Be wary of Sigma lenses which you may be tempted to buy as they are cheaper than sony's lens. They can be great. But just a word of caution.
I am just getting my lens back from warehouse express because it had to be repaired.
The sigma lens drum motor or whatever, runs too fast for a Sony ( I have the A300) which results in the camera vibrating, making one heck of a noise. Not good for the camera.
Warehouse say they have fixed the lens, drum. So I will let you know how that goes once I get it back.
Does your sony have just the standard lens or did you get a lens specifically for the type of pictures you are going to take like wildlife.
I find I have to pretty close to the subject before getting a decent shot with the standard lens, the sports type lens gives you better clarity at a longer distance.
But it's always best to stick to "rule of thimb" as they say.
Taking pics from a good distance wont give good results unless you have a very expensive zoom lens.
Don't want to put you off, but make sure you get a good lens and although it costs more, it's best to stick to the camera's brand.