Will
first things first. It seems you are unsure if your lens is front or back focusing in relation to your camera. You need to know what the problem is before you can comment on its sharpness, so I suggest you do a test or get someone with experience to do a test for you and that person should have experience of looking at images at 100% when viewed on a monitor. You may be comparing your images to those on the net that have been processed exceedingly well and thinking your lens is not up to scratch when in fact your lens may be just fine.
If you are considering other lenses you need to determine if you need a zoom. If you need a zoom then other lenses should not feature in your considerations. If a zoom is what you need then a zoom is what you should get. Photographers choose zooms for reasons of composition and are confident that they will get close enough to the subject at the varying focal lengths the zoom can offer. There may be a small depreciation in image quality compared to an equivelant prime but that should not be a deal breaker.
If you are cropping heavily regularly then perhaps you should consider a prime. Once you have cleared up these issues then you can consider other specific lenses.
It would be useful to know what environments you photograph in and how close the birds are that you photograph.
A few images posted of what you have done recently may also help a little.
As your questions stands you are likely to get replies that "this lens is best" or "that lens is best" and whilst these replies may be true for those individuals they will be utterly meaningless without knowing the whole context of your requirements, abilities with the camera , your fieldcraft skills, your processing abilities and your general aspirations.
All of the lenses you have suggested are good lenses....it is a matter of what is best for you
Finally, I think there is a lot to be said in mastering what you have before moving on and spending hard earned cash on better lenses. There are images in the galleries and on the net elsewhere taken with bridge type cameras that exceed what many produce with top Canon and Nikon gear. The truth is, at the end of the day, it's the photographer that makes the biggest difference. A good photographer will maximise the potential of what he or she has and blow others out of the water with good planning and skill.
