Long-term changes in the food resources of a guild of breeding Ardeinae (Aves) in Italy

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The breeding herons and egrets in northwestern Italy derive a large portion of their food resources from rice fields. In order to investigate the possible effects of changes in rice cultivation techniques on the trends of the breeding population of these birds, we studied their diet and the prey availability in rice fields, and we compared the results for 2013 with the situation in past decades, by taking advantage of similar investigations conducted by our research team since 1978. The diet of five species of herons and egrets changed sharply after 1995, with the bulk of the diet now made up of completely new prey species, in particular the allochthonous Louisiana crayfish and Oriental weatherfish, that only recently colonized our study area, and with a lower frequency of the amphibians that were a staple prey before 2000, but whose abundance in rice fields has declined. Since 2000, the decrease in the populations of some breeders, particularly the most abundant Grey Heron and Little Egret, has coincided with their sharp change in diet, with the reduced availability of amphibians and with the diminished availability of flooded rice fields. However, the proportion of food resources obtained from rice fields versus the natural habitats remained high, or was only slightly lower for some predator species. Several issues remain open about the value of rice fields in supporting the foraging herons and egrets, and about the effect of changing agricultural techniques on the bird population. We need to assess more accurately the proportion of food obtained from rice fields, and to check whether the foraging herons and egrets can satisfy their energy requirements while foraging in agroecosystems.