Diomedean Evolution

A small arboreal carnivorous glider inhabited a large forested tropical island far from the extreme polar seasonal changes. Rapid geological changes on the low density planet deflected ocean and air currents which, because of the great axial tilt and large fluid masses, bear considerable heat and cold. Drought reduced the forest to woods scattered across pampas.

The gliders:

developed wings to fly between woods;
grew in size to prey on large grass-eaters;
spread into different environments;
but, because of mobility, remained a single species;
developed intelligence to cope with environmental diversity;
flew north and south from the home continent;
found good hunting territories but could not survive the polar winters;
therefore, returned to the tropics where, however, they could not survive indefinitely;
consequently, learned to migrate regularly.

(To be continued.)