Do I need a run?

[rosemarythyme]

At the coldest, about 10 degrees fahrenheit? We've only been in the Catskills one winter, but it's definitely not as bad as where I'm originally from (Wisconsin). Lots of snow though. You know, there must be ventilation - the previous owners kept chickens in this thing successfully. I thought ventilation meant windows but I might have missed openings at the eaves/higher up in the coop. Will check and add along with windows if none is there. Thank you!

10F isn't that bad. We don't get as cold here but my chickens have spent nights at 12F pressed up against open windows in the coop without issue. Usually when I see folks talking about confining chickens through the winter, they're talking about arctic climates, where you're looking at extended periods at -20, -30F or more.

Optimally you will want to have a covered or climate proofed run so they can get outside even in winter weather, just to reduce any chance of behavioral issues popping up from confinement, even if you plan on culling down the flock each winter, which some folks in colder climate areas do practice.

 

[aart]

There must be ventilation high up that I missed

Look at the eaves, gables, soffits etc.
Pics inside and out could help us help you.
Just because they kept birds in there before doesn't mean it's a great scenario.

Catskills in NY

Welcome to BYC! @khoehne
No, probably not as bad as WI, but still can get pretty nasty there.
Here's how to add your general geographical location to your profile.
It's easy to do, and then it's always there!

 

[khoehne]

If you want to protect the chickens, but still have something movable, you might consider a dog kennel for a run (the kind that are about 10 feet square and covered with chainlink fencing, plus add something over the top.)

Or you could consider a cattle panel hoop coop for the run, with a tarp on top to provide shade from the sun and some shelter from rain and snow. You would need to secure the tarp tightly enough that it cannot flap and rip loose in the wind, and be aware that heavy snow can make the whole thing collapse.
The "articles" section of the site has quite a few examples, such as this one:
https://www.backyardchickens.com/articles/cattle-panel-hoop-coop.74636/
I would use something sturdier than the chicken wire that is pictured in that article. Chicken wire is fine for keeping chickens in, but is weak enough that many predators are able to rip it open and kill the chickens.

This is such a good idea - thank you!