The 400mm focal length permitted close enough working distances to avoid obstructions, and that focal length combined with the f/2.8 aperture blurred the abundant foreground and background distractions.
Then, this barred owl showed up. Obviously, owls are much smaller than deer, and suddenly, significantly more focal length was needed.
Fortunately, the friend I was shooting with solved that problem via a pair of extenders in his pocket. He opted to use the 1.4x on his lens, kindly loaning me the 2x.
The capability to go too 800mm was indispensable in this case.
After getting sharp insurance shots at faster shutter speeds, the exposure time was increased, decreasing the ISO setting for less noise. The keeper rate dropped, but only 1 sharp image was necessary, and the results surpassed that requirement.
A larger version of this image is available here.