Like migratory birds, cruise ships regularly shift from one part of the world to another as the seasons shift -- from the Caribbean to Europe or South America, or from Alaska to Asia or the South Pacific, for example. These usually twice-annual exoduses are so huge that they could rival a naval armada. They offer exceptional bargains, as well as lots of languid days at sea, and, occasionally, even unusual ports of call.

Cruise-book author Kay Showker considers repositioning cruises "about the best value in cruising," compared with a regular cruise on the same ship. Many old salts not only find these voyages relaxing because of a large number of days spent crossing the ocean but also because they usually are not filled to capacity.

Travel specialist Nancy Kelly, head of Kelly Cruises in Chicago, sees other benefits.

"Some cruise lines," she said, "will offer a special theme on repositioning cruises, so you may be traveling with like-minded people for a big band cruise, jazz cruise, wine and food festival, film and theater or perhaps health and fitness."

With this year's race across the oceans, Kelly noted, "there are hundreds of repositioning cruises to choose from." She spotlighted Princess Cruises' 16-night Venetian Passage in October and Royal Caribbean International's 14-night westbound transatlantic sailing in November as "so reasonably priced, you may feel giddy." (Fares start at $1,270 and $864 per person, respectively.)

"Crystal Cruises is top-of-the-line six-star luxury," Kelly added.

Crystal is offering free round-trip airfare, plus two-for-one rates, prepaid gratuities and a $2,000 shipboard credit on its 10-night Big Band Celebration cruise from Lisbon, Portugal, to Miami in December, starting at $4,000. Other deals:

Celebrity Cruises (www.celebritycruises.com): Celebrity Century, 14 nights from Miami to Barcelona departing May 20, from $899 for an inside stateroom; 14 nights from Barcelona to Miami departing Oct. 25, from $799; Celebrity Eclipse, 13 nights from Southampton to Miami departing Oct. 31, from $1,199.

Crystal Cruises (www.crystalcruises.com): Crystal Symphony, from London to New York departing Sept. 5, from $5,710. From New York to Los Angeles, departing Nov. 2, from $4,930. The line is offering shipboard credits of as much as $1,000 per person.

Disney Cruise Lines (www.disneycruise.disney.go.com): Disney Magic, 14 nights from Barcelona to Port Canaveral, Fla., sailing Sept. 18, $1,649 for a Deluxe Oceanview Stateroom.

Holland America Lines (www.hollandamerica.com): MS Noordam, 13 days from Fort Lauderdale to Barcelona departing May 14, from $899.

Norwegian Cruise Line (www.ncl.com): Norwegian Epic, seven-day transatlantic crossing, a maiden voyage, from Southampton to New York departing June 24, from $859.

Regent Seven Seas Cruises (www.rssc.com): Seven Seas Navigator, 18 nights from Seward, Alaska, to Tianjin, China, departing Sept. 15, from $8,775.

Seabourn Cruise Line (www.seabourn.com): Seabourn Legend, 13 days from Malaga, Spain, to Fort Lauderdale departing Nov. 20, from $2,999; Seabourn Odyssey, 13 days from Nice, France, to Fort Lauderdale departing Dec. 6, from $3,599.

Silversea Cruises (www.silversea.com): Silver Whisper, 12 days from Southampton to New York City departing Sept. 2, from $4,598; Silver Shadow, 16 days from Vancouver, British Columbia, to Tokyo departing Sept. 12, from $7,197; a free economy promotional air package also is available from select cities.

Windstar Cruises (www.windstarcruises.com): Wind Surf, 14 days from Lisbon to Barbados departing Nov. 14, from $1,749; Wind Spirit, 14 days from Lisbon to St. Martin departing Nov. 20, from $1,849.