imo the 7d is a better birding camera overall than the mk3, the mk3 does feel as though its working faster and it might be, but the 7d does not lag behind when it comes to getting the shot .
iv yet to say when using the 7d that i had wished id had the mk3 with me instead.
Rob.
The 7D is an excellent camera in its own right but I would not compare it 'directly' with the MK3 or MK4. If one thinks a $5000 camera should take 'better' photos than a $1,700 camera by a factor of 2.94, one would be bitterly disappointed. After a certain level of features and build a camera possesses, 'additional' performance come in SMALL increments and the skill of the user is paramount in bringing out the perceived advantages. I have stated, in my opinion, the 7D is the best value in Canon's current lineup of digital DSLR, but for me, it is not a competitor to the MK3 or MK4. I bring both on most of my outings and use them as appropriate.
The difference between the MKs'3 and MK4's performance is even tougher to appreciate. I have used the MK3 extensively for 2 1/2 years but am comfortable to make a comparison between the two after putting the MK4 through some grueling conditions. I do not use Live View or the HD Movie modes, so I will not comment on them. The difference for me is almost indiscernible except for the 45 active AF points in the MK4. It is very helpful to be able to shift from the left/right to 16 points to 45 points in a couple of seconds while still panning the camera to chase wildlife as the conditions change. For me, it meant the difference between getting a better focused image vs. a slightly blurred one. The increased pixel count and slightly higher quality help in cropping images when the wildlife is far away.
If I own a MK2 N or if I can sell my MK3, I'd buy the MK4 in a flash. If I cannot sell the MK3 and do not need/want the HD Movie mode, I'd wait for a good, used MK4 to come on the market.
I used the 70-200mm f/2.8 L IS and 400 DO lenses for the posted photos. Extenders were used except on the hummers. The hummers were shot with f/2.8 @ ISO 3200 under dark, drizzly sky. Capuchin was darting swiftly between branches and leaves. Humpback Calf breached suddenly, out of nowhere. Gannet was bobbing up and down in the waves, to get away from the strong head winds. All challenging conditions for the MK4's AF to lock on and shoot at 10 fps and showoff its low light capability.