Blackcap - Sylvia atricapilla
For this bird my ID is 100% by song, appearance and location. They are common in our area.
This one was seen in Heverleebos in April, 2016.
I have other pics where you can see its black head but I am not posting them unless asked because they are of lower quality.

The Blackcap is a common and widespread sylviid warbler which breeds throughout temperate Europe, western Asia and northwestern Africa, and winters from northwestern Europe south to tropical Africa. Its colour pattern is unique in the genus "Sylvia"; the Blackcap's closest living relative is the Garden Warbler which looks different but has a fairly similar song.