This bird, shown above, is the smallest crane in the world, weighing only 4-7 lbs (2-3 kg) and measuring 3 feet in length. Yet one population of this species must perform the herculean task of crossing the Himalayas to reach its wintering grounds on the Indian subcontinent! To me it doesn't get any tougher than that.
I learned of this migratory route from watching the DVD of the BBC series "Planet Earth". It is on Disc one in the episode entitled "Mountains". The segment on the Demoiselle Crane shows them initially trying to cross as a storm begins to blow. As the narrator informs us they must turn back or risk death. They return the next day and push on through to success, despite what must be incredible exhaustion. As if the weather and altitude are not enough of a peril they must also face Golden Eagles waiting to make a meal of them. The hunting efforts of a cooperative team of two Golden Eagles is shown. The BBC does a tremendous job of documenting this migration and I highly recommend it to interested parties. (As an aside this episode also contains some of the most extraordinary nature footage I have ever seen. This is breathtaking footage of the mythical Snow Leopard shot in the Karakoram mountains of Pakistan. It was a thrill to behold!).