Conservationists have expressed concern over the decreasing number of Sarus cranes an endangered species in the Tram Chim National Park in the Mekong Delta province of Dong Thap.
Nguyen Hoai Bao, an ornithologist and lecturer at the Ho Chi Minh City University of Science, told the Saigon Times Daily the number of birds migrating to the park has gradually declined.
In 1988 more than 1,052 of them had been recorded in the park, but since 2006 no more than 200 migrate to the park each year, he said.
Last year only 67 Sarus cranes were spotted, he said.
Every year the cranes -- the world's tallest flying birds -- flock to Tram Chim in March and April when the floods recede.
Last week three Sarus cranes were spotted in the park. They are an advance party and will be followed by larger flocks in the next few months.
A group of experts is always present in the park to keep a watch on the migration of cranes to report to the International Crane Foundation.
Nguyen Van Hung, the park's director, blamed the shrinking wetland areas, the cranes' habitat, and the species' reduced birth rate for the decrease in numbers.
The park is home to 231 water bird species, of which 32 are protected.
The Sarus crane is considered an iconic species of open wetlands, but is listed as "vulnerable" by the International Union for the Conservation of Nature.