I'd bet those are birds. The fact they can only be seen through heat detecting infrared lens is a good indication. I've never seen nocturnal birds through an infrared system, but have seen them through NVGs. Their flight patterns were similar to what I see here.
Would be interesting to contact a birder group or ornithologist in Kielce City to see what night birds, if any, are native to the area. I just found an ornithologist who lives in Warsaw and leads birdwatching groups throughout that part of Poland. I'll send him an email, with the video attached, and get his opinion.