With the arrival of spring, things on the planet start to move. Two North American bird species, the sandhill crane and snow geese, have massive migrations through the United States, on their way to home ranges in the north. Wildlife managers estimate that 650,000 sandhill cranes take up residence for a few weeks in late March, along 80 miles of the Platte River in Central Nebraska. Here they rest and eat before resuming their flight to the northern US, Canada, Alaska and Siberia. Snow Geese, who winter in the Chesapeake region, stop and recharge at open water lakes, including the Middle Creek Wildlife Management Area in eastern Pennsylvania. As many as 200,000 geese have congregated there, and produce an extraordinary spectacle when they commence massive group flights. Both are prime examples of nature at work and are a marvel to witness.