It is frustrating how slowly they grow at first. Even if they double in size when they're really tiny, that still makes them pretty darned small. It takes such patience to get them to the point that they are feeder size. But once they do, the wait will be worth it. I've had to buy mealworms four times this winter because even with all my bins, they just don't grow fast enough for the number of birds I have to feed. That is slowing down now, and hopefully I can keep up feeding them with my own bins, now that their numbers are down. They are hunting for a lot of their own food now that the ground isn't covered in snow.