The spotting scope is a nice handy tool.
But for your application I think you need a variable focal length. In my personal opinion, the iPhone or any other mobile phone is not the tool for the job. It might do in certain circumstances but usually, with windows, you are far away and so the iPhone is not going to do the job most of the time.
If you're really far away then you're basically down the road of a proper DSLR and a really nice long lens - say a 600mm with x2 doubler for backup.
Good thing with a DSLR (mirrorless, these days) is you can swap out the lens for a shorter one if you're not that far away. And the quality will be 100%. You can even set it up on a nice tripod and set it going for the night, visit back every time you get a beer to check it's still recording.
A good, much cheaper alternative to that would be a SONY handicam or similar device. But you won't be getting the zoom required for really long distance with that. Some of them have night modes though which aren't really good enough for the pitch black dark but will certainly light up a dimly lit window very well.
Another alternative - again for the not so far away scenario - is a raspberry pi with the HQ camera and the lenses you can get for it. You can also buy a mount that allows you to fit the Rpi to the back and the lens to the front, so that you basically have a proper camera. The HQ camera comes with a tripod mount as well.
The RPI quality will be OK if the right lens is used, but definitely not as good as the handicam or the DSLR. But one advantage of the Rpi is you can use a tool called "motion" which will allow you to set it up to only take footage when motion is detected in the window. So if your install location is out of sight, this would be a fire and forget option for the entire night. And in the Morning you just review what it caught.
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