2d make sprite "bobble" on the Y whilst using velocity on x

Hi

But of a noon question but wondered if some could help me....

I am trying to make a bird move across the screen....currently using velocity. These birds are spawned. My current code looks like:

flyPrefab.rigidbody2D.velocity = new Vector2(flyPrefab.transform.localScale.x, 0f) (direction.x gameController.currentLevel.flySpeed);

Now what I would really like to do is have the bird move up and down a bit (so its harder to tap to kill) as it flies across. But I can't for he life of me work out how to do it.

I have tried what I thought would be a simple solution and made a animation of it move on the Y. However this does;t play well with the physics.

Any pointers would be very much appreciated.