One of the largest nesting colonies of water birds in India is located in the mangrove forests of Bhitarkanika National Park, Orissa. Eleven species of water birds nest in this colony spreading over an area of approximately four hectare. A total count of nest trees and number of nests carried out in the second fortnight of August 2004 revealed the presence of 13,704 nests in 3839 trees. In decreasing order of abundance they are Asian Openbill, Large Egret, Little Cormorant, Intermediate Egret, Purple Heron, Night Heron, Grey Heron, Blackheaded Ibis, Oriental Darter, Cattle Egret and Little Egret. Asian Openbill accounts for nearly 66% of all the nests counted in the heronry. The count revealed a decline in the number of nests of Oriental Darter over the years, the reasons for which are yet to be ascertained. Seventy nine percentage (79%) of the nest trees were Excoecaria agallocha. Heritiera fomes, Cynometra iripa, Hibiscus tiliaecius and Tamarix troupii were the other tree species that were found to be used by the birds for nesting. Composition of nest trees, their structure, nest selection by the breeding birds and the association among them are discussed.