I think these shots look as striking in monochrome as they did in color. Robin's tend to be fairly easy to photograph as they will sit still for longer periods and if you don't move to quickly they wont fly off right away like many other birds. I have watched them looking very poised, observing their surroundings, taking the time to consider their next move and its a great look for photos.
Nikon D750 1/500 sec. f/5.6 ISO-200 at 500mm
Nikon D750 1/500 sec. f/5.6 ISO-200 at 500mm
Nikon D750 1/200 sec. f/5.6 ISO-200 at 500mm
Nikon D750 1/320 sec. f/5.6 ISO-200 at 500mm