The government has designated Long Valley, a 37-hectare stretch of wetland near Sheung Shui, a natural ecological park.
The bird haven was spared dissection by the Lok Ma Chau rail line in 2000 after public opposition.
In late 2010, birdwatchers reported that there were at least 261 species of birds in the area.
The Development Bureau will also preserve part of the rice paddies in the north and south of the area for conservational value.
The wetland has the largest rice paddies in the city. Farmers grow rice and keep most of the crop unharvested for the birds' benefit.