Occasionally, you have people with 300 f/4 or equivalent (100-400, 70-200 with TC etc) complaining that their lens is too short for photographing birds.
My own take is that 300 f/4 focal length is adequate for quite a few birds especially with the 1.6x FLM. Also remember that the focal length of 10D + 300 f/4 + 2xTC is LONGER than that of 1D+500 f/4+1.4x TC. Of course, the latter will give you better image quality.
I've photographed lots of species with 300 f/4, not just Sunbirds. The above 5 pics are a few sample pics. By current standards not fantastic, but I still like them and find the quality ok. If you cannot photograph birds with a 300 f/4 equivalent focal length (as mentioned earlier, 10D+300+2x TC is actually longer than 1D + 500 f/4 + 1.4x), are you sure that getting a longer lens will help?
I salute clubsnappers like Madcat who push the limits of their equipment (sigma 50-500) rather than complain 'my lens not long enough'
p.s. The first 4 are taken with a Nikkor and the last with a Canon.