Aspects of breeding biology of Cattle Egret Bubulcus ibis in a Grosseto province colony (Tuscany, central Italy)

Dragonetti M., Giovacchini P.

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In this paper we have studied a heron colony near a small artifcial lake in Grosseto province (Tuscany, central Italy) during 2004, 2005 and 2006. The colony was almost mono specifc (Bubulcus ibis 94.9%; Egretta garzetta 3.1%; Ardea cinerea 1.2%; Ardeola ralloides 0.8%) and was settled in a flooded area inside a thick willow wood; therefore we had to adopt a non invasive census method (perimeter count) in order to avoid an excessive disturbance to the nesting pairs. The nesting pairs of Cattle Egret increased from 106 in 2004, to 240 in 2006. A similar increase was recorded also for the other three species. The whole population of the colony (nesting and non nesting birds) grew from a total of 390 herons in July 2004 to 860 in July 2006. In spite of this increase of colony population, the wood area occupied by nesting birds did not increase: it was 0.90 ha in 2004, 2005 and 0.87 ha in 2006, while the number of trees with nests increased from 54 in 2004 to 86 in 2006. Moreover the mean number of nests per occupied tree increased from 2.06 ± 1.69 (range = 1-9) in 2004 to 2.94 ± 2.85 (range = 1-20) in 2006, showing a tendency to an increased crowding, even if the total wood area theoretically available for the colony was about 8 ha. Cattle Egrets started nest building at the beginning of April; onset of laying was estimated on 21 sample nests between 10 and 15 April; about 80% of those pairs were incubating their eggs between 7 and 25 May. After 15 June there were no more sample nests with birds incubating; hatching started between 7 and 25 May; at 9 June 68% of broods were already hatched. At the end of July all newborn Cattle Egrets were fledged. These data allowed us to estimate the average length of the incubation period in 23.8 days. The mean number of hatched Cattle Egrets per nest was 2.5 ± 0.95 (range = 1-5; N = 34) the average fledging success was 2.2 fledglings/nest, a value relatively high if compared to the bibliographic data.