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 or the West Coast to Asia or the South Pacific, for example. These usually twice annual exoduses are so huge that they could rival a naval armada, and they offer exceptional bargains, as well as lots of languid days at sea, and, occasionally, even unusual ports of call.

Veteran 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 longer voyages relaxing because of a large number of days spent crossing the pond or the Pacific but also because they usually are not filled to capacity, and, therefore, Showker notes, "the whole tempo of the ship is more low key." Travel specialist Nancy Kelly, head of Oak Brook-based 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." For instance, Disney Cruise Lines brings aboard extra performers such as cast members from some of its Broadway shows or renowned musicians such as Boyz II Men to brighten days at sea.

This year for the younger generation, Disney will offer a few new programs specifically for trans-Atlantic crossings, including a slumber party with camp-out games, a singalong and various fun foods throughout the night. "It's a true sleepover, with the children spending the night at the Oceaneer Club and Lab," said Disney spokesman Jason Lasecki.

Of course, there also is the silly stuff that makes a repositioning cruise more like a fortnight at camp. As part of its tradition on trans-Atlantic and Panama Canal sailings, for instance, Disney lets passengers enter a boat-building competition.

With this year's race across the oceans, Kelly noted, "there are hundreds of repositioning cruises to choose from." Kelly spotlighted Princess Cruises' 16-night Venetian Passage in October and Royal Caribbean International's 14-night westbound trans-Atlantic 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, and offering free round-trip airfare, plus two-for-one rates, prepaid gratuities and a $2,000 As You Wish Shipboard Credit on its 10-night Big Band Celebration cruise from Lisbon to Miami in December, starting at $4,000.

Other lines too are offering deals sure to lure you to sea; here's a sampling:

Azamara Club Cruises

( azamaraclub.com)

Azamara Journey -- From Rome to Miami, departing Nov. 15; fares for veranda staterooms from $1,799. Special offers: as much as 50 percent off brochure prices on certain sailings; Club Veranda stateroom for the price of a Club Oceanview stateroom, based on availability (booking must be made by Wednesday. )

Celebrity Cruises

( 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 Equinox -- 15 nights from Rome to Fort Lauderdale sailing Oct. 31, from $1,299.

Celebrity Constellation -- 14 nights from Barcelona to Fort Lauderdale departing Nov. 6, from $999.

Celebrity Eclipse -- 13 nights from Southampton to Miami departing Oct. 31, from $1,199.

Costa Cruises

(costacruises.com)

Costa Atlantica -- 17 nights from New York City to Copenhagen departing May 5, from $899.