Call it a wild goose chase: A 30-day search through a southern Louisiana swamp was called off yesterday after some of the world’s top ornithologists failed to find the elusive ivory-billed woodpecker. The bird, once the largest North American woodpecker with a wingspan of up to 33 inches, has not been seen for certain since shortly after WWII. But three years ago, a forestry student said he saw a mating pair, and the claim was credible enough to launch a major search. Traveling by foot and canoe through mosquito- and alligator-infested swamps, and using state-of-the-art technology, six scientists combed 35,000 acres of land northeast of New Orleans. The team turned up some evidence that the birds might not yet be extinct — but not the definitive sighting they’d hoped for.