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Origin of birds. Evolution from reptiles. Characteristics. Distribution. Migration.

Birdsí main characteristic: they can fly.
It is said birds are living dinosaurs due to their evolution from reptiles.
Birds maintain a constant internal temperature as mammals do.
Birds have beaks and feathers.

Penguins and ostriches have lost their ability to fly.
Many birds have adapted the way in which they fly according to their necessities.
Birds have developed aerodynamic shapes and airbags connected to their lungs and reduced their body-weight.
Birds have changed their teeth for bills.
Birds have developed their lower extremities into bones which have been welded together.
Their upper extremities have formed wings.
Some birds moult and change their feathers once a year.
When birds beat their wings to fly their energy consumption is enormous.
Birds have an extremely well-developed sense of sight.

Their reproduction is of a sexual nature.
Males catch the femalesí sight by using the most varied techniques.
Most birds have only one mate during their breeding season.
Just after the eggs have been fertilised, the female lays them in a hollow in a rock or a nest.
Different species lay different egggs.
Development and growth of eggs.
Some fledgings stay in their nest where they are fed by their parents.
Other chickens abandon their nest and follow their parents shortly after birth.
Most newly-born fledglings go through a special phase where they will follow their parents.

During the Jurassic Period the first feathered being: the Archacopteryx (half bird, half reptile)
In the Terciary Era there existed birds similar to present-day ones. In Madagascar lived the Aepyornis.
Nowadays, there are around 8,500 different species of birds, half of them are quite small.
Birds have adapted themselves to a great variety of ecosystems.
Humid tropical areas.
The Arctic Tern.


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