The massive annual spring migration of America's birds – millions of them – is underway. Here's how to follow along at home.

Forbes contributors publish independent expert analyses and insights. I write about biodiversity and the hidden quirks of the natural world. Most birds fly at low altitudes, relatively speaking. But ...

Migratory birds change how high they fly over deserts and seas. Wing shape, feather color, and heat all play a role in helping them survive.