U.S. retailers are shoring up their e-commerce operations, playing catch-up as budget-conscious shoppers defect to online merchants such as Amazon.com Inc., Bloomberg said.

Walgreen Co.  and Toys “R” Us Inc. are among the chains rushing to open distribution centers, revamp websites and integrate their stores more seamlessly into their online operations. Amazon, meanwhile, plans to open 15 distribution centers this year, Bloomberg said.

“Many brick-and-mortar retailers have had their ‘aha’ moment as Amazon continues to grow and the Internet is getting more viable,” said Mick Mountz, CEO of Kiva Systems Inc., which has seen orders surge this year for the warehouse robots it sells to retailers.

Some chains were caught off-guard by surging online orders in the 2010 holiday shopping season, Mountz said. One customer lost “millions of dollars” of Web orders it couldn’t fulfill because of inadequate online infrastructure, he said.

Last holiday season, online sales reached a record $32.6 billion, up 12 percent from the previous year, according to market researcher ComScore Inc.