Correct me if I am wrong but bird flu is spread between birds in a flock, and so industrial, heavily concentrated flocks are breeding grounds for bird flu? So backyard chickens being more diverse, smaller flocks, would increase food security and reduce spread?

You have to contend with wild birds passing diseases to your chickens.

Not exactly. Free birds can still spread the flu from one location to another, e.g. by dropping excrements.

Right, but if you don’t mix flocks, how do they get their excrements from one backyard to another that is distant?

Chickens aren't the only birds that carry bird flu.

I meant wild birds.