I'd get the 7D...
Except even with that there's no such thing as a "long enough" lens for birding! Even if you get a 1000mm lens some day, and use it on a crop camera, there will always be small subjects just out of reach. Those are the times to just put the camera down and enjoy the show... wait for your subjects to come closer.
The 400/5.6 is a fine lens, sharp with great color rendition and fast AF. But it won't give you more reach than your 300mm + 1.4X added (actually not quite as much: the zoom + TC is effectively 420mm equiv.) It also isn't stabilized and it's awfully nice to have IS on longer lenses. Of course you can add the 1.4X to the 400/5.6, for an effective 560mm f8... but good luck getting a steady shot with it! You'll probably want to use a tripod or at least a monopod most of the time.
I use 300/4 IS a lot, with and without 1.4X.... It's easily handheld. This is on 7D (at high ISO)....
And this is the same lens on 5D Mark II (lower ISO, but ridiculously high shutter speed because I had been shooting in shade when this hawk showed up, didn't have time to change my camera settings)...
One of the most affordable longer lenses is the Sigma 150-500 OS. It sells for $1000-1100 US new. It's a fairly large lens... about 10 inches long (at the shortest/when retracted at infinity focus), uses 86mm filters, and weighs about 4 lbs. But I mention it because, even with a 7D, shooting birds you will still find yourself wishing you had more lens at times. I guarantee it!
A 7D and/or a longer lens will only partially help with examples like you show... You still need to get closer, fill the viewfinder. You may need to use a blind, lots of patience, and/or some sort of attractants to bring the subject closer. Study your subjects and learn their habits. The hungry waxwing and hunting redtail hawk in the above photos let me get unusually close. I know the waxwings tend to come to a certain group of trees each Winter, feeding on the berries for a few days until there's no food left.
For water birds, some photographers use floating blinds. You also might look for birds acclimated to people being around, or work with them to acclimate them to your presence. I sometimes shoot in busy parks, on active piers, and other similar places where the birds largely ignore and tolerate us flightless critters...
Though sometimes the subject might get too close. This Canada goose in a local park was begging for handouts and bit my finger a moment later...
Alternatively, you can gradually accustom animals to your presence. I first saw a coyote regularly coming around my neighborhood in September or October last year. She was fairly accepting, but still a bit wary. By being quiet, parking and using my car as a blind, I was able to get in range to use the 300/4 and, in this case, the 300 + 1.4X...
Over several months, she got to know me and allowed me closer, never was the least bit threatened or threatening toward me. I'd wander out back, no coyote to be seen... Then I'd spot her a short distance away. Often when she saw me, she'd come trotting over to see what I was up to. At times she'd yawn, then lay down and nod off while I sat nearby. Patience paid off. Early in January I was able to photograph her with 135mm and even 70mmm focal lengths...
A few days later she disappeared from the neighborhood. I'm hoping she returned to the nearby hills looking for a boyfriend. After all, Spring is in the air!
Like has been said before get closer, also with a 7D you're not gonna get any extra reach just a cropped image.
You can do that in post and get the same magnification with the full frame.
Sorry, but that's not entirely true. With an 18MP 7D you get more "pixels on target" than you would cropping down an image from a 21/22MP full frame camera. The 7D has more than twice as many pixel sites per square millimeter as the FF cameras. If you do the math you'll find it doesn't work out the same as the lens factor (1.6X). It's more like a 1.3X teleconverter effect with the APS-C camera... "Free" teleconverter (i.e., no loss of light, as will occur with an actual teleconverter).
There is some "reach" advantage to using the 1.6X crop camera. It's why I keep a pair of 7Ds and use them alongside a FF camera (5DII in my case, so it's also a question of focus with action vs stationary subjects, and the crop cameras get about 10X as much use as the full frame!)
Even with long lenses and a crop camera, you still need to get as close as possible and try to fill the viewfinder, to get the best possible image. A flash with an Xtender (i.e. Better Beamer) can be a valuable addition, too.
