hmmm for something that long you may to look at some of the old 4/3 offerings. I am not sure if there's any there but those are big lenses as well. May as well just keep using your 1DIV
There is a 300 2.8 that is the same size and price as a Canon 300 2.8 L, which would be 600mm with the crop factor. Don't need anything that elaborate. Even just a 300mm 5.6 prime with excellent autofocus, sharpness, and compact size would be a perfect travel birding setup.
I would love to get one of theses:
180-500mm equivalent, and it looks like a bargain compared to the 200-400 F4 L IS!
*edit* They did make a 50-200mm 2.8-3.5 zoom, that is about a pound lighter than Canon's 100-400 and a bit shorter, seems to be common used on Ebay for $500-600. Have to read how it performed.