For all of you who might be skeptical to extenders. Look at Joel´s Red Bellied Woodpecker!
Thanks! Just as your post appeared I was finishing this image which was taken with the Version 1 500mm and the Version 3 2X teleconverter. This was with the 1DX at ISO 3200 f/8 1/320 sec. using a Gitzo tripod and Wimberly II Gimbal