Male mountain bluebird in Yellowstone National Park, USA

Hello, harbinger of spring

Welcome back, little mountain bluebird. Although this bright fellow may be enduring the last of this year's snow in Yellowstone National Park, USA - his presence means winter will soon be ending there. Bluebirds are a symbol of happiness and a welcome sign of spring in many cultures. Soon other migratory birds will arrive in the Greater Yellowstone Ecosystem—an 18-million-acre tract that includes Grand Teton National Park and surrounding national forests. It's the world's largest intact ecosystem in the northern temperate zone. What exactly does that mean? Since wolves were reintroduced to Yellowstone 25 years ago, all the animal species that were first documented by European explorers can still be found here.