Urban waste-water treatment plants as hotspots for birds : an environmental assessment highlights the role of a single dominant gull

Waste-water treatment plants (WWTPs) are still little explored in their ecological role. This work reports data obtained from standardized sampling for the two largest Italian WWTPs, to obtain seasonal patterns (late winter, spring, and autumn) of univariate diversity metrics in bird communities. The ecological conditions, linked above all to the high availability of water, with the presence of mud and trophic resources, and heterogeneous features (buildings, trees, and hedges), allow the presence of water-related and synanthropic bird species, using the site as a seasonal stopover and wintering sites. The large availability of biomass of invertebrates, linked to the sludge from the water-waste treatment plants, can provide an important trophic resource during spring migration. The richness, diversity and evenness did not differ significantly between the two plants analysed, both having comparable size and heterogeneity. The highest Simpson dominance values were recorded in autumn with species frequency concentrated in a few abundant species. Detrended Correspondence Analysis (DCA) shows a close association between the autumn period and the dominant Black-headed Gull (Chroicocephalus ridibundus), in both treatment plants, with possible implication on the spreading on zoonosis.