Entire flock of 50+ chickens became very weak in matter of few days.

[Danielpn]

H

Could they have gotten into something toxic on the property?
- I am not entirely sure. As I said they roam around so maybe?

Roaming around it could be as simple as antifreeze carelessly poured out onto the ground. I suggest confining them for several days, force fluids on the ones that have the best chance of recovery, and hope for the best.