U.S. airlines collected $5.7 billion in fees for checked luggage and changed reservations last year, according to data released by the Department of Transportation on Monday. Here, a by-the-numbers guide to the airline industry's "staggering" haul:

$3.4 billion
Baggage fees collected by U.S. airlines last year. The practice of charging $25 for the first checked bag and more for a second "began as a revenue bridge when travel fell sharply during the 2008-09 recession and fare increases were hard to pull off," says John Crawley at Reuters. Those "ancillary fees" remain, "and are now an important part of the revenue stream for airlines."

Percentage increase in revenue from baggage fees from 2009 to 2010. In 2009, the industry collected $2.7 billion.

More than $8
Average additional baggage charge "for each of the 416 million passengers who boarded an airplane last year," says Scott McCartney at The Wall Street Journal. Travelers "hate the nickel-and-diming they get at the airport these days," so maybe the industry would be better off if everyone just paid $8 more for their ticket, says McCartney. "You don't pay extra for a television in a hotel room — it's an expected part of the service."

$952 million
Baggage fees collected by Delta last year, by far the most of any airline. "Delta has sent a very loud message to the rest of the industry: Y'all got a lot of catching up to do," says Chris Morgan at The Consumerist.

$482 million
Baggage fees collected by Delta in 2009

$581 million
Baggage fees collected last year by American Airlines, coming in a distant second to Delta

$2.3 billion
Reservation-change fees collected by all U.S. airlines last year

$699 million
Reservation-change fees collected by Delta last year, again the most of any airline

3.2 percent
Drop in industry-wide reservation-change fees from 2009. "One might suspect that travelers are getting more careful about reservations and are a bit better at avoiding nasty reservation change fees," says McCartney in The Wall Street Journal.

Sources: CNN, The ConsumeristDepartment of TransportationReuters,  The Wall Street Journal