Hi Renate For this conditions exposure is the key Probably more than with any other !!!!
On heavy overcast you need to overexpose but close to two or more It will give great detail in the bird and blow the bg They look good !!! Would also suggest bumping up the ISO The Mk3 does well with noise Stopping down a lot more would have giving you more dof !!!
When composing I would try the first image at a wide open aperture Look to see what is in focus What you see in the viewfinder (in focus) will also be in the final image Then stop down for an alternative !!!! I like this image a lot !!!