Chicks need about 1/2 to 1 square feet per chick. So if you have less, you will need to find something bigger. When they get over crowded, they can turn on each other or starve others out of the food and water.
The problem with brooding very young chicks outside is that the temp continues to drop all night long. So the temp you leave them at at sun set, is going to be far colder by 4 am. So if you do take them outside to finish brooding, you will need to use a building or structure that has a constant temp or is heated to remain at a stable temp. Doesn't matter what this buildings temp is, but it needs to remain constant so that the brooder temp is not falling in to dangerously cold temps during the night. 10 days old is way too young for becoming chilled.
Enjoy your cutie pies! If you have any further questions, feel free to ask around the forums.
and Welcome To BYC!
