United Nations-backed events celebrating the annual migration of an estimated 50 billion birds are taking place?last weekend.

The events?mark one of nature's great wonders and highlight the negative effects that human activities can have on migratory birds and their habitats.

The theme of this year's World Migratory Bird Day (WMBD) is "Land use changes from a bird's-eye view" and events, including bird festivals and bird-watching trips, will be held in more than 50 countries to mark the occasion.

"Although migratory birds face many serious threats, the way humans use the land around them has by fare the greatest negative effect," said Bert Lenten, Deputy Executive Secretary of the Convention on Migratory Species and initiator of the WMBD campaign.

"Unsustainable human land use, whether through deforestation, intensive agriculture, bio-fuel production, land reclamation, urbanization and mining directly removes or damages the habitats of migratory birds, affecting their populations on a global scale."

Each year an estimated 50 billion birds – representing around 19 per cent of the world's 10,000 bird species – migrate across vast swathes of the planet. But the loss and degradation of their natural habitats has become an increasing occurrence, mainly caused by the pressures of rapid urbanization and a steadily growing human population.

Initiated in 2006, WMBD is an annual UN-backed campaign organized by the Convention on Migratory Species (CMS) and the African-Eurasian Waterbird Agreement (AEWA) – two intergovernmental wildlife treaties administered by the UN Environmental Programme (UNEP).

CMS and AEWA bring together governments and other stakeholders to coordinate and develop global flyways policies and work towards establishing a viable network of sites where migratory birds can breed, rest and refuel during their long journeys.