Wonders of God's Creation - why migration?

I just watched "Wonders of God's Creation: Animal Kingdom" on Amazon Instant Video. Apart from providing inspiring insights into the complexity and ingenuity that God has put into living creatures, it also made me think about a repeated theme of the animal kingdom - migration. It seems that migration occurs in much of the fauna - birds, flying insects, fish, sea mammals - all have a cycle of migration, why is that? Why has God put that theme of migration throughout creation? Maybe it's to remind us humans, who are the ones observing this trend, that we too are on a journey. Each species is equipped with the necessary attributes that make traveling thousands of miles possible, sometimes without stopping such as in flying over oceans - another testament to that there had to be a creator!

In the video it shows how Atlantic Salmon exhibit great strength and perseverance as they migrate out to the Atlantic and then back to their home to lay eggs and then die. They have to survive predators all along the way and literally jump over obstacles (they can jump as high as 10 feet). The salmon have extremely sensitive sense of smell which guides them back to their home waters - apparently every body of water has it's particular essence (another fascinating fact of creation).

I don't have the answer to "why migration" but it's something interesting to ponder at night when sleep eludes one!