NEW YORK ( TheStreet) -- Delta (DAL - Get Report) kicked off December traffic reporting for the airline industry, saying its unit revenue declined by 4.5%. The carrier also boosted its fourth-quarter margin estimate.

The decline in consolidated passenger revenue per available seat mile (PRASM) reflected calendar quirks, Delta said. The Sunday after Thanksgiving, the year's busiest travel day, fell in November 2014 after falling in December the previous year. 

On the plus side, Delta said, "Atlanta and Seattle continued their strong performance in domestic markets in December," resulting in a 1% unit revenue increase for the December quarter.

Additionally, the carrier also boosted its fourth-quarter margin guidance to between 12% and 13%, up from previous guidance of 11.5% to 12%. In a note, Cowen & Co. analyst Helane Becker said the increase in guidance is largely related to lower fuel costs. She maintained an outperform rating on Delta and a $58 price target.

"We expect margins to continue to expand in 2015 as jet fuel costs have declined significantly," Becker wrote. "We continue to believe the airlines will need to maintain pricing (that doesn't mean increase fares) for multiples to continue to expand. Delta will be one of the industry leaders in pricing going forward and we expect the airline to maintain fares given demand is relatively strong."

In mid-morning trading, Delta shares were down 48 cents to $48.70.

Earlier Monday, Deutsche Bank analyst Mike Linenberg forecast that U.S. airline industry December PRASM would fall by 3%.

Linenberg forecast that Delta PRASM would fall by 4%. He expects Alaska (ALK - Get Report) and United (UAL) to also fall by 4%, with American (AAL - Get Report) flat and JetBlue (JBLU) down 7%.

Meanwhile, in a report released Monday, trade journal Airline Weekly ranked Delta as the 11th most profitable airline in the world with a 12% operating margin. The publication ranked airlines by operating profit or loss margin for the 12 months ended September 2014. The top four were Copa with a 21% margin, Spirit (SAVE) with an 18% margin, Ryanair with 16% and Alaska with 16%.s

-- Written by Ted Reed in Charlotte, N.C.

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This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.