When going out into the field I try to travel light -- either the Canon 20D with the 400mm lens on a tripod w/1.4x & 2.0x teleconverters - or - the Canon XTi with the 300mm lens + the 1.4x teleconverter. If, instead of walking, I'll be doing more driving, the, of course, I will take both bodies and even add the 70-200mm lens. Springbrook is for walking - so last week I only had the 300mm lens with me. Of course, that is when I wish I had the longer lens, such as in this instance when I came across these two Green Herons on the opposite side of the pond. I did put the 1.4x on, but I sure wish I'd had the 400mm instead.
To try to solve this predicament I bought a medium-size backback that will hold the other body & both of the extra lenses, but little else. It's quite a bit of weight to carry, but at least I'll have the right lens with me.