Low-light Telephoto for Alaska Bears

I'm going to the Katmai Peninsula in July to photograph brown bears at Lake Clark NP, and am concerned about lenses. I own a Tamron 200-500 f/5-6.3, but am very concerned about it during overcast/rainy days and early and late in the day, when brown bears are usually more active.

The Tamron isn't a bad lens for under $800, but it's pretty slow. I don't want to weight myself down with a Canon 400mm f/2.8 IS or some other comparable "anchor." As a trade-off, since I'll be using a good tripod and ball head, which of the following (if any) would do you recommend?

Canon 300mm f/4 IS
Canon 100-400 f/4.5 IS
Sigma 120-300 f/2.8

I'll also have a 1.4x extender with me. I'll be shooting a Rebel XT and a Rebel XTi. My other lenses will be a Sigma 10-20 f3.5 and a Tamron 18-250.


What would y'all recommend? I can always rent.

Never mind. I bought the Canon 300mm f/4L IS.

Seems like a great choice. Won't you be killing your lens speed with that extender though??

Amy

Sure, by one stop....but I'll only use it when absolutely necessary. I can always up the ISO to compensate early and late. During mid-day hours, it shouldn't be a problem....I hope.