Possible mechanism of nest density regulation in Gull-billed Tern colonies. - E.V. Barbazyuk. - Berkut. 16 (1). 2007. - P. 119-130.
Nesting territory structure and territorial behaviour of the Gull-billed Tern was studied at Shalkar and Ayke Lakes (southern Russia) in 2000, 2001 and 2003. To examine the defended area around nests, field experiments were conducted during which one nest was moved gradually toward the nearest neighbouring nest. The area surrounding a Gull-billed Tern nest was shown to consist of at least three territorial units, which are not visible by direct observation. Immediately surrounding the nest is a small area designated as the Core Area; the Core Area is surrounded by a larger area called the Conflict Zone, consisting of an aggressively defended Inner Layer directly bordering the Core Zone, and a less aggressively defended Outer Layer. Agonistic interactions between birds increased as the distance between the nests was reduced. In the Core Area aggression was greatest, as expressed by absolute intolerance of other individuals. The Core Area is thought to play the prime role in nest density regulation in Gull-billed Tern colonies. Analysis of the distribution of nearest-neighbour distances in colonies in the study area as well as literature sources support this hypothesis. The size of the Core Area is suspected to correspond to that of the individual distance maintained by each bird around itself throughout the year. [English].
Key words: Gull-billed Tern, Gelochelidon nilotica, behaviour, nesting territory, colony, aggression.
Address: E.V. Barbazyuk, Institute of Steppe, Pionerskaya str. 11, 460000 Orenburg, Russia;
e-mail: bev@esoo.ru.