elk and reindeer both migrate hundreds if not thousands of miles in the far north. On the plains of Africa wildebeest and antelope, buffalo and elephant all migrate from one grazing ground to the next, but none manage the migrations that some birds achieve.
These movements have fascinated humans for millennia, there are even references to migration in the Bible and in the writings of Roman/Greek philosophers such as Aristotle and Homer.
Birds live in all parts of the world, some areas are very much more seasonal than others, and the further north or south from the equator, the greater the changes in the seasons. These can be temperature, daylight length and such things as snow cover, all of which are important to birds.
Location Matters - Seasons
In geographical areas closer to the equator the temperature and day lengths are less variable and so birds that breed in these areas tend to be sedentary, meaning that they do not tend to leave their ranges, although they may have to migrate to keep up with a specific food source.
But birds that live in areas where there is large variation in the seasons, deal with those changes in one of two ways. A very few birds, and many species of reptiles and insects can enter a state of torpor known generally as hibernation. Nightjars are thought to do this; however the great majority of birds solve the problem by migrating to another geographical area to winter.
Some birds achieve spectacular migrations of thousands of miles without rest and sometimes over oceans, although they are unable to swim or float. The longest known migrant is the Arctic Tern, who migrates from the North Pole to the South Pole in the autumn, returning north again in the spring. The Steppe Eagle can migrate 8000 miles from the Russian Steppes to South Africa, and it has been satellite tracked and found to cover as much as 500 miles in one day in good thermaling weather.
Most birds that are moving in the autumn are looking for warmer weather and thus generally better food supplies. Although it would be a mistake to think that birds start to move because they are getting hungry, quite the opposite in fact, because birds need to build up their body reserves to be able to make their long migration. It is known that many small migrants can even double their own body weight, to be able to survive the migration. They store up enough fat so that they can fly non stop for incredible lengths of time – on occasion up to 90 hours – that is over three and a half days.
Migration Triggers
No one really knows what triggers the start of migration for a bird. We do know however from research on Turkey Vultures that for a few days prior to starting their move, their heart rates goes up (similar to ours in anticipation of a long journey) and once started, their heart rate settles again (also similar to ours once the plane has taken off and we can relax!)
An estimated 5 billion land birds leave Europe and Asia each fall heading for Africa and a similar number leave North America for Central and South America. In total, individuals from nearly 400 species migrate each year. In the autumn of 1992, researchers in Vera Cruz, Mexico counted over 2.5 million migrating raptors!
Migration has its costs and benefits. Unlike hibernation or cold season dormancy, migration allows birds to have year-round activity. They can exploit various food resources when they are plentiful and they can inhabit warm climates year-round. As there is “no free lunch”, the costs associated with migration are great as well. The energy resources required for such long journeys are large and the physiological demands on the birds’ systems are also great. The dangers associated with such a trip are also significant. According to ‘Gill’, more than half of the small land birds in North America never return from their migration.
Why Migrate?
The evolution of migration is a complicated and often misunderstood one. There are many questions that complicate the complete story of migration such as: Why do some populations of a species migrate and some not? Why do some individuals migrate extreme distances while others take fairly short trips?
Food
One of the main driving factors of migration is food. Migrating birds exploit seasonal abundances of food, but not only food, they also may move to gain extra daylight hours so as to utilise that food source to the utmost.
Feeding Habitat
The temperate zone, often a destination during the breeding season, offers a great deal of suitable breeding habitat for many bird species, and also longer daylight hours. This allows reduced density of breeding sites which may aid in successful breeding (less predation on nests, more prey availability)
YOU CAN HELP
None of the above would be possible without the help and support of others. You can make a difference and help us to help them. Whatever you can afford is much appreciated.
We are eternally grateful for those who leave a legacy to the ICBP, as they make a huge difference to our conservation work. For further information please contact us
