Think Like a Weasel
Pretend you have beady eyes, sensitive whiskers and can squeeze through any gap the size of a hot dog- not the bbq size either- the measly ordinary kind. Imagine you can smell chicken from a mile away.
Look up at the eaves and roof of your coop or barn. Are there any rotted wood, small cracks, knots that have fallen away from the planks? Is there an uncovered vent? A cupola that needs some wire on the inside. What about hay vents under the fascia? At the very least, plug these and better still, cover with mason wire (aka hardware cloth, 1/2").
Poultry wire? Why some predators will reach through, grab a hen by leg, wing or head and drag it through in the most agonizing of deaths. The horror.
Then creep down on the floor. Can anything, even something small, crawl up a drain? Dig a tunnel? Even concrete floors can have cracks and chunks missing. Drains can be covered with small-slotted caps, and dirt floors can be wired, paying attention to the edges. Think weasel. Do you really want your chicks ripped at neck and breast and left flailing and terrorized? The survivors will be waiting for it to happen again and again. This mesh will keep out all but baby snakes too. Think like a weasel. Chain link is merely an inconvenience, and a climbing tool.
Go out to your run, poke, check, pry. Is it roofed or covered with netting? Are the edges sealed. Could something slither in? Chew through? Marry the edges of seams with little piggie nose rings, or wire them shut. Has anything corroded? Maintenance. Think like a weasel, imagine the bloodlust and the carnage, night after night. Then think what it would be like to be your hens. Do you feel safe?
Think like a weasel. Please.