Millions will attend. And a few hundred binocular-bearing humans will show up, too, this weekend when two of Alaska's top birding festivals go head-to-head Thursday to Sunday.

Homer hosts the 19th-annual Kachemak Bay Shorebird Festival while Cordova is the site of the 21st-annual Copper River Shorebird Festival.

So how's a Southcentral birding buff to decide whether to commune with millions of western sandpipers in the Copper River Delta or loons and a variety of ducks in Homer?

And why pack so much birding activity into a single weekend?

"There is a reason, an unfortunate reason," said W. Keys, president of Anchorage Audubon. "The peak of the migration right around May 6. For both areas, it's the same weekend.

"The accepted wisdom is that in Homer you'll get probably a larger variety of shorebird species. In Cordova, you'll get fewer species but you'll get tonnage. Just with western sandpipers and dunlin -- you'll get like 90 percent of them in the world right there. It's movie birding, huge numbers of shorebirds.

"You don't get that many, maybe 10,000, in Homer. Plus, Homer is easier, you can just drive. And the festival is a real festival, usually with great speakers."

Audubon Alaska vice chairwoman Sirena Brownlee said she alternates between festivals year to year. Taking the fast ferry from Whittier to Cordova turns transportation into a bird-watching excursion crossing Prince William Sound. Keys has seen jaegers flying beside the ferry for stretches.

"But if you go to Homer, you can take your car so can get to more birding," she said. "Cordova sometimes has more numbers. Homer sometimes has more diversity."

Some birders may let weather help them decide. Cordova is expecting rain on Thursday, with a good chance of cool showers on Friday and Saturday. The forecast for Homer, however, is just clouds. But crummy weather can mean superb birding.

"Sometimes you'll get birds that will really pile in because of the weather," said Nils Warnock, a shorebird expert who's executive director of Audubon Alaska and a speaker in Homer this year. "When they start running into bad weather and the winds are strong, you can see some numbers."

Warnock has an affection for Cordova, though.

"It's a small intimate festival in a great spot," he said "It's truly one of the premier shorebird sites in the world."

Reach reporter Mike Campbell at mcampbell@adn.com or 257-4329.