NEW YORK — Robotic toy hamsters, the latest Barbie dolls and stylish boots are disappearing from store shelves as holiday shoppers start to get serious. But don't confuse this with the days of Tickle Me Elmo.

Instead of a throwback to great buying binges of the past, the empty shelves are just another sign of bad times.

The shortages come from stores that are terrified of ordering too much and are keeping their inventories thin.

"I guess if you see it, you should get it," said Martha Frey, who was surprised when she couldn't find a specific style of boots in a popular size for her 17-year-old daughter recently at a Top Shop in Manhattan's SoHo district.

Shoppers are spending a little more these days, but they aren't going on buying sprees. Stores, remembering how Americans snapped their wallets shut last holiday season, didn't order big piles of merchandise in the first place.

The result, with seven weeks to go before Christmas, is that popular toys are already hard to find.

In fact, the holiday season's early hit — the Zhu Zhu Pets hamster, an interactive mechanical rodent by Cepia Inc. that sells for $9.99 and is being compared to Furby a decade ago — is almost impossible to nab.

Other toys that are already becoming hard to find include Mattel's Mindflex, which measures brain activity through a helmet, a Nerf dart thrower called Nerf N-Strike from Hasbro Inc. and Barbie Fashionista, who can twist her hips and strike other poses.

"Stores just under ordered across the board," said Jim Silver, an analyst at Timetoplaymag.com, who predicted shortages of the top 100 toys by early December. In a typical year, only the top 15 are in short supply that early.

In recent weeks, toy makers have dispatched executives to China to make sure they get enough products to keep shelves full, Silver said. But production times can be long, and chances look slim that people who put off buying a coveted toy until Thanksgiving will be able to get one by Christmas.

Shoppers are starting to notice.

Tami Megal, a 36-year-old mother of girls ages 5 and 9 from Melville, N.Y., had to go to Toys R Us five times before she got her hands on Zhu Zhu Pets a month ago. But she's having a hard time finding the accessories, like the car, pet carrier and bed.

"It's no use to just get the hamsters. You need the habitat," she said. Megal noted that overall worry about shortages has made her start her holiday shopping early. She's almost finished.

The barren shelves are in stark contrast to last year, when stores ordered too much and had to slap big discounts on merchandise as soon as it hit the floor. Holiday sales posted their biggest decline in at least three decades, and the results cascaded into poor profits and even the closings of prominent stores like Circuit City.

This year, inventory is 8 to 13 percent smaller for mid-price clothing, and 10 to 15 percent smaller for home furnishings, said Antony Karabus, CEO of Karabus Management, a retail advisory firm.

Stores would rather be out of stock than stuck with lots of leftovers. But they also know that merchants who carry goods shoppers want will have an edge.