The RSPB do a "kit" for £150. Colour camera by day. B&W Infrared camera by
night. Cable into house.
Oof, that is expensive. And requires setting up and hoping you get an occupant - can't be fitted to an existing box. (Not that you would if it was occupied anyway, but might be good if there was a box that's regularly used which you can adapt during winter)
Then there's wired and wireless IP based CCTV. A wireless IP camera is cheap, but oh my, isn't there a lot of complete dross of insecure, unreliable and incompatible stuff in this sector? Those can stream to your phone and computer (some only do phone, really - do beware when buying these, it's one of the hardest areas to get a good value camera in)
The RSPB version is obviously going to cost a lot, but I can see why it would appear. Nice, simple, turnkey house. I'm no fan of the RSPB but if you've got the money and don't want the faff, why not?