Even when it was -18F out the other day, the chickens weren't in their coop at all except to roost and lay so I wondered how much the behavioral issues would come into play. I don't want to integrate just one chicken, but I also don't want to build a new coop so I thought I'd get some advice from others who may have a similar setup. We would definitely build the new part so that they would have a small opening that would connect the two 1/2's of the run that I could block off so that we could raise the new chicks in the new half of the run until they were all roughly the same size and used to each other. The run would be predator proof (hardware cloth sides with hardware cloth buried in a trench all the way around the run, welded wire on top, it's own access door), but I'd probably only build a small shelter in the other half of the run that wouldn't be sufficient for a bird in the winter - just a place that could eventually serve as a second nesting box and some additional shelter. I will attempt to build the new part of the run with a slanted wire topped roof so I could put a tarp on it during the wet seasons and have it not collect water or snow. But could take it off as well.
Our bird that died is a Mereks suspect (lost the use of his legs before he died) so we have to get vaccinated chicks. Who knows, maybe we will get a hen and a roo and be left with four birds. HA HA. We don't really want more birds than that - too many eggs!!!. We wanted to wait a year and add the new birds in year three, but the three birds we have now are just barely meeting our egg needs for breakfast with little leftover for baking, hard boiled egg lunches, fritatas, sharing, etc and we are getting 2 a day (with a few more days off now that it is winter).