I used my 300mm F/2.8 permanently coupled up with the 1.7x Teleconverter, giving a useful 500mm focal length at maximum aperture of F/4.8.
This adult male Pallid Harrier was stumbled upon at a reservoir knee deep in water, bathing. It then flew to this path where we drove up to it. We were about 25 yards away as it continued to preen for some ten minutes. The smallest image at the bottom of the page is the original size while the others are cropped and zoomed in.