The occurrence and density of three sympatric corvids in a Mediterranean agroecosystem explained by land use

Agroecosystems harbour common species besides farmland birds, which are often understudied because they are not appealing to researchers. However, common species are fundamental to the structure of most ecosystems, so if the preservation of an overall pattern of species richness is a valid conservation goal, we need to improve our knowledge of them. Corvids are some of the most common species in agroecosystems, and are considered in some countries to be farmland birds. The aims of this research were to estimate the densities, assess the habitat selection, and measure the niche overlap of three corvid species, the Eurasian Jay (Garrulus glandarius), the Eurasian Magpie (Pica pica), and the Hooded Crow (Corvus cornix), in a Mediterranean agroecosystem. To collect data, a stratified random sampling design was used; 264 linear transects were walked during the winter, and 301 point counts carried out during the breeding season. The densities of the three species were estimated through the distance sampling method, whereas the evaluation of habitat selection was performed by a binary logistic regression following an information-theoretic approach. To define the niche overlap of the three species both during the winter and the breeding season, the Hurlbert index of niche overlap was used. The results showed that, in the Mediterranean agroecosystem, the most abundant species is the Eurasian Magpie, which occurs in built-up areas at lower densities than in the countryside. The Eurasian Jay typically selects woodland during the breeding season, but can shift its niche to agricultural land. The estimated density for Hooded Crows was higher here than those recorded in other climatic regions, even though the scarce presence of open water has been described as a limiting factor for this species. Finally, our results showed that there is some overlap in niche between these three species. In particular, the niche of the Eurasian Jay and Hooded Crow overlap both during the winter and during the breeding season, whereas the Eurasian Jay niche overlaps with that of the Eurasian Magpie in the winter, which in turn overlaps with that of the Hooded Crow during the breeding season.