Response of specialized birds to reed-bed aging in a Mediterranean wetland: Significant changes in bird biomass after two decades

Traditional fish farming carried out in wetland is declining in many countries of Mediterranean Europe. This decline can lead to a lack of management of the reeds that tend to age progressively. In this work we compared, through a wide temporal range (2001-2019), the densities of four habitat-specialized birds (warblers), strictly linked to Phragmites australis reed-beds in a coastal wetland on the Tyrrhenian central Italy. In this wetland, following the abandonment of fish farming, the average density of reeds significantly decreased, and both the average reed diameter and habitat heterogeneity showed a significant increase. Comparing 2001 to 2019, we observed an increase in the total density of breeding warblers. The two species of Acrocephalus ( scirpaceus and arundinaceus ), and Cettia cetti showed a marked increase in density, while Cisticola juncidis showed a clear decrease. More particularly, a significant increase in Cettia cetti ( p < 0.001) and a decrease in Cisticola juncidis ( p < 0.05) emerged when comparing bird biomasses. Species diversity and evenness were more high in 2019 than in 2001. Our data suggest that: (i) these species could be considered indicators of long-term reed-bed changes and (ii) their biomass may be used as a more effective metric when compared to abundance.