Small birds are some of my favorite subjects. But to fill the frame, you have to get very close. One I took recently was of a flycatcher, at 560mm f/8 and about 3 meters. (crop body)
As probably just about anyone else here, I lust for the fast supertelephotos. With fast-moving subjects, a fast aperture certainly sounds attractive.
But it occurs to me, they might not be all that useful for the tiny guys. (Bear with me, I've never used one)
First, in the previously mentioned conditions, I calculate DoF (with a standard CoC) at 7mm. If I had an f/4 lens I would have been at f/5.6, with a DoF of 5mm, or the thickness of 3 nickels! You can't even have the whole head in focus with that little DoF. So, I wonder if I had such a lens, if I would be forced to stop down all the time anyway. (Optical sharpness would be superb, of course) The one possible mitigation I can think of here is smaller birds will probably be printed smaller, which increases the size of the CoC & therefore perceived DoF in the final print, thus affording the use of big apertures.
Second, I just noticed the MFD of the 400 DO II is 3.3 meters! The rest of the long supertelephotos all have similar max magnification factors, too. Smaller subjects + smaller prints allow cropping, of course. But I find myself thinking of the guy I ran into, visiting a vagrant warbler, who missed all his best shots because the subject came right up to his blind, way inside his MFD!
Of course, there's still plenty of advantages too. The aforementioned optical sharpness. Focusing speed/accuracy. The ability to use 2x TC's and retain AF. The option of sacrificing DoF when you'd otherwise have no shot due to weak light. Option of more background blur, many little birds like dense vegetation and often have busy backgrounds.
A long f/4 lens is not in my near future, but I got to wondering just how useful they actually are for little birds, and hoped to get your comments!