Little chickens, whose quarter-pound breasts fit perfectly inside a bun, are proving essential to the war effort. In the process, they’re getting harder to come by.

A shortage of the smaller birds derailed the Popeyes challenge to reigning champion Chick-fil-A last summer, and most petite poultry are sold in grocery stores, not in chain restaurants. Now the supply will be further tested as more competitors jump into the fray. McDonald’s Corp., the world’s biggest restaurant chain, is testing new fried-chicken sandwiches in four U.S. cities with the added pop of MSG, a controversial flavor enhancer it says it doesn’t use in its national menu. Wendy’s Co. is going all in, spending $30 million to beef up its chicken supply chain.

“Consumers don’t want tough and tasteless big chickens,” said Scott Sechler, owner of poultry producer Bell & Evans. There’s “increasing consumer demand for smaller, premium-quality birds.”

Chickens have been the most popular meat in America for a long time. More chicken is devouredin the U.S. than anywhere else in the world — an average of 93.5 pounds per person last year, according to the National Chicken Council. The birds have been getting progressively bigger over the years. Today’s broiler varieties, the ones raised for meat, average more than 6 pounds (2.7 kilograms) each. In 1925, they weighed 2.5 pounds.

So how did these fast-food chains so quickly find success with their chicken sandwiches? Birds lighter than 4.25 pounds.

Small is beautiful

The summer skirmish focused Big Chicken’s attention on the smaller and less plentiful variety. Breasts from wee birds recently reached nearly triple the cost of breasts from a “jumbo” nine-pounder, a historically wide difference, according to Russ Whitman of commodity researcher Urner Barry.

Jumbo boneless breasts on Tuesday were at a record low of 87 cents a pound, while prices for the smaller version jumped to $2.53, the highest in a year, Urner Barry said.

When it comes to sandwiches, small is beautiful. Cutting up a bigger portion takes labor, and what does one do with the trim? Restaurants want product that comes ready to go at the right specifications, said David Maloni, executive vice president of analytics at supply-chain consultant ArrowStream.

“It’s getting harder and harder to get that” smaller bird, “so they’re paying a premium,” Maloni said. The fast-food chains won’t settle for bigger birds, he said.

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