Unity keeps changing NavMeshAgent to UnityEngine.AI.NavMeshAgent and "Target" field doesn't appear.

I'm getting a bit frustrated with this. For some reason, after I save the script and re-open Unity, it throws up a couple of errors:

Assets/Scripts/BirdMovement.cs(15,31): error CS0118: BirdMovement.birdAnimator' is afield' but a type' was expected Assets/Scripts/BirdMovement.cs(15,18): error CS0029: Cannot implicitly convert typeT' to `UnityEngine.Animator'

… then changes both instances of NavMeshAgent to UnityEngine.AI.NavMeshAgent in the script. The Target field doesn't appear in the Bird inspector.

Here's my script in full:

using System.Collections; using System.Collections.Generic; using UnityEngine;

public class BirdMovement : MonoBehaviour {

[SerializeField]
private Transform target;
private UnityEngine.AI.NavMeshAgent birdAgent;
private Animator birdAnimator;

// Use this for initialization
void Start () {
    birdAgent = GetComponent<UnityEngine.AI.NavMeshAgent> ();
    birdAnimator = GetComponent<birdAnimator> ();
}

// Update is called once per frame
void Update () {
    // Set the bird's destination
    birdAgent.SetDestination (target.position);

    //Measure the magniture of the navMeshAGent's velocity
    float speed = birdAgent.velocity.magnitude;

    //Pass the velocity to the animator component
    birdAnimator.SetFloat ("Speed", speed);
}

}