This is the lovely eastern marsh harrier (Circus spilonotus), a bird of prey spotted and identified for me by birder Mike Kilburn in Mai Po at the start of 2021.
This is a migratory bird, so a winter visitor here in Hong Kong, with the breeding range covering north-east China, Mongolia and south-east Siberia and the wintering range including southern China, Taiwan, Korea, southern Japan, northeast India, Bangladesh and Southeast Asia as far south as the Philippines, Borneo and Sumatra. Large numbers of birds migrate along the Chinese coast with thousands passing through sites like Beidaihe during the autumn.
It prefers to hunt in open country including marshland, paddy fields and grassland, so Mai Po is ideal, and as the photo above shows, while hunting it flies low over the ground with the wings held in a shallow V-shape. Its prey includes small mammals, birds and frogs.
IN the photos below it scared a couple of small ducks.