This is exactly what I worry about! My child wants a new chicken, I told her no b/c you can't introduce just one new chicken, plus it's winter and only going to get colder. Then I'm thinking well, if someone goes broody then we'll see what we get, BUT then I worry that they won't accept the babies. I guess I'll deal with it when it comes to pass. I'm kinda hoping we don't get a broody, but also kinda hoping we do. Plus I don't want to deal with any boys I might get. Then I think maybe I can have a coop of just boys, but I really don't want that. I think if they were nice then someone might want them, but as it is now there's a 50/50 chance the father would be a leghorn or bantam BBOE. Willie is with them the most, but when Odin gets to mingle he fertilizes everyone he can catch. I wonder how that works.. like after a hen is fertilized, how long will she lay fertilized eggs.
I separate the boys from the girls for 30 days, prior to setting up the breed pens. I do this for two reasons. 1) Many have said, and I have witnessed that a chicken can remain fertile for up to 30 days. 2) I feed the boys and girls a special blend of feed that is designed to improve hatch rates. Once the boys join the girls, they go on the girls feed and I don't use any of the eggs for hatching for the first week. After that, everything goes into the incubator.