a Photographer's Dilemma -- suggestions requested

I've signed up for a photography workshop in Churchill, Manitoba, Canada for June. It has already turned out to be much more expensive than I expected, due to flight costs. The instructor has encouraged me to buy or rent a 500mm or 600mm lens so that I can get good photos of the wildlife we'll encounter. Those lenses weigh 10-11 pounds and cost several thousand dollars. To rent one costs about $700 for a couple of weeks. I found descriptions of a 400mm f/2.8 lens that is a bit cheaper and can be used with a teleconverter to get the reach of the bigger lenses. But it's still super heavy. I'm not a very big person, and the idea of lugging around all this equipment seems intimidating. However, I don't want to go on the trip with an inferior lens and be unable to get decent photos. That would seem like wasting the investment I've already made.

What do you recommend?

+photography +cameras´╗┐
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