300 birds would put off enough heat to keep any barn warm!
I have about 30 birds in my coop, and it's toasty when they're all in there.
If you only have a couple of adult birds, you
might want to add a little heat when it gets really cold. Now, for me, REALLY cold is anything below 0°F. Which is what it is right now, and all the birds are outside with the exception of the ones on the nests. But if you have a dozen or more, they will huddle and pool their warmth. A heat lamp on their waterer will keep it from freezing, but some people don't even do that... they just knock out the ice and refill it.
If you have chicks, then a heat source is a must until they're fully feathered. My 6 week old chicks went outside when it was in the 40s, and slowly acclimated. Now they're 15 weeks old, and out there with the rest of them right now. So they really will do fine. If you give them heat, and then something happens like the power goes out, or the bulb burns out at night, and it's below freezing, what will your birds do then?