That if you buy birds from an auction or swap, you have to be prepared that you've increased your chances of bringing diseases home with you.
That depending what your birds catch, depopulateing might be your only option, especially if you want to bring in new birds.
That hen to rooster ratios are a real thing, pay heed to them especially in a closed environment where the hens can't get away.
That hawks will try to steal your babies from literally right in front of you.
We experienced that, too. My wife had her chickens in a chicken tractor in the yard while cleaning the coop. She heard commotion and "screaming" and turned to see a large blackish/gray bird draped around the tractor, just 20 feet from her. She had a tough time shooing it away. We learned later it was an adolescent bald eagle. (Everyone survived!)