I think the key may be in what breed of chicken this actually is. A BR female has the same colouring as what you have the, but very dark legs. I am not as familiar with the specific characteristics of barred Marans. Maybe some of the characteristics seen here are common for the breed, (or maybe it's a mixed breed). For instance, female feathering on roo or spur-buds, large wattle on hen, or late laying. The weight thing seems to be my favourite possibility for the late start so far, but thought I'd ask about how much you know about the breed anyway.