text: function Bird(game, stage) {
    this.game = game;
    this.stage = stage;
  
    this.height = 50;
    this.width = 50;
  
    this.x = 80;
    this.y = 150;
  
    this.velocityY = 0;
    this.terminalVelocity = 400;
    this.gravity = 1000;
  
    this.color = 'yellow';
  
    this.stage.addEventListener('click', this.flap.bind(this));
  }
  
  Bird.prototype.flap = function() {
    this.velocityY = this.velocityY - 900;
  }
  
  Bird.prototype.update = function(delta) {
    this.velocityY = this.velocityY + (this.gravity * (delta / 1000));
    if (this.velocityY > this.terminalVelocity) {
      this.velocityY = this.terminalVelocity;
    } else if (this.velocityY < -this.terminalVelocity) {
      this.velocityY = -this.terminalVelocity;
    }
    this.y = this.y + (this.velocityY * (delta / 1000));
    if (this.y > this.game.height) {
      this.velocityY = 0;
      this.y = this.game.height;
    } else if (this.y < 0) {
      this.velocityY = 0;
      this.y = 0;
    }
  };
  
  Bird.prototype.draw = function(ctx) {
    ctx.beginPath();
    ctx.arc(this.x, this.y, this.height / 2, 0, Math.PI * 2, true);
    ctx.fillStyle = this.color;
    ctx.fill();
    ctx.closePath();
  };
  