var tick = function(dt, paused) {
	birdDelayCounter++;
	if(birdDelayCounter % 10 === 0) {  // delay so it doesn't spawn a bird on every frame
		birdDelayCounter = 0;
		birds.spawn();
	}
}

var spawn = function() {
	var birdBMP = new Bitmap("images/bird.png");
	birdBMP.x = Math.round(Math.random()*500);
	birdBMP.y = -30;
	birdBMP.regX = 25;   // important to set origin point to center of your bitmap
	birdBMP.regY = 25; 
	stage.addChild(birdBMP);
}

box2d.createBird(birdBMP);

var createBird = function(skin) {
	var birdFixture = new b2FixtureDef;
	birdFixture.density = 1;
	birdFixture.restitution = 0.6;
	birdFixture.shape = new b2CircleShape(24 / SCALE);   // half of bird.png width divided by world scale for right size
	var birdBodyDef = new b2BodyDef;
	birdBodyDef.type = b2Body.b2_dynamicBody;
	birdBodyDef.position.x = skin.x / SCALE;  // divide skin x and y by box2d scale to get right position
	birdBodyDef.position.y = skin.y / SCALE;
	var bird = world.CreateBody(birdBodyDef);
	bird.CreateFixture(birdFixture);
	bodies.push(bird);
}

var actor = new actorObject(bird, skin);
bird.SetUserData(actor);  // set actor as userdata of body so we can get at it later if we need to

var actorObject = function(body, skin) {
	this.body = body;
	this.skin = skin;
	this.update = function() { // translate box2d positions to pixels
		this.skin.rotation = this.body.GetAngle() * (180 / Math.PI);
		this.skin.x = this.body.GetWorldCenter().x * SCALE;
		this.skin.y = this.body.GetWorldCenter().y * SCALE;
	}
	actors.push(this);
}

// within physics loop before world.step
for(var i=0, l=actors.length; i<l; i++) {
     actors<i>.update();
}

// before step
for(var i=0, l=bodiesToRemove.length; i<l; i++) {
	removeActor(bodiesToRemove<i>.GetUserData());  // get the actor object in the user data of the body and send to removeActor function
	bodiesToRemove<i>.SetUserData(null);
	world.DestroyBody(bodiesToRemove<i>);
}
// after step
if(bodies.length > 30) {
     bodiesToRemove.push(bodies[0]);
     bodies.splice(0,1);
}

var removeActor = function(actor) {
	stage.removeChild(actor.skin);
	actors.splice(actors.indexOf(actor),1);
}