The individual posting this video speculates that it could be a strangely shaped balloon, but the classic hat shape with the tight brim around the bottom edge would make it a strange balloon indeed. Interesting that the comments (in Dutch) are so civilized compared to the ones that appear on American UFO videos.
So this isn't a bird then?
When I freeze this clip at 14 seconds, I notice what appears to be a line or appendage extending from the bottom of the object. When I continue with the clip, it moves with the object.
...still in the 'it's a bird' frame of mind, I thought the streak might be the bird having a poop (!) after seeing a large predator approaching (i.e. the plane taking the film)...but after looking at the video from frame to frame at the 14 second point, I am not too sure...