G'day Sharnee.
Why do you want that macro? Would a longer lens help you get into birds more?

I have no idea about the Sigma 105mm 2.8 macro. If you don't get to many replies about that lens post again in Gear Talk ....... http://www.ausphotography.net.au/for...php?120-Lenses. While it's been a bit quiet there some will help.