How can I fix rotation of Z axis to 0?

Hello,

I am trying to right a bird controller.

In this script I am rotating object on Vector.up axis.

Also during elevation I rotate forward axis of object around transform.right axis to make birds head look towards upward or downward. (I limit that rotation with 45 degree)

In theory in shouldn't mess with Z axis (transform.right) of object. It should stay same. But after I run script for a little time, orientation of bird starts to breaking down.

Here is the script:

     void Update () {
 
         float hor = Input.GetAxis("Horizontal");
         float ver = Input.GetAxis("Vertical");
         float elev = Input.GetAxis("Elevation");
         float xAngle = transform.rotation.eulerAngles.x;
  
 
         bool eliminateHeight = false;
         if ((xAngle > 0 && xAngle<180 && transform.position.y < minHeight) || (xAngle < 360 && xAngle > 180 && transform.position.y > maxHeight))
         {
             eliminateHeight = true;
         }
 
         float prevHeight = transform.position.y;
         cc.Move( -transform.forward*ver*speed);
 
         if (eliminateHeight)
         {
             transform.position =new Vector3(transform.position.x, prevHeight , transform.position.z);
         }
 
         transform.Rotate(Vector3.up*hor,Space.World);
 
   
 
         if (elev == 0)
         {
             if (xAngle < 180f)
             {
                 if (xAngle > 1f)
                 {
                     transform.rotation = Quaternion.Slerp(transform.rotation, transform.rotation * Quaternion.Euler(-1, 0, 0), Time.deltaTime * 40);
                 }
                 else if (xAngle > 0)
                 {
                     transform.rotation = transform.rotation * Quaternion.AngleAxis(-xAngle, transform.right);
                 }
             }
             else
             {
                 if (xAngle < 359)
                 {
                     transform.rotation = Quaternion.Slerp(transform.rotation, transform.rotation * Quaternion.Euler(1, 0, 0), Time.deltaTime * 40);
                 }
                 else if (xAngle < 360)
                 {
                     transform.rotation = transform.rotation * Quaternion.AngleAxis((360 - xAngle), transform.right);
                 }
 
             }
 
         } else 
         {
             if(  ((xAngle>360-45||xAngle==0) || xAngle<45) || ((xAngle < 360 - 45 && xAngle>180 && elev < 0) || (xAngle > 45 && xAngle < 180 && elev > 0)))   
             transform.Rotate(-elev, 0, 0);
 
         }
         
     }

So I decided to write a if condition that checks z axis rotation and when z axis is not 0 it should set it to 0. But I couldn't manage to set only z axis to zero.

Here is the if condition

         if (zAngle != 0)
         {
             print("fixing z");
 
             if (zAngle > 180)
             {
                 transform.rotation = Quaternion.AngleAxis(-(360 - zAngle), transform.forward);
             }else
             {
                 transform.rotation = Quaternion.AngleAxis(-zAngle, transform.forward);
             }
           
 
         }

How can i set z axis rotation to 0 without altering other axes.

Edit: I fount the reason of why z axis rotation is changes. It is due to obstacles that bird encounter during its flight.