Cuckoo
A variant of Marans (Gallus gallus domesticus)
Irregular blue-grey barring across the whole bird, from the sex-linked barring gene. Historically the classic French market Marans and still widespread in Europe.
Keeper tips for this variant
Barring is sex-linked, so a Cuckoo hen x non-barred cock gives auto-sexing chicks (barred sons, solid daughters) — handy for sorting at hatch.