birdsoup

In some mountainous areas migration may be measured in terms of vertical feet as opposed to the number of horizontal miles flown.  Certain species spend the breeding season high up on the mountain.  In fall they do not fly south to warmer weather.  Instead from the higher elevations at the top of the mountain they simply move down the mountainside to the valleys below, thousands of feet below their normal breeding area.  A difference of a 1,000 vertical feet may be equivalent to flying south hundreds of miles.  And it takes less energy. 

In spring, they again move higher up the mountain.    Up and down, not north and south.