Breeding ecology of the Cattle Egret (Bubulcus ibis) in Guerbes-Sanhadja wetlands of Algeria

Abstract

Information about breeding ecology and nest site selection in the Cattle Egret (Bubulcus ibis) at newly established colonies are still lacking in many North African heronies. This study aimed at determining nest site selection and assessing breeding parameters of Cattle Egrets nesting, for the first time, at the Ouajaa marsh in Guerbes-Sanhadja wetlands (northern Algeria) in relation to nest site and nest traits. Breeding chronology of Cattle Egret in the region was monitored onsite, since the use of the wetland as a dormatory till the end of the breeding season. Egg morphometrics (length, breadth, weight and volume) and breeding parameters (clutch size, hatching, fledging and breeding successes) were measured and compared between nests built on different tree species (Salix alba and Acacia cyanophylla) and at different locations within crowns of nesting trees. The effects of nest site characteristics (nesting height, trees and positions) and nest traits (nest surface, outer and inner diameters) on breeding parameters were investigated using generalized linear models (GLMs). The breeding period of Cattle Egret at the Ouajaa Marsh lasted from 12th April to 20th July 2014. Egg morphometrics were similar between central and peripheral nests. Overall, Cattle Egret’s eggs measured 46.18 ± 2.03mm. Clutch size was on average 3.43 ± 0.91 eggs (N=53 nests). Overall, hatched eggs averaged 2.72 ± 0.91 per nest, whereas the number of fledglings totaled 134 chicks with an average 2.53 ± 0.85 per nest. The reproductive success at Guerbes-Sanhadja wetlands was 73.6%. However, it was higher (84%) in three-egg nests. GLMs showed that the number of hatchlings increased significantly with the increase of clutch size (P=0.032), and it was significantly higher (P=0.028) in central nests (2.74 ± 0.96 chicks) compared to peripheral nests (2.68 ± 0.82 chicks). GLM revealed that the number of hatchlings positively influenced the variation of the number of fledglings (P<0.001), while the effects of nest site characteristics, nest traits and clutch size were not significant. Our findings indicate that cattle egrets prefer relatively shorter nesting heights above the water surface, which reflects that colonies built above water have more secure aspects compared to inland nesting sites. Consequently, vegetation structure is an important criterion for the choice of a specific nesting site among ardeids.