Delta Air Lines fell after the carrier curbed its forecast for a key revenue measure for the first quarter. The news also hurt other airline stocks. Delta said that unit revenue, a measure of pricing power and the extent to which planes are filled, rose at a lower pace in March than in the previous two months. The company partly tied the weakness to less last-minute travel from government clients and lower than expected demand. Delta said it now expects a rise of 4 to 4.5 percent.