As it happened, I ended up about 30 feet from the bird without time to take one of the extenders off. One extender would have been sufficient to get the bird nearly full frame, the image quality would have been better, I would have had a higher shutter speed and I would have had auto focus.
Sometimes it's better to stick with basics rather than using gimmicky set ups to gain an advantage. In any case, here is one of the shots, cropped only to exclude empty space to the left of the bird. Obvoiusly, shooting vertically would have been a better choice, but, I didn't want to take the time to re-orient the camera and maybe lose the shot altogether.
|Canon 50D, EF 300mm f/4L IS USM, f/8, 1/180 sec., ISO 800|