DL flights 1-4 are all between JFK and LHR. None of the other numbers from 5-9 appear to be in active use. For comparison, UA 1/2 are SFO/SIN, UA 3 is ORD-ZRH, UA 4/5 are IAH/LHR, UA 6/7are IAH/NRT, and UA 8/9 are unused. AA 1-4 are JFK/LAX, AA 5 is DFW-HNL, AA 6/7 are DFW/OGG, and AA 8/9 are unused.