Had that situation recently. BR rooster over 30 hens. For awhile, most of the eggs were fertile, then it began to drop off with some of the hens. Seemed the BR eggs stayed fertile. Had his brother in with him for awhile, but he refused to share and I got tired of the fights so Dutch got them all to himself. Now, since he's almost two years old, I put him and his BR ladies into a separate coop so he could keep up with the fertilizing job. My younger Delaware rooster is doing well with his 25 or so women. He's not quite a year old.