FEEDING BLACKBIRDS on the wing in the cab of his moving locomotive is the everyday experience of J.E. Flynn, 346 E. Jessamine avenue, St. Paul, engineer on the Northern Pacific yard engine in the Mississippi street yards in St. Paul. It all started last spring when Flynn stopped his engine to take on water at a tower where the birds loitered. He started feeding them with the engine at a standstill. Now, whenever he passes the spot, the birds fly in the cab window for food, then fly out again.