Our beautiful planet: Spring whale migration

It's officially springtime in the northern hemisphere — although, the weather in Germany, at least, hasn't gotten the memo yet. But usually the start of the new season heralds a reawakening in nature: daffodils start to bloom, birds begin nesting and butterflies emerge. 

Spring is also the time of year that a great migration takes place along the West Coast of the US. 

In March, gray whales start the journey back from their Mexican breeding grounds to their summer quarters off Alaska. Hugging the coast as they swim, these gentle giants occasionally surface for air and whale watchers turn out in droves to catch a glimpse. 

Their yearly migrations are an odyssey — and one of the longest of any mammal. The round trip is around 11,000 miles (17,700 kilometers) but these formidable filter feeders, which grow up to 50 feet (15.24 meters) in length, have to avoid dangers, such as ships and orcas, on the way. 

In the peak years of the whaling industry from the 17th to 20th centuries, the animals were hunted to the brink of extinction for their meat and blubber. 

Numbers have recovered to an extent, particularly along the Eastern North Pacific. But threats, such as oil and gas drilling, climate change and a drop in food supply, remain.