Does anyone know what are these birds?
Looks like European Starlings (Sturnus vulgaris).  They are often seen in large numbers and are native to most of Europe, including Serbia.  They were introduced to North America (Central Park, New York City) as a project to introduce species of birds from works of Shakespeare and are now widespread.  



Source: https://www.allaboutbirds.org/guide/European_Starling/id