We have 2 of these window mounted bird feeders and we love them!
https://www.amazon.com/Aspects-ASP155-Window-Feeder-Variety/dp/B000CSJ1VE
It takes a while for the birds to notice them but once they start coming it's amazing. We get pretty cardinals and finches.
We use safflower seed that doesn't attract squirrels. The bottom part slides out easily to refill without having to remount the suction cups.
Good luck!
You're saying if it's too big they'll poop in the food? So this one would be a bad idea? https://www.amazon.com/Aspects-ASP155-Window-Feeder-Variety/dp/B000CSJ1VE/ref=sr_1_1?s=lawn-garden&ie=UTF8&qid=1510712313&sr=1-1&keywords=Aspects+ASP155&dpID=518BuRKUMML&preST=_SX300_QL70_&dpSrc=srch
I've had a 'Window Café' for about 5 years; I lived on the East coast of Canada for the first couple of those years (usually about -5C, sometimes -10C in the winter) and I don't think it's ever fallen off, even staying attached during a hurricane or two. The roof is secured by 3 suction cups and the reservoir/trays slide out for easy cleaning & refilling.
Be sure to clean the part of the window thoroughly before you stick it on. Make sure the suction cups are clean, too. Rub each cup with your finger for a minute or so to transfer oils from your finger, which helps it stick longer and stay stuck. I've also read you can rub the side of your nose to get more oils, too. If you've been missing any of those steps, then doing them should help any suction cup stay stuck for much longer.
I have this one:
https://www.amazon.com/Aspects-ASP155-Window-Feeder-Variety/dp/B000CSJ1VE
unsure how small she wants... I do have blue jays that will come and try to eat from it, but it's not easy for them. cardinals fit ok. finches, chickadees, and tufted titmouse love it.
this one looks like it might make it make it even harder for larger birds - but I can't speak to the quality or any experience with it.
https://www.amazon.com/Birdscapes-Clear-Window-Feeder-348/dp/B000NVM2L6/
maybe that at least gives you a starting point!