With the worst-ever U.S. outbreak of bird flu leaving almost 45 million dead chickens and turkeys, egg costs are climbing to records.

Prices for wholesale consumer-grade eggs, sold in grocery stores, more than doubled in the past month to reach an all-time high on Tuesday, according to commodity researcher Urner Barry, which has been tracking the industry since 1858.

More than 10 percent of the country's laying hens have been wiped out by the spread of avian influenza across the Midwest. Iowa, the top U.S. egg producer, was hardest hit. Prices for breaker eggs — those cracked and sold in liquid form for use by industrial food manufacturersm wholesale bakers and restaurants — started reaching records last month. To make up for supply losses, buyers are snapping up consumer-grade fresh eggs, normally sold in cartons to shoppers, and driving costs higher.