Ardea
Official journal of the Netherlands Ornithologists' Union

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Ruiz X. (1985) An analysis of the diet of Cattle Egrets in the Ebro Delta, Spain. ARDEA 73 (1): 49-60
In this paper the diet composition of a population of Cattle Egrets feeding in the rice fields of the Ebro Delta has been analyzed. Prey contents of 118 stomachs collected during the whole year were studied for this purpose. The importance of each prey category in the diet of the whole population of Cattle Egrets for the different parts of the year was estimated by means of Simpson's index of dominance. Inter-individual differences in food-choice were estimated by a comparison between different values of dietary diversity, thus permitting the detection of the existence of an aggregation of certain prey types in the stomach contents. The interpretation of the results is based on both the changes in the rice field food supply, and on certain morphological characteristics which limit the opportunities for foraging of this heron. In autumn and winter the vertebrates make up the basis of the Cattle Egret's diet. They are represented by Rana perezi which is the predominant prey. In spring and summer the arthropods contribute to a larger extent to the Cattle Egret's diet, corresponding to the Orthoptera (spring) and Coleoptera (summer) prey categories. In autumn, a segregation of feeding habits can be observed within the population, which implies that rodents or a group of arthropods are taken as alternative prey to the amphibians. During the rest of the year all Cattle Egrets tend to utilize the same staple food, which is complemented by a wide variety of secondary resources. In winter, frogs are complemented with fish and arthropods, while in spring or summer arthropods are complemented with vertebrate prey (frogs, fish, reptiles and rodents). The observed substitution of the prey group with large biomass (vertebrates), by another with much smaller biomass (arthropods) is related to changes in the food supply of the rice fields.


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