But his high-octane lifestyle critically depends on the summer sun. And in Yellowstone, that's never a guarantee. 

 

In the mountains surrounding the plateau, summer has not yet arrived, /at this altitude, it is still bitterly cold. These mountains barricade Yellowstone in a ring of ice. And if the sun is swallowed by cloud, the cold can drive down from the mountains, bringing with it a punishing echo of winter. It can snow in any month in Yellowstone.

 

The warm nursery meadows of a few days before, are now buried by a June blizzard.

 

A curtain of snow dulls senses. This mother grizzly bear must now be especially careful. A male grizzly will often kill cubs if he finds them. And though she would rather sit out  the storm, she must move. Panicky cubs could easily be lost in the blizzard.

Even in Yellow Stone it is extremely rare to see wolf pups like this.

 

Before long they are summoned back into the forest by the pack. It will be winter before they are regularly seen again.

 

As May turns to June, Yellow Stone sees its nursery season. 

 

By early June, summer is finally here. In the flower meadows, plants turn sunlight into the sugar of nectar, and that's what Yellow Stone's smallest migrating bird has been waiting for. The Calliope Hummingbird has made the journey all the way from Mexico. He’ll make this flower patch his own. And he’ll fight to defend it. He’ll even take on a far larger Rufous Hummingbird.