Breeding biology of the Cattle Egret Bubulcus ibis in southwest Spain

The Cattle Egret has expanded its breeding range worldwide since the 19th century, although the explosive phase occurred principally between 1950 and 1970. The species is not a new colonist to Europe and was a common breeder in Andalucía (southern Spain) during the last century. It is known to have colonized later in other areas of Spain (Alicante-Murcia, later Extremadura and after that Cataluña) and the south of France (Voisin 1991). Although Extremadura has only recently been colonized by Cattle Egrets, the similarity and proximity between Andalucía and Extremadura suggests that they have not needed to adapt their breeding to completely new environmental conditions there. The breeding biology of this species is welldocumented for most of its traditional range in Africa and Asia (Hellebrekers & Hoogerwerf 1967, Siegfried 1972), as well as in its extended range in America (Lowe-McConnell 1967, Jenni 1969) and Australia (McKilligan 1997). However, in the western Palearctic, only data from the Camargue (H. Hafner unpubl. data), the Ebro Delta (Ruiz et al. 1981) and the Valencia Albufera (Prósper & Hafner 1996) are available. Factors affecting the breeding success of the species have been widely studied in these recently colonized areas (Baxter 1994, McKilligan 1997). However, few data exist from those areas with well-established colonies (see however Ruiz et al. (1981) and Maddock & Baxter (1991)). Here we describe the breeding biology of Cattle Egrets in mixed colonies in southwest Spain (Extremadura) and we analyse the implications of nest location within the colony (nest height and density) on breeding success. Because nest height is essential for success in temporal marshes, where it confers more protection from terrestrial predators (Perennou et al. 1996), we expected a higher breeding success in these sites. For many colonial birds, density-dependent effects on reproduction have been detected (see review in Wittenberger & Hunt 1985). Here we test for breeding success in relation to nest density and we compare breeding parameters in southwest Spain with others from the western Palearctic to test for any latitudinal patterns. The study was carried out in three mixed colonies of Extremadura (southwest Spain): Montijo (38°55′N–6°23′W), Badajoz (38°53′N– 6°58′W) and Morante (39°03′N–6°41′E). The Badajoz and Montijo colonies are in highly modified habitats, while the Morante colony has more traditional habitats with Holm-oak Quercus rotundifolia dehesas and pasturelands. The Badajoz and Montijo colonies are on islands on the Guadiana river covered with Tamarix sp. , Phragmites sp. , Populus sp. and Eucalyptus sp. Both colonies contained Cattle Egrets, Little Egrets Egretta garzetta and Blackcrowned Night-herons Nycticorax nycticorax. The trees found in the Morante colony were mainly Fraxinus sp. and Quercus rotundifolia, and only Cattle and Little Egrets nested there. Data were collected during the 1996 and 1997 breeding seasons (March–August). Before birds began nesting, we installed wooden stakes of known height within the colonies as reference points for measurements of length and height. Large-scale photographs of the colony sites Bird Study (2001) 48, 367–372