Five decades of breeding populations census for 12 species of colonial waterbirds in northwestern Italy

Abstract

Colonial waterbirds, a major biodiversity element occurring in the core of ultra-anthropized Europe, are ideal indicators of the wellness of inland wetlands. Nonetheless, there is a critical knowledge gap in their trend and population status. We present an uninterrupted 47 years-long dataset of the breeding populations of 12 species of colonial waterbirds (Ardeidae, Phalacrocoracidae, Plataleidae, Threskiornitidae) throughout a 58,000 km² agricultural region in the higher Po basin (NW Italy). A trained team of collaborators censused with standardized field techniques the number of nests of each species at 419 colonies in the 1972-2018 period, summing up a total of 236,316 records. Data cleaning and standardization were performed for each census year, ensuring robust and consistent data. This dataset is among the largest ever collected for a guild of European vertebrates. It has already been used to describe the factors influencing population trends, and still offers opportunities to explore a wide range of key ecological processes such as biological invasions, global change consequences and biodiversity impact of agricultural practices.

Fig. 1

Study area in NW Italy where all the colony sites were monitored from 1972 to 2018. Dots colors group the colonies in the “Paddies” (yellow), in the “Rivers” (grey) and in the “Uplands” (red) sectors. Dot size classifies the colonies by the average number of nests over the entire monitoring period (small: <50 nests, medium: 51–250, large: >250). The hydrological network is visible on the map, with the main river crossing the area from west to east being the Po. Black contours represent Regional borders, white areas are non-Italian territories. Elevation model from.

Fig. 2

Examples of potentialities of the dataset. (a) comparison of population trends among Grey Herons (grey line) and Black-crowned Night-Herons (purple line). (b) comparison of population trends of A. cinerea over the three different sectors of the study area. (c) spatial expansion of Grey Herons over the study period; dots show the location of all the active colonies, and dot colors correspond to sectors as in Fig. 1.

Fig. 3

View of an area of intensive rice cultivation in in the ‘paddies’ sector. In 2001 all the cultivations were rice paddies with different levels of irrigation. The wood in the center hosted a mixed heronry during the whole period of monitoring and has been protected as Nature Reserve since 1985.

Fig. 4

Proportion of colonies where the number of nests was estimated during the study period. (a) proportion of the active colonies where the number of nests was estimated (deep red), ad where only the occurrence of each species was reported (orange). Complete censuses (100% of active colonies exhaustively censused) were realized in the years 1981, 1985, 1986 and 2002. (b) yearly use of the different census techniques (see Methods) among the colonies in which nest counts were performed.

Fig. 5

Example of an aerial photo used for nest counts. Monospecific colony with 25 nests of Grey Herons at egg hatching stage, on trees (Alder Alnus glutinosa).