I believe so. Within ~12s (15 tops).Is any mob off-screen guaranteed to despawn eventually?
Did I see elsewhere that you thought birds were a special case (de-spawn resistant)? Because I didn't see any reason they should be. Sometimes you get an annoying little tweeter stuck on-screen in a little nook, or mostly off screen but periodically renewing their active status by nipping briefly on-screen. ("On-screen" here meaning within the inner 'safe zone'/no spawn box, not necessarily visible, depending upon resolution.)