Buy her a really nice dinner, listen to her hopes and dreams, then take both her wings in your hands and tell her that she's a wonderful duck and that you wish nothing but the best for her, but that you really see her more as a duck than anything, and that you value her duckship but you're not here to complicate her life. Tell her that the drake who she meets in the future who looks at her and sees what she wants him to see will be the happiest drake on earth, but that you and she are looking for different things in life. Let her know that you'll always be there for her but you don't want to be unclear about your feelings for her or throw off mixed signals. Then take her back to her coop but don't give her an opportunity to kiss you goodbye or anything else that gives her false hope, and fade out of her life a bit for a little while so she can move on and you won't be confusing her. After that, NEVER BRING UP THAT CONVERSATION AGAIN, pretend like nothing ever happened and let her have her dignity, and don't get all butt hurt if she starts spending less time with you.
She might be hurt, and she might be a little undignified or unfair about expressing that hurt, but remember that it's about what she's going through and not about you or who she is, and have a forgiving heart without standing for any unkindness or dishing out any yourself.
