Diet composition of syntopically breeding falcon species Falco vespertinus and Falco tinnunculus in south-western Slovakia

Abstract The red-footed falcon and Eurasian falcon represent two syntopical falcon species. While the Eurasian falcon is considered a common and numerous species in Slovakia, the red-footed falcon population has undergone a considerable decline during the past few decades. Nowadays it nests in a single locality in Slovakia, the Sysľovské polia Special Protection Area, which forms the northern and fragmented border of the species distribution area in Europe. By analysing prey remains from 9 nests (from 1998, 2001, 2013, 2014 and 2016), we identified 433 prey items belonging to 35 taxa and 9 orders. Every year, invertebrates made up the major part of the diet spectrum, in which Calosoma auropunctatum, Tettigonia viridissima, Zabrus tenebrioides, Anisoplia aegetum and Rhizotrogus sp. were the most frequent species of prey. Of the vertebrates, Microtus arvalis was the most hunted prey species. By supplementary analysis of 21 photos, we extended our knowledge on the diet by other 6 taxa. The peak of the M. arvalis population growth in 2014 did not manifest itself in the red-footed falcon diet composition. In 1998, 2014 and 2016 we also studied the diet of a syntopical species, the Eurasian kestrel. By analysing prey remains in 22 nests, we identified 1,151 prey items belonging to 37 taxa and 7 orders. In 1998 and 2014 vertebrates predominated, especially the common vole, however in 2016 invertebrates prevailed. This fact could be a reaction to the M. arvalis population peak in 2014 and its decline in 2016. These results suggest that this variability in the foraging behaviour of the Eurasian kestrel, an opportunistic predator, during the hunting of invertebrates increases the diet similarity and overlapping of the food niche of both studied falcon species.