You could try a windowsill feeder like this from Amazon. (Has a crazy amount of positive ratings). I am impressed with the security of the suction cups.

Best Window Bird Feeder with Strong Suction Cups & Seed Tray, Outdoor Birdfeeders for Wild Birds, Finch, Cardinal, Bluebird, Large Outside Hanging Birdhouse Kits, Drain Holes + 3 Extra Suction Cups https://www.amazon.com/dp/B00YSU3WG2/ref=cm_sw_r_cp_api_i_03RGCbBKQWP66