Great article by Glen Martin in today's San Francisco Chronicle about the beauty of spring bird migrations:

Each year, songbirds, shorebirds, waterfowl and raptors transit the Americas, flying to their wintering habitat in the South in autumn, then heading north, back to their breeding grounds, in the very late winter and spring. It is an epochal phenomenon, one that has conformed to the same basic routes and timing since the end of the last ice age.