3rd party teleconverters

I recently purchased the Canon 400mm 5.6 and am just getting started with avian photography. I would eventually like to get a 1.4x teleconverter. A photo friend has recommended looking into 3rd party teleconverters such as Tamron or Kenko that will allow autofocus. I am starting with mostly BIF handheld, but don't know where all will lead and well equipped with tripod and ballhead from my landscape work, as needed. Any general or specific thought would be helpful.

Roger. Adding a 1.4 converter will make your lens into an f/8 lens. It may still focus with a 3rd party converter, but it will likely be very slow. Trying to do birds in flight will be made even more difficult. I'd suggest seeing how it goes with your 400 f/5.6 and cropping rather than adding a converter.

The extra 160 effective millimeters gained with the converter may not be worth the loss in speed in autofocus.

Keith, thanks for the good thoughts. To clarify, if it matters,I was thinking of using the teleconverter as more of a supplement, and would still be using the 400mm 5.6 as my basic lens. I know the loss of a stop and slow auto focus would be a limiting factor for general use.

Roger, I use the Kenko 1.4 with both my Canon lenes and my Sigma 150-500. Most of the time the auto features will continue to work. As you indicated, I only use the extender to suppliment, not as a standard set up.