Brian, from the test report I have just read on the Sony mentioned by Robin it states it's excellent at 70 but still good at 300 but with a bit of corner edge drop off. You are not going to get a zoom which is 'best' at the long end.
I shoot my 80-400 at max 400mm 90% of the time and the only time the images are not sharp enough for web viewing it's due to me, not the lens.
The more you spend the better things will be but I'll suggest you need a minimum of 300mm max FL for those subjects and working distance but ideally 400mm if budget allows