python中如何判断pygame.Sprite.Group()中包含几个list,要想判断Group中已经把list都

python中如何判断pygame.Sprite.Group()中包含几个list,要想判断Group中已经把list都移除了该怎么做?
BirdGroup=pygame.Sprite.Group()
BirdGroup.add(bird1(bird)
for birdlist in BirdGroup.sprites():
screen.blit(birdlist.image,birdlist.rect)
if planeSprite.rect.colliderect(birdlist.rect):
BirdGroup.remove(birdlist)
如何判断BirdGroup中是空的?