February results from Southwest Airlines ( LUV) and other carriers show that they are no longer packing planes, raising concerns that first-quarter results could be lackluster.

On Thursday Southwest announced that traffic, as measured in revenue passenger miles, rose 8.8% year-over-year in February, while capacity, as measured in available seat miles, rose 9.3%. With supply rising slightly faster than demand, the low-cost carrier said that load factor, or the percentage of seats filled on every plane, came in at 62.2%, down from the 62.5% it had a year ago.

Reacting to Southwest's results released before the bell on Thursday, Bear Stearns told investors that the airline was on track to match, but not exceed, its earnings estimates. That has been an emerging theme on Wall Street this week. After two months of decent but unspectacular results, analysts expect March results to appear better because of easy comparisons but say first-quarter estimates may not have much upside.

"The rather lackluster performance of pricing coupled with stubbornly high fuel prices points to less of a likelihood of first-quarter positive earnings surprises," wrote Deutsche Bank analyst Susan Donofrio on Tuesday. "For this reason, we think that investors need to be selective with respect to owning airline stocks over the next six months."

Southwest shares rose 6 cents, or 0.4% to $14.21, on its news.

In addition to Southwest, a number of other carriers released February results late Wednesday.

Delta Air Lines ( DAL) said February traffic rose 8%, against a capacity increase of 8.1%, which left load factors unchanged from last year at 68.6%. As with other high-cost, legacy carriers, Delta's international operations provided a notable bright spot, with traffic up 6.3% against a capacity increase of 5.3%. Latin American and Pacific routes were particularly strong.

Daniel Hemme, airline analyst at Prudential Equity Group, said that Delta should see revenue growth outpace capacity additions in March. But he also said the company's solid performance will be offset by high labor costs, and lowered his price target to $12 from $16. Delta shares rose 5 cents, or 0.6%, to $9, on the traffic boost.

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