For the elephants that are returning to southern Angola, after herds were devastated during the country's civil wars, the battle is far from over.

Old land mines are threatening the lives and limbs not only of people, but also of the growing elephant populations that are crossing into Angola from northern Botswana on ancient migration routes that continue into Zambia.

Mines are a particularly stark example of how humans interfere with migratory journeys that have linked breeding and feeding sites across the globe for millennia.

Up to 10,000 animal species are thought to migrate. Yet, increasingly, air, water, and land routes are being destroyed by barriers, ranging from roads, fences, dams, and power lines to unsustainable hunting or fishing practices, habitat degradation, pollution, and climate change.

One example is the critically endangered Irrawaddy dolphin, found in the Bay of Bengal and Southeast Asia. Barriers to its migration range from entrapment in fishing nets to conditions caused by gold mining and dam building.

In North America, one of the world's fastest land animals, the Pronghorn antelope, faces obstacles such as highways and fencing.

The harsh winter in 2010 left herds stranded and hungry, blocked by fences while they burned up their fat reserves searching for ways through.

Similarly, in South Africa, 12 percent of Blue Cranes, South Africa's national bird, and 30 percent of Ludwig's bustards are dying annually in collisions with a growing number of power lines.

Climate change

Climate change is also having a severe impact on the world's most peripatetic animals. Migratory species, from Monarch butterflies to humpback whales, are suffering as a result of shifts in temperature and the disruption of the traditional timing, abundance, and location of food sources.

The trend looks bad. But some countries are taking action. Since the Convention on the Conservation of Migratory Species of Wild Animals entered into force in 1983, its membership has grown steadily to include 116 countries in Africa, Central and South America, Asia, Europe, and Oceania.

To date, the CMS has concluded agreements and memoranda of understanding to conserve more than 26 migratory species.

Thanks to the CMS, Papua New Guinea and Mozambique, for example, recently agreed on cooperative arrangements to conserve migratory dugongs, animals once thought by seafarers to be mermaids. Likewise, a 20-year agreement has recently helped to increase the number of harbor seals in the Wadden Sea, shared by Germany and the Netherlands.

Protecting migratory species benefits not only the animals concerned, but humans as well.

A 10-year program to restore and conserve seven million hectares of wetlands in China, Iran, Kazakhstan, and Russia has improved conditions for the critically endangered Siberian crane, as well as drinking-water supplies, inland fisheries, and carbon storage.