You have given them the proper spacing, temperature, feeding and brooding care. At 21days, they are set to the range and just have shelter for them to run to when raining. They are allotted the suggested 1 sq meter per bird for ranging area.
You forgot to count the days for harvest time....Now you walk through the ranging area and you notice that the area is almost bald. Take some to get average weights. You'll be surprised that they will now be close to your 1.5k...must be about 50days now?
Be observant...senses will still be your best guides.