more like:
List<GameObject> allGameObjectsInTheEntireUniverse = new List<GameObject>();
allGameObjectsInTheEntireUniverse.Add(new Building(x, y, 1));
allGameObjectsInTheEntireUniverse.Add(new Bird(x, y, 2));
allGameObjectsInTheEntireUniverse.Add(new Building(x, y, 1));
allGameObjectsInTheEntireUniverse.Add(new Submarine(x, y, -1));
allGameObjectsInTheEntireUniverse.Add(new Flower(x, y, 0)); // draws under buildings
allGameObjectsInTheEntireUniverse.SortByZThenByY();