Video cameras shoot at a max resolution of 1920px X 1080px so most dslr cameras down sample the image. If you have a camera and take a picture and zoom in and it is sharp that is one thing, IT IS NOT THE SAME AS VIDEO. Unfortunately there is no cheap way to get there from here. I have a nikon d800 which is a full frame dslr for stills and video Nikon D800 Digital SLR Camera (Body Only) 25480 B&H Photo Video
about $3000 which will shoot hd video but your big investment is the glass. Think about what a good spotting scope goes for a good large telephoto like nikon or cannon 500mm fixed 2.8 is $8000-$10000.
Nikon AF-S NIKKOR 600mm f/4G ED VR AF Lens 2173 B&H Photo Video
I would recommend finding a spotting scope your camera can mount to. I believe this is how they shoot most of the long range stuff. A telephoto that large is very expensive and has limited use.
If it was me I would get a
-Panasonic lumix GH3 $1000 +/-
-a novoflex adapter for nikon or cannon glass $350+/-
- buy a good spotting scope with a lens mount attachment
- I would search ebay for used manual telephotos (you don't want auto focus anyway)
-you can get a doubler which will double your magnification but low light performance will suffer
I shoot video and stills for a living if you want more description and advice pm me