Veðrfölnir is the hawk which lives in the World Tree Yggdrasill.
Yggdrasill, the gigant ash is the spill of the universe, which unites the worlds of the gods, the people, the giants and the dead. The ups and downs of the tree reflects the ups and downs of the universe; the tree shores the world, which it is protecting and it shares her distress.
Sources don't reveal the origin of the tree. It already existed before the world began, like it is destined to survive Ragnarök (Doomsday)
The roots are the connection between the different worlds Yggdrasill united:

• One of the roots extends in to Niflheim,the underworld;
• another into Jötenheim, land of the giants; and
• the third into Åsgard, home of the gods.

At its base are three wells:

• Urdarbrunner (Well of Fate), from which the tree is watered
  by the Norns (the Fates);
• Hvergelmir (Roaring Kettle), in which dwells Niðhögg,
  the dragon that gnaws at the tree's roots;
• Mímisbrunner (Mimir's well), source of wisdom.

Several animals lives among the branches and around the tree:

• a wise eagle Vidofnir,which houses in the upper branches, with a hawk - Veðrfölnir - between his eyes,
• the squirrel Ratatoskr which brings the insults between the eagle and the monstrous Niðhöggr the dragon that lies between the roots, and
• Dáinn, Dvalinn, Duneyrr and Duraþrór the four stags that live among the branches and nibble at the saplings.

After Ragnarök (Doomsday), the world tree, though badly shaken, will be the source of new life.