Following the edge of the oak woods, you suddenly realize that you have been hearing a bird song without even noticing it. The song is a continuous series of widely separated rough phrases and it seems to be coming from high up in the oak branches. You catch a hint of movement up there, but it takes a while before you can find the bird with your binoculars. All you can see of it is the bright yellow breast as it slowly feeds among the leaves. What kind of bird is it?