Had a first on an O'Dark thirty(05:30AM) flight on 29th. Us self loading cargo could not board due to absence of a FA who apparently overslept. The Delta ground crew borrowed a FA from a flight leaving 30 minutes later so they could "legally" board pax. The missing waif showed up around door closing time and we got off 20 minues late, but arrived on time in ATL. This is the first that I have heard that a minimum number of FA's is required to start the proceedings. News to anyone else?