Siskin?

More ID assistance required, this time with this shot from yesterday afternoon in a beautifully sunny north Norfolk. I'm plumping for juvenile Siskin, but with very little confidence I'm right. Perhaps those who are much better at this than I am could enlighten me...

Canon 1Ds MkIII< Canon 100-400mm IS L lens, 1.640th sec, f/9, focal length: 400mm