I have about 25 hens and they are only laying about 2-3 eggs a day. They are 18 months old, and were laying about a dozen a day, but their egg production suddenly dropped in early November. They are perfectly healthy, no lice or mites, they have clean water, clean coop, feed, grit, oyster shell. I haven't changed the feed and they are receiving additional lighting at night. Three of their roosters have recently died, but they dropped their egg production before that happened. Does anyone know what might be the cause?