The ruby-throated hummingbirds have not yet returned to fly about the yard and drink nectar, but spring migration is underway nonetheless. The largest North American migratory beast is clanking northward, up the highways on a rolling bed of straw, nervously perched, eyes ablaze, twitching as the air gets cooler with every mile. The surest sign of spring is the arrival of the racehorses at Aqueduct.

All the big horses have been south for the winter, pasturing and racing in balmy environs such as Florida and Louisiana. Now, the march to the Kentucky Derby is on, and the next stop is out in Ozone Park at the Big A, where the most significant three-year-old Derby prep in New York will go off this Saturday: The 84th running of the Grade 1, $750,000 Wood Memorial for three-year-olds at nine furlongs on Aqueduct’s main track.

The Wood has historically been an important race on the Road to the Roses. Five years after the race was inaugurated at the old Jamaica course, Gallant Fox won it before going on to win the Triple Crown. He was the first of 11 Wood winners that got their picture taken in the winner’s circle on the first Saturday in May. Three more of those winners went on to take the Triple Crown. Secretariat is not on that list; he was upset in the 1973 edition of the Wood by his stablemate Angle Light. Within the last decade alone, a partial list of horses that have gone to the gate in the Wood and gone on to figure heavily in the Triple Crown includes Fusaichi Pegasus, Monarchos, Funny Cide, and Empire Maker. There are no certainties, of course. Bellamy Road blew the doors off the Big A and set a stakes record in 2005 before flopping as one of the favored ponies in Louisville.

More than history should draw the attention of race fans, however, as this year the Wood is poised to play an especially important role for two reasons: dirt and a horse named War Pass.

This is the age of the artificial surface. The new, engineered surfaces are touted as more forgiving, and while the industry is swept up arguing the merits of the fake dirt — many pencils are being pushed, many screeds drafted — it seems that fewer horses break down running over it. It’s a good move, well-intentioned and focused on the welfare of the thoroughbred, even when that welfare comes at great expense.

I’m all for it.

It does, however, change the way one reads the Racing Form. Keeneland and the California tracks — as well as many others — are racing over artificial surfaces. When you scan the past performances of the Derby contenders, you see quite a few little circled A’s on the performance lines. Many more than in the past.

The Derby, however, is still run over dirt, and so is the Wood.

So while the Blue Grass Stakes, run next week at Keeneland, and the Santa Anita Derby, run Saturday, will remain important races, attract good horses, and always have a heavy influence on spring racing, they may not be as applicable as in the past. A race over artificial dirt is just that. I don’t know that the difference is as great as the apples-to-oranges comparison between turf racing and dirt racing, but we’re well within the difference between, say, Granny Smith and Red Delicious.

Two turns, Grade 1 company, over dirt — the Wood is going to figure.

And in this running of the Wood, War Pass will figure.

War Pass came into his last race, the Tampa Bay Derby on March 15, undefeated, the shining star. He had closed out his juvenile season with a victory at the Breeders’ Cup Juvenile and won two-year-old of the year. He had come back in a Gulfstream Allowance on February 24 and easily opened up 7 1 /2 lengths to take it. Then: disaster.

Owner Robert La Penta said that “he only ran about five furlongs!” He finished dead last. It goes without saying that this wasn’t the route to the Derby that La Penta and trainer Nick Zito had mapped out for him. Will he return to form? Even a game finish on the board in Saturday’s race will prove he’s back. If you throw out the last race, he should annihilate the field.

I’m not laying any money on him — at least not until I see him in the paddock.

It’s important to know upon how little a horse’s reputation is built, and how little it takes to change the status of a favorite. This was wonderfully, if inadvertently, pointed out by Richard Dutrow, who trains the winner of the March 29 Florida Derby, Big Brown.

After that race he said: “I don’t see any three-year-old that can beat him anywhere in the world and I’ve watched them all.” That’s easy to say from the winner’s circle. Dutrow said that he had “totally misjudged him before his last race. I didn’t know he was that good.”

Big Brown went from “totally misjudged” to the best horse in the world in the span of nine furlongs.

War Pass has made two starts in 2008. He romped in one and he flopped in the other — you can’t get any more unpredictable than that. And there will be company for him up on the pace Saturday. Giant Moon has a quick jump out of the gate. He finds himself in a similar spot as War Pass, having won four in a row on or near the pace before he flopped last time out in the crazy fog at the Gotham Stakes. First Commandment will also be near the pace. He’s taking a huge step up to run with these horses, but he’s headed in the right direction.

The horse to watch is Bill Mott’s Court Vision. He took the tough road of debuting in a graded stakes, the Fountain of Youth at Gulfstream Park on February 24. He ran like he needed a race. He was running seven wide and he finished over six lengths off of the lead, but he was still running at the end. Maybe his loss will keep the odds up.

mwatman@nysun.com