A tropical species called the magnificent frigatebird, famous for its 7 1/2-foot wingspan and penchant for aerial piracy, has appeared in New Jersey several times this year, surprising experts and delighting birders.

Frigatebirds prefer far warmer climes and generally do not venture past North Carolina. Before this year, only 10 had been seen in New Jersey since 1926, according to the New Jersey Audubon Society. But the New Jersey Bird Records Committee confirmed four sightings this summer and is looking into another five since then, including three this month. All have been on the Shore, from Cape May to Raritan Bay.

Experts offered several possible explanations, including a frigatebird baby boom in the Caribbean, or a food shortage down there, or even global warming-induced wandering. But a favorite theory was that hurricanes and other tropical storms blew or scared the birds up the coast.

"They're storm-riders," Eric Stiles of the Audubon Society speculated. "They hit the leading edge of the storm and they just tried to stay out of harm's way. Plus they were probably getting blown all over hither and yon."

Birders have flocked to the Shore after each sighting, hoping to add the rare visitor to their "seen in New Jersey" lists.

Most have left disappointed. Frigatebirds fly high and far, covering lots of ground in little time.

Hobbyists who caught a glimpse were treated to a striking sight. Frigatebirds, also known as frigate pelicans or man-of-war birds, soar on columns of warm air like sleek, black kites, rarely flapping their wings. They have the highest wingspan-to-weight ratio in the bird kingdom, enabling them to glide with almost no effort, according to whatbird.com and other online references.

But when they maneuver, it's as if a turbojet kicked in. They swoop down on lesser aviators like gulls and terns, intimidating them into dropping their catch. Then they might jet after the falling fish, snagging it before it hits the water.

"The Frigate Pelican is possessed of a power of flight which I conceive superior to that of perhaps any other bird," John James Audubon wrote in "Birds of America." "However swiftly the Cayenne Tern, the smaller Gulls or the Jager move on wing, it seems a matter of mere sport to it to overtake any of them."

Frigatebirds don't survive exclusively on piracy. They occasionally fish for themselves, skimming the sea and plucking prey with their long, hooked bill. But they do not dive, and they never alight on water. Theirs is largely an airborne life.

In fact, they have trouble taking flight from the ground. Wings that long are hard to flap, experts explained. They roost on low shrubs or short trees, so they can leap off and catch a breeze.

Another striking frigatebird behavior comes during courtship, when the male inflates a large red throat sac, then wobbles it back and forth to impress passing females. But that is unlikely to happen in New Jersey; these birds will probably find their way home before they nest in the spring.

Pete Bacinski, director of Audubon's Sandy Hook Bird Observatory and a birding columnist for The Sunday Star-Ledger, said a birder in Union Beach had called him at work on Nov. 9 with word of a frigatebird sighting.

"He said, 'Pete, I wouldn't interrupt you at work if I didn't have something really good,'" Bacinski recalled. "We bolted out the door."

Bacinski photographed a frigatebird not far from the spot. He was astounded to compare pictures with the birder later and discover that he had seen an entirely different bird.

Those may have been the seventh and eighth sightings this year. The Bird Records Committee has voted on and approved four others this summer: July 19 at Cape May, July 20 at Town Bank, July 29 at Stone Harbor and Aug. 20 in Holgate. It will examine evidence this spring of sightings Sept. 21 and Nov. 6 from Barnegat Light, Nov. 7 from Cape May, in addition to the two sightings on Nov. 9.

Bacinski speculated that this year's higher sea temperatures, widely linked to climate change, had lured the birds north. Some experts doubted that, though.

Rutgers behavioral ecologist Joanna Burger said birds get blown off course during every big hurricane season.

"There's no way of knowing exactly what's going on," she said. "It is remarkable that there are so many records this year as opposed to other years, but you sometimes get that in looking at bird numbers over the years. There are years when there seem to be a lot more wanderers than other years."

_____________________________________________________________________________________________ Alexander Lane covers the environment. He may be reached at ala ne@starledger.com or (973) 392-1790.

1. A volunteer with the N.J. Audubon Society caught this image of a magnificent frigatebird over Union Beach. INFO GRAPHIC: Flying pirate: frigatebird The magnificent frigatebird has the longest wingspan relative to its weight of all birds. A mostly tropical seabird, it often lives in colonies and can be found year-round in southern Florida. When not breeding, they can be spotted from the coast of North Carolina, south to Florida and west to Texas, and also on the coast of California. The male puts on an impressive courtship display by inflating its throat sac into a very large red balloon. The females do not breed every year and usually lay just one egg, which must be guarded constantly from other birds in the colony. Despite its reputation as a bully and pirate of terns' and other seabirds' food, the magnificent frigatebird does on occasion dive for its own prey. Length: 37 to 41 inches Weight: 3.3 pounds Wingspan: up to 71/2 feet Diet: Fish, crustaceans, jellyfish, eggs, young birds and turtles.

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