The development and lateralization of prey delivery in a bill load-holding bird

Laterality or the preferential use of one or the other side of the body is associated with differential brain activity and increased specialization of brain function. Right or left biases are widespread among vertebrates and have been documented among some birds. Bill load holding or the delivery of whole prey in a bird's beak is common among terns (Charadriiformes) and many other birds. We recorded more than 2000 Caspian tern, Hydroprogne caspia, chick-feeding events for analysis of lateralization of prey within the bills of adult birds, size and shape of prey items delivered to chicks in relation to chick age, and the development of prey-handling skills among chicks. No initial lateralization of prey head position was found as adults flew into the colony, but head position of successful feeds was found to be significantly lateralized with a bias to the right side. Adults switched prey orientation significantly more often from left to right than from right to left without dropping the prey item, suggesting an adult bias. Adults appeared to adjust the length but not the shape of prey delivered according to chick age. Ability of chicks to manipulate prey items increased with age. Lateralization was most obvious in the first week of a tern's life, and adults modified their delivery behaviour to respond to these changes. Bill load-holding birds are excellent subjects to study lateralization of feeding behaviour in the wild and they may allow integration of lateralization with developmental changes.