Back on March 12 or 13 I got some day old polish chicks and about 4 weeks later integrated them to my coop(August hatch). One of the polish turn out to be male. The Silkies went broody and two weeks ago I put some fertile eggs from Craigslist under them. That left just one hen laying and I was not harassing her sisters to collect them often. Well last night when I collected the "infertile" eggs I found an inch long embryo. Apparently the three month old polish cockerel has been fertile at least a week.