Bird migration is one of the great wonders of the natural world. Migration is a huge feat of endurance requiring great strength and stamina. However, today birds face additional threats caused by human activity. Hungry, exhausted birds may arrive at a stopover site, only to find that it has been destroyed by farming or urbanisation. Every year, millions of birds are illegally killed by hunters, or collide with man-made structures such as powerlines. Climate change is causing habitats to shift or disappear.