Should I expect to see them laying right at 18 weeks? Thank you! Good info! I have shavings but need straw
Honestly, we humans are usually fussier about the precise makeup of the nesting material than the chickens are.

18 weeks is about the earliest that you might see eggs from production-type layer strains. 20-24 is common, but some slower-maturing breeds might not lay until they're over 6 months old.
