Digiscoping or telelens?
As mentioned before, digiscoping is making photographs through a telescope. There has been much discussion about 'digiscoping versus telelens photography'. I have no experience printing digiscoped photographs, but as far as I can see, digiscoped images for internet use can be as good as 'normal' bird images. An interesting article (test report of a comparison between a prime telelens and a Swarovski telescoop plus camera) is here. Advantage of digiscoping is the possibility to make photographs at much larger distances from the object, the bird.
To photograph birds with a telelens, you will have to be very close to the bird. That's why bird photographers often use small tents and/or camouflage netting and sit in the same spot for hours, waiting for the birds to arrive. I have done that a couple of times, but I don't like it much. It is also possible to photograph birds from your car. I sometimes do that. The car then functions as a mobile 'tent'.
I use the cargo compartment of my van as a mobile tent. I sit comfortably in a chair before the open side door. I place a camouflage net in the door opening so that birds cannot see me sitting in the car. Most of the time birds will fly away when a car stops close to them, so it's best to arrive when there's no bird closeby and wait till the birds come close enough to make a photograph.
Equivalent tele length
A 600 mm (35 mm equivalent) telelens does already count as a very long one. For digiscoping this is not long at all. To calculate the 35 mm equivalent tele length of a camera-telescope combination, use the formula: M*F*C, in which M is de telescopes magnification, F is the focal length of the camera lens and C is the crop factor of the camera. My Swarovski STX 95 mm telescope's minimum and maximum magnifications are 30x and 70x. Most of the time I use a 20 mm lens on my camera and the camera's crop factor is 2. So the minimum and maximum tele length equivalents are 1200 and 2800 mm respectively. When I use my 30 mm Sigma lens even 1800 - 4200 mm So 3 - 7 times the 600 mm telelens. That's an object magnification of 36 - 84 times (divide tele length by 50).
Drawback of digiscoping is, that it is - compared to telelens photography - much more difficult, or even impossible, to photograph fast moving birds, especially birds in flight, because a camera-telescope combination cannot be used hand-held. So a tripod is always necessary.
Calculating the distance to the bird
Especially when birds are far away it is difficult to measure the distance directly. The distance to the bird can be calculated with this spreadsheet
It is possible to use DSLR camera's for digiscoping, but most people use either a compact camera or a system or '4/3' camera. That's a mirrorless camera with exchangeable lenses. Examples are the Panasonic DMC GX1,G3/G5/G6, GH3, GH4, GX7 and other Panasonic camera's, the Sony Nex-6/7, the Nikon V-1 and the Olympus E-PM2 and other Olympus camera's. Examples of compact camera's used for digiscoping are the Sony RX100 and Sony RX100 II and the Nikon 8400 and Nikon P6000. Newer cameras like the Panasonic GX7 and GH4 have 'focus peaking', showing the spot(s) of best focus on the object when focusing manualy and autmatically. Especially handy for manual focusing. These are only a few examples and an internet search will give you many more results. Not only for the compacts but also for the other camera types.
Shutter shock is the vibration that occurs when the mechanical shutter of a mirrorless (Micro 4/3) camera closes. It's in a way comparable to 'mirror slap' of a DSLR camera and causes unsharp images. Shutter shock occurs with shutter speeds less then 1/200 seconds. So it can be a problem under low light conditions and at high magnifications (long tele- length equivalents). Some camera's have the possibility to use an electronic shutter instead of the mechanical, preventing shutter shock from occuring.
It's important to choose a good to very good telescope to get good results. They are relatively expensive, but still less expensive then a DSLR camera with a long telelens.