I took these pictures of a Nutria from the bird blind at TRNWR back on Dec 18 with the rented 600mm lens. Not only does this lens have much more reach than my 70 - 300mm zoom, it also produces sharper images. My dear wife is starting to understand why I want to spend a ridiculous amount of money to get a lens like this!
Nutria are actually an invasive mammal brought here to Oregon from South American back in the 1930's to replace the Beaver when its numbers where diminishing from over trapping. They destroy a large amount of native plants and can damage ecosystems as they only consume about 10% of the plant, eating only the stems and leaving the foliage to die.