California shell egg prices jumped by nearly one-third between Oct. 25 and Nov. 1.

“It’s squarely bird flu,” said Mike Weber, a producer near Petaluma, Calif.

A recent outbreak of avian influenza in California affected seven commercial flocks, one backyard flock and more than 1.57 million birds combined, according to the U.S. Centers for Disease Control and Prevention. The last reported detection was Nov. 2. U.S. detections in commercial flocks included one in September and nine in October, according to the CDC. Backyard flock detections included two in September and seven in October. Total birds affected surged to 4.37 million in October, up from about 70,000 in September.

Detections are higher in fall and spring as wild birds spread the virus as they migrate, according to CDC.

Weber believes bird flu arrived as a result of climate change in that, “when water sources dry up, birds are at fewer places” and more of them congregate together, he said.

In the most recent outbreak, the first detection in a U.S. commercial flock was in February 2022. Bird flu so far has affected more than 104 million poultry in 48 states. The U.S. has more than 378.5 million egg-laying hens.

Bird flu historically was seasonal in that the virus survived in cold, wet conditions but did not respond well to dry heat, Idaho state veterinarian Scott Leibsle said. But with the most recent virus, “we started to see it losing that seasonality, and persisting.”

In Colorado, for example, “it’s plenty hot and dry in the summer, and at the end of June and July they got hammered with positive dairies and poultry,” he said.

Egg production drops slightly when daylight hours are reduced seasonally, said Allie Taylor, store manager at McIntyre Pastures near Caldwell, Idaho.

Perishable eggs must be sold within about a month, which means supplies aren’t built up when a production loss occurs, Weber said. His family’s farm is a member of a cooperative, “and when we lost chickens, we had to buy eggs to supply supermarkets. It can be costly.”

Demand also rises at the end of the year as people eat heartier meals, he said.

Weber Family Farms — a small to mid-sized producer from a U.S. industry perspective, Weber said — tested positive for bird flu Dec. 15. Months of facility work and flock replacement at the cage-free farm followed.

“It’s kind of back to basics,” Weber said.

Basics include buying chicks when they become available, raising them for 18 weeks and then moving them into production buildings, where they will spend five to six weeks before they start laying, he said.

For a dozen Grade AA eggs in cartons, California shell egg prices Nov. 1 were $5.03 for jumbo and $4.92 for extra large, according to USDA’s Agricultural Marketing Service. Prices on Oct. 25 were $3.81 for jumbo and $3.70 for extra large.

Large eggs were $4.85 on Nov. 1 and $3.63 Oct. 25.

The prices reflect negotiated sales not including discounts or other contract terms.

Consumer demand for shell eggs over the past week remained limited ahead of the November holiday demand season, but interest was expected to rise as Thanksgiving neared, according to the Nov. 1 USDA Egg Markets Overview.