It's unlikely that your project will succeed without a vertical fin. Are you aware that there are countless radio-controlled model airplanes and gliders that feature a fixed vertical fin but no rudder? The fin is much more than just a thing to attach the rudder to. The rudder itself is not very necessary if the aspect ratio of the wing is not too high. (High-aspect-ratio designs tend to suffer from severe "adverse yaw".)
But, the birds seem to be able to pull it off, so you never know. The distribution of the "twist" or washout in the wing seems to be a key factor for them.