Nest site selection and breeding success in an expanding species, the Cattle Egret Bubulcus ibis

摘要

Native to the Indo-African area, the Cattle Egret Bubulcus ibis is now a cosmopolitan species, still in great range expansion. The factors affecting breeding success of the species remain however poorly known, and here we investigate the breeding biology of the Cattle Egret in a new breeding population in the Soummam Valley (Petite Kabylie, Algeria). The installation chronology and nest site selection are described, and their effect on breeding success investigated. The nesting season lasted generally from the end of March to the end of July. Clutch size, number of hatched chicks and breeding success were low compared to other breeding areas in the Mediterranean. The first Cattle Egrets arriving at the colony in March selected the highest trees and the highest nest supports close to the trunk. With new arrivals during the course of the season, a gradual decrease was observed in the height of the nests which were progressively built away from the trunk. This selective behaviour allowed the first installed pairs to have a better breeding success. This phenomenon is discussed here within the framework of territoriality and the breeding advantages of being high ranking in the 'territorial hierarchy' for the individual.