Using 2IS zoom - getting lack of definition

I'm using my 2IS zoom to the optical max for bird pictures. I seem to be losing some definition in the feathers. The day was cloudy, I tried ISO of 100 and 400 with speeds of 1000 to 1250. Is zooming to the optical max causing some loss of definition? I'm concerned if I get a teleconverter it will only magnify the problem. Any suggestions? Thanks.

You might have better luck in the
"Small Compact Digitals by Canon" section of the forum. This area mostly relates to DSLR's

Jon, The Elder wrote in post #3044951
This area mostly relates to DSLR's

???
Since when was the "Talk About Photography" section mostly about DSLRs? Or has a Mod already moved this post (wrongly) once?

Anyway, to timc, in situations like this a picture is worth 1000 words. Show us an example of one of your "problem" images, preferably with EXIF data intact, and you'll get lots more opinions.