The pattern of egg production is changing not only in Iowa and the midwest but nationally. Here's a summary of changes over a recent 10-year period and an explanation of some reasons for the change.
Eggleton, Leonard Z.
"Egg Production: Trends and Outlook,"
Iowa Farm Science: Vol. 19
, Article 3.
Available at: https://lib.dr.iastate.edu/farmscience/vol19/iss12/3