For birds and anything at a distance Image Stabilization is a plus, unless a tripod is being used. A fast focusing lens is important 2.8 being best for low light but as you probably know the dollar factor always has something to do with it.Since birds are not always in ideal light conditions. My opinion is save the money to buy good glass makes all the difference.Nikon has great glass, if I had a Nikon camera I definitely would have Nikon glass.