The Milky stork is a large bird named for its pale creamy yellow plumage during the breeding season. Outside of the breeding season, the bird is completely white except for black flight feathers of the wing and tail, which also have a greenish gloss. The bare facial skin is greyish or dark maroon; with black, irregular blotches. During breeding, the bare facial skin is deep wine red with black markings on the lores by the bill base and gular region, with a ring of brighter red skin around the eye. Soon after courtship, the facial skin fades to paler orange-red. Breeding birds also show a narrow pinkish band of bare skin along the underside of the wing. During courtship, the bill turns deep yellow, with a greyish tan on the basal third; and the legs become deep magenta. The sexes are similar, but the average male is slightly larger with a longer, thinner bill.
Milky storks are widely but patchily distributed in Southeast Asia. They occur in Sumatra, Java, Sulawesi, eastern Malaysia, Cambodia, southern Vietnam, Bali, Sumbawa, Lombok, and Buton. These birds inhabit mangrove, freshwater and peat swamps, and estuaries. They forage on tidal mudflats, in shallow saline or freshwater pools, freshwater marshes, fishponds, rice fields, and on backswamps along river floodplains up to 15 km from the coast.
Milky storks are highly gregarious birds and usually soar in flocks on thermals at great heights to travel between areas. They feed by day using various feeding mechanisms. These birds locate and capture prey predominantly by the sense of touch, usually by bill groping or direct bill probing around deep holes in the mud. The groping method consists of walking slowly through shallow water with the partially open bill submerged in the water. The stork rapidly shuts its mandibles when a prey item touches the groping till raises its head and quickly swallows the item whole after some tossing. After swallowing a large fish, the stork may rest for up to one minute before resuming foraging. The bird may also stand passively at the water's edge with its half-open bill steadily submerged in the water where waves are present, so that water flows through the bird's mandibles. The stork also sometimes draws its bill through the water in an arc side-to-side when standing or walking, until the bill touches a prey item. Other foraging methods are herding mechanisms including foot stirring; the stork stands on one leg and disturbs the river bed with the foot of the opposite leg. Milky storks also forage in flocks at high prey density, whereby they cooperatively flush fish in shallow water to divert them to their half-open bills. They may also sometimes detect aquatic prey by direct visual searching. Between foraging activities, Milky storks spend their time standing in shaded spots or in the sun adopting wings drooped position. During high tide, they often roost in mangrove trees or in remnant trees on rice fields. Outside of the breeding season, Milky storks are usually silent; at nests, during display, they utter a 'fizz' call and the young produce a froglike croak when begging for food.
The mating system of Milky storks is unknown, however, storks generally are monogamous and form pairs. Breeding typically occurs after the rains during the dry season that can last from April to November. During this time Milky storks perform courtship display that consists of repeated bowing and bill-raising from both partners, who stand opposite each other and perform this display in a mirror action. The male at the nest advertises to the arriving female using display preening, whereupon the female responds with a balancing posture and gaping. An up-down greeting display from both partners follows the arrival of one partner at the nest, and the male adopts a flying-around display upon female arrival. Milky storks breed in colonies that can range in size from 10-20 to a few hundreds nests. Nests are usually built in dead or dying mangrove trees; these are sturdy, bulky structures measuring about 50 cm in diameter and predominantly comprising medium live sticks on which many leaves are still attached. The female lays 2-4 eggs and both parents incubate them about 27-30 days. The chicks hatch helpless and are fed by both parents. Young storks begin to leave their natal breeding grounds at 3-4 months of age and can become reproductively mature from 3 months of age; however, actual breeding age is probably slightly older.
The global Milky stork population has been declining substantially, especially since the late 1980s. It has been largely attributable to habitat destruction and disturbance through mangrove deforestation for human activities such as fish farming, tidal rice cultivation, timber exploitation, and in Indonesia, human resettlement. Deforestation has led to a lack of suitable mature trees for the birds to nest in and thereby affected breeding success. Breeding colonies have also declined through increased illegal international trade in this bird from the mid-1980s and, especially in South Sumatra, poaching of eggs and juveniles for human consumption. Juveniles, in particular, have been sold to and purchased by zoos in Singapore, Kula Lampur, Brunei, and several European zoos. Milky storks are also generally vulnerable to human disturbance, which may also explain the marked widespread decline of this species. Another potential threat to these beautiful birds is the contamination of their natural habitat with copper, zinc, and lead. Sources of this observed contamination include the use of agrochemicals in Kuala Gula, corrosion, and runoff from jetties and boats coated with these metals, and aquaculture development. On Pulau Rambut (Jakarta), breeding colonies might also be threatened by increasing sea pollution.
According to the IUCN Red List, the total Milky stork population size is around 1,500 mature individuals. This includes around 1,600 in Sumatra; around 500 individuals on Java; less than 100 birds on the mainland of South-East Asia and around 100 birds in Rawa Aopa Watumohai National Park, Southeast Sulawesi. Overall, currently, this species is classified as Endangered (EN) on the IUCN Red List and its numbers today are decreasing.