Thanksgiving birds are smaller and more expensive this year after the worst outbreak of avian flu in U.S. history wiped out nearly 8 million turkeys.

The U.S. Department of Agriculture reports that per-pound turkey prices are up 21 percent from last year. Frozen turkey supplies are at the lowest in three decades.

But not all retailers are passing those costs on to consumers. Many stores discount turkeys to get shoppers in the door during the holiday, taking a loss to boost sales of side-dish items.

Walmart is selling the birds for 64 cents less than last year, according to the Associated Press.

The turkey shortage has meant fewer purchases for many food banks this year.

Purdue University Economist Corinne Alexander said small even price increases can hit low-income families hard.

"For these families, any food price rise is significant," Alexander said in a release. "We should remember those who are less fortunate and share our food bounty."

Read More:

• Thanksgiving Turkeys Cost More Than Ever After Bird Flu Wipeout (Bloomberg)
• Fear Not: You'll Get Your Thanksgiving Turkey, Pumpkin Pie (Associated Press)