Out of curiosity, what lens are you using?
You probably have two or three distinct issues with photography through a window. Several have been mentioned, but to summarise:
1. The glass: optical flatness (or as close as poss) will help, but it's not the biggest issue by far. I asked about your lens:
assuming it's being used wide open (or close to that), the larger the aperture, the larger the front element, and the less that any one part of the glass (and the lens) contributes to the final image. So you can get images "through" solid objects such as chain link fencing or twiggy bits on trees, if the point of focus is way away from the obstruction. Obviously this doesn't work with twigs your target bird is perching on, as those'll be in focus, but wildlife photography is possible through netting, etc. This works with glass too, but only to an extent.
You want flatness, but also thinness in the glass, to keep the light transmission as high as possible, it also helps to cut down the obviousness of multiple reflections (see below). You also want the lens as close as possible to the glass, so an individual blemish only affects a proportion of the light from object to sensor, not all of it.
2. Reflections: You get these from bloomin' everything and they are probably the worst issue photographing indoors-to-outdoors. The lens, the camera, and anything behind those will be reflected in the glass. If they net-out to be the same distance away as your intended point of focus, you'll get a sharp image of them too. For example, if your bird table is eight feet from the house, the focal distance is probably about nine or ten feet: anything at that point in the room, if bright enough, will also be thrown into sharp focus. In Victorian theatre, it was how they did "Pepper's Ghost", but that's no help here!
And, of course you probably also get two reflections, one each from the front and back surfaces of the glass (the nearest surface is the brightest by far). If it's double-glazed, there will be four. If looking into the sun, or towards any other bright point of light, you also have internal reflections of the lens itself to consider. For that reason I take off the protective "UV" filter that lives on the front of the lens, as it's two fewer air-glass boundaries to worry about.
How do you mitigate these? The only practical (impractical?) way is to stop the reflections happening. So fix up some black cloth around the lens (so you can still move it about), that goes up to the frame of the pane you're using and is fixed round it.
Get the front of the lens as close to the glass as possible,and make sure both inside and out are as spotless as possible. Tape-up any shiny bits of lens and camera. You might also contrive something like a cap peak sticking out from that pane, to stop light falling just inside (onto the camera gear). You see this idea used on surveillance cameras of different types, and it also has the advantage of keeping rain off.
It's worth mentioning here, too, that out-of-focus reflections look like lens flare or muck on the window - they will degrade the image, mainly making it low-contrast, and unless you do some experiments you may be puzzled as to why!
Finally, aperture: I mentioned this in passing. You can't beat a good, fast, prime lens in this context. It doesn't need to be Canon and it doesn't need to be posh, either. For years I used a truly ancient Tamron 400mm drainpipe with a three-element lens (the-back end was a doublet). Canon have the widest lens mount in the business*, which leaves room for adaptors to other brands of lens. Watch out for stuff sticking out from the back of the lens into the mirror box though. I've already dinged the mirror mount on my 6D by forgetting to remove the stop-down lever from a Pentax lens! But on an APS sensor there is usually sufficient clearance for most lenses - there's a table on the Net somewhere that I found, to save you causing damage by experimentation.
With a brand-converting mount adaptor you lose infinity focus, and obviously the lens system becomes fully manual, but that shouldn't be an issue in this context. The longer the focal length, the less the infinity-focus thing matters. It's hardly noticeable on my 400mm lenses, and if you add on doublers, etc., the issue goes away for all practical purposes.
You do, however make the mount mechanically looser, so supporting the lens-camera at both ends is sensible. I have a long-lens support, that has a 1/4" tripod thread at one end for the camera and an adjustable-height "Y" at the other for the front of the lens. There's a row of 1/4" threaded holes at the rough centre of gravity of the system. It works very well, but it's heavy and prone to losing bits when you travel with it, but I can't complain: it was made by the mechanical engineering workshop that used to build bespoke camera hardware for the BBC Nat. Hist. Unit
. It's aluminium and heavy - any nice hardwood would work just as well...
So, if you have specific target spots to aim at, the above will be helpful. If you want to hand-hold and just point+shoot, you may continue to be disappointed.
Hope that helps. I'm only a novice at all this though, and I've seen some really nice wildlife stuff on this forum in the past - more knowledgeable people will be along soon, I'm sure.
Here are three of mine from a day out with the kids (and grandkid!) last November:
https://photos.app.goo.gl/UjmD3UiAEcMf1CZTA
Note the issue with the Osprey pics - even with a reasonably fast lens (300mm @ f/4), you can't separate the bird from the background very well (it was across a lake around 200ft away). The perched shot is OK, but the just-taken-off one is too cluttered, IMHO. A longer lens and/or a wider aperture would help, but both would make handheld operation impossible. I used a monopod for the Osprey, but the Egret is handheld and a lot closer**. It looks decent mostly because the background is lake water in shadow, so doesn't distract the eye - happenstance, not skill!
Have fun, E.
PS: The mouse in my avatar was taken about eight feet from the dining room window (closed), near dusk. It's got many of the issues discussed above ;-)
*dunno about the new "mirrorless" mount - changing the mount is a sales lock-in gimmic, IMHO.
**It wasn't at all bothered by park visitors - it had seen a frog, I think!
