Nest Space Partitioning among Colonial Nesting Waterbirds at Bhitarkanika Mangroves, India

Bhitarkanika mangroves in Orissa harbours one of the largest mixed-species heronries in the country with more than 30,000 birds of 11 species breeding annually in a small mangrove patch of c.5 ha area between June and December. Patterns of spatial segregation was studied among nesting waterbirds as competition for space in waterbird colonies is known to be mitigated through habitat partitioning. Both vertical and horizontal associations among the nesting waterbirds in the heronry were studied. It was observed that Asian openbill (Anastomus oscitans), Large egret (Ardea alba), Intermediate egret (Egretta intermedia), Little cormorant (Phalacrocorax niger) and Little egret (Egretta garzetta) were associated more frequently than they would be expected at random. There was a significant avoidance trend between Grey (Ardea cinerea) and Purple (Ardea purpurea) herons and between Darter (Anhinga melanogaster) and Asian openbill. Interestingly, Black-headed ibis (Threskiornis melanocephala) was observed to nest away from most of the species within the heronry forming sub-colonies on its own. Results of the current analysis on vertical alignment of nests did not support the body mass-nest height hypothesis which postulated a direct positive correlation between body weight and nest height among colonial waterbirds. There was a significant radial zonation of species in the heronry with Asian openbill storks preferring the central portion of the heronry (KW 2=8.54, P<0.05) whereas Darter and Grey heron nests were observed more towards the periphery of the heronry (KW 2=6.40, P<0.05). On the other hand, nests of Little egret (KW 2=11.11, P<0.05), Purple heron (KW 2=11.53, P<0.05) and Night heron Nycticorax nycticorax (KW 2=10.61, P<0.05) were found to have clumped distribution being restricted to select blocks of the heronry. The study supported the hypothesis that edge nests suffer higher level of predation and demonstrates that the highest level of predation occurs in the periphery and central nests have higher reproductive success than the peripheral nests.