14 Then the Psalm goes on in its text, “Upon them the fowls of the heaven shall inhabit”....Upon the mountains, then, the fowls of the air shall have their habitation. We see these birds dwell upon the mountains, but many of them dwell in plains, many in valleys, many in groves, many in gardens, not all upon mountains. There are some fowls that dwell not save on the mountains. Some spiritual souls does this name denote. Fowls are spiritual hearts, which enjoy the free air. In the clearness of heaven these birds delight, yet their feeding is on the mountains, there will they dwell. You know the mountains, they have been already treated of. Mountains are Prophets, mountains are Apostles, mountains are all preachers of the truth....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)