The Eurasian Jay (Garrulus glandarius) is considered as the most important factor in the dispersal and spread of oak species. We conducted studies in oak stands in four countries (Poland, Hungary, Romania, Ukraine) in the autumn of 2015 and 2016. To identify the preferences of Jays for both acorns and trees, we compared the size of acorns and tree characteristics between each selected tree and the closest unused oak. We found that acorns from selected oaks were smaller (narrower) than those from unused trees. We found no differences in the characteristics of selected and unused oaks. These results indicate that the size of acorns can be an important indicator determining the choices of foraging birds. The Jays’ preferences for specific trees may influence the composition of oak populations. Trees with certain phenotypic and genomic characteristics may be favoured and dominate in the ecosystem.
