How to make my flying object amplitude up and down on the current y position?

I want to create 2d flying object that can waving up and down and move forward. I had put my object with position.y = 7 on the scene, but when i start the game, the object will BLINK to position.y = 0 and start amplitude up and down. Can anyone help me to fix t$$anonymous$$s code so my object will start amplitude by its current y position? Below is the codes i put to my object:

 using UnityEngine;
 using System.Collections;
 
 public class BirdMovement : MonoBehaviour {
 
     public float horizontalSpeed;
     public float verticalSpeed;
     public float amplitude;
     private Vector3 tempPosition;
 
     // Use t$$anonymous$$s for initialization
     void Start () {
         tempPosition = transform.position;
     }
     
     // Update is called once per frames
     void Update () {
         tempPosition.x += horizontalSpeed;
         tempPosition.y = Mathf.Sin (Time.realtimeSinceStartup * verticalSpeed) * amplitude;
         transform.position = tempPosition;
     }
 }