Question for the 400 f5.6 owners

What do you consider a maximum distance to render best detail on particularly small, and then medium size birds with the 400 f5.6 L what is the ideal range? thanks

I use mine always with my 1.4 extender fittted.
Depending on the size of the bird anything over 50/70 meters
(165/230 ft) is not ususally worth taking, unless just for bird sighting record purposes.
But I will be interested in others comments here

I found that anything over 30 feet almost wasn't worth the effort for Robin sized birds or smaller. Eagles in flight are good out to about 100 yards. I never had any luck with the teleconverters. Even the 1.4x reduced image quality terribly and the 2x was worthless. Others get good results, but I found the canon brand a terrible waste of money. I got more actual use from a set of cheap Kenko tubes than ever the TC's. A set of tubes will magnify the image quite a bit and you keep auto focus, but they won't work at maximum focus distance but will lower the minimum to about 3.5 feet instead of 12.

The trick is to use a hide, such as a hunting tent or your car and shoot out the window. The car window has been one of my most productive shooting positions to date.

If you can, set up your own yard with feeders and perching sticks to bring the local birds to you. Plan the angles of sunlight, shooting position from a window or buy a little tent hide for the purpose.

Agree about Mitch's appraisal of the tc's being worthless. Shots are blurry. Period.
With a tripod and good lighting, you can get fair detail out to 50'. And sometimes you get lucky and can get soaring hawks way higher than 50'.
Karen

Thanks for confirming that everyone I just wanted to know the lens's capability and make sure it's not me or the lens/camera

All lenses have a break off point. While you could shoot the moon with any of them, looking at the moon, it's bigger than a small bird at 30 or 40 feet. You can shoot small birds way out there, but you would have to crop the image so much to expand the bird that the quality would degrade to the point it makes no sense to do it. Get close, or entice the birds closer to you.