Travelling and hiking, it's hard to beat the 100-400 IS. And the 7D is an ideal companion for it. (I certainly wouldn't give it up for a full frame camera with that lens!)
Any prime lens of equal or longer focal length, you'd likely want a tripod or at least a monopod.... and the big, fast super teles are more of a chore to haul around, too.
You might be best served by learning some stalking skills, investing in some camo clothing and/or using some sort of attractant to get the birds to come to you.... and just using the lens you already have.
The most hand-holdable alternatives are....
Canon 300/4 IS with a 1.4X teleconverter. Though it's a good combo, this would not give you significantly more reach or any faster aperture. It might be less convenient, too, than a zoom. Only two focal lengths: 300mm, plus 420mm when combined with the TC.
Canon 400/5.6. One focal length. Can't use with a TC, unless you give up autofocus or use some work-around, but then will still be limited by slower focus in good light. Nice and sharp, but no IS, so harder to handhold. You'll need to keep to 1/640 or faster shutter speeds.
Sigma 150-500 OS... I'd like to try this lens sometime... I'm not a fan of the push-pull zoom design of the Canon 100-400... but the image quality doesn't appear as good. Other comparisons I've seen (aside from that link above) have been more favorable. It would give you 100mm more reach, but is larger and heavier. Technically it's not usable with a teleconverter, either.
Sigma 120-300/2.8 OS with a 1.4X and/or 2X teleconverter. This is a whole lot more expensive (I'm guessing it's out of the budget)... and bigger, heavier. Quite versatile, though a monopod at least might be useful.
Sigma 120-400 OS.... no gain compared to the 100-400. It's a lot less expensive, about the same size and some think Sigma's OS is at least as good, maybe even better than the IS on some of Canon's lenses (for example, the 100-400 has an older, less capable version of IS).
There simply is no such thing as a "long enough" lens when shooting birds. If you have a 300mm, you'll want 400mm. If you have 400mm, there will be times you'll wish it were a 500mm. Got a 500mm? Well time to start saving up for a 600mm or 800mm!
And, with each increase in focal length, it's harder to get a steady shot, the more you might want to use some sort of support... Plus you are shooting through more atmosphere, which can have negative effects on image quality at times.
So, you might want to just use what you've got and work to get closer to your subjects. Some will always be just out of reach... maybe those are the times to put down the camera and just enjoy the moment.
