Foraging behaviour and feeding success of the black-necked stork (Ephippiorhynchus asiaticus) in Dudwa National Park, Uttar Pradesh, India Gopinathan Maheswaran* and Asad R. Rahmani Centre for Wildlife and Ornithology, Aligarh Muslim University, Aligarh-202 002, India (Accepted 17 October 2001) Abstract Foraging behaviour of the black-necked stork (BNS) Ephippiorhynchus asiaticus was studied in 1996 and 1997 in Dudwa National Park (DNP), Uttar Pradesh, India. The storks were observed using tactile and visual techniques to catch ®sh. Of the 929 ®sh seen caught in 2 years, 894 (96%) were caught by a tactile mode of feeding and the remaining 35 (4%) by a visual mode of feeding. The rate of foraging attempts by BNS ¯uctuates with that of season in DNP and coincides with prey abundance. Immediately after the monsoon when the water level was high, BNS had to search for prey more often, as the prey became widespread. Whereas in summer when the water level decreased, the concentration of the ®sh was higher, which helps BNS to catch ®sh in quick succession. Prey behaviour and the condition of the wetland determined the selection of the tactile foraging technique among the BNS in DNP. The black-necked storks were more successful in the early hours of the day (06:00±10:00) and they were generally more successful or preferred to feed on medium-sized ®sh (i.e. 5±10 cm) in DNP. Prey pro®tability was highest for larger size ®sh and decreased as the prey size decreased. Principal component analysis showed that prey size, handling time of each prey, the month and water depth determined foraging success in 1996 and 1997. Key words: black-necked stork, Ephippiorhynchus asiaticus, foraging, India, feeding success, foraging attempts INTRODUCTION Models of optimal foraging behaviour often assume that individuals forage independently (Stephens & Krebs, 1986); however, when many wading birds con- gregate in a particular place, competition between species increases and the chances of an individual getting food is reduced (Maheswaran & Rahmani, 2001). Birds use different postures to increase their foraging ef®ciency in different types of habitats. The use of a particular posture may be ecologically signi®cant as it is correlated with habitat or prey availability, and methods of prey capture differ among wading birds and to some extent among individual birds (Kushlan, 1978). A bird is more likely to choose a behaviour based on its success rate or on the time between successes rather than on net energy return. Certain feeding patterns are apparent within the ciconiiform group of waterbirds (Kushlan, 1978). The tactile technique of groping, probing and head-swinging is exempli®ed by wood storks Mycteria americana, ibises and spoonbills, respectively (Kushlan, 1978). The foraging repertoire of wading birds can be conveniently divided into postures, which are generally physical attributes and feeding behaviours consisting of actions directed toward obtaining prey (Kushlan, 1978). The foraging behaviour of the black-necked stork (BNS) Ephippiorhynchus asiaticus was studied in Dudwa National Park (DNP). The hunting strategy of the BNS in DNP can be divided into two types: (1) tactile foraging: by walking (slow/fast, sometimes running) here and there in water and pecking the bottom of the wetland continuously until a ®sh is caught; (2) visual foraging: standing still at one place where the bird can see the movements of the ®sh. It was hypothesized that the DNP wetland in which the BNS foraged could have played an important role in the storks' selection of the tactile mode of foraging. A wading bird can select the most appropriate foraging behaviour for its needs, and the choice of a foraging behaviour that is successful should lead to repeated use. According to Kahl (1962), the ability of most tactile foraging species to use vision, also enlarges their funda- mental foraging niche. Despite their mutual importance, the effect of prey availability on feeding behaviour and *All correspondence to: G. Maheswaran, Bombay Natural History Society, Hornbill House, S.B. Singh Road, Mumbai-400 023, India. E-mail: gmaheswaran@yahoo.com J. Zool., Lond. (2002) 258, 189±195 # 2002 The Zoological Society of London Printed in the United Kingdom DOI:10.1017/S0952836902001309