Nine years after California voters decided that egg-laying hens must be given more space, animal-rights activists on Tuesday filed papers for a new initiative requiring the birds to be kept cage-free.

The initiative’s language, submitted to the state by the Humane Society of the United States, addresses the group’s concerns that 2008’s Proposition 2, which required hens to have more room in their habitats, did not achieve the right conditions for farm animals, including pigs and calves.

An economic study commissioned last year by the egg industry for a similar ballot measure in Massachusetts estimated that switching to cage-free farming would add one or two cents to the price of each egg purchased by consumers.

“Californians know that locking farm animals in tight cages for the duration of their lives is cruel and compromises food safety,” said Wayne Pacelle, president and chief executive of the Humane Society. “All animals deserve humane treatment, especially those raised for food.”

The state attorney general’s office must now approve the initiative’s title and summary before supporters can begin circulating petitions. They need to collect 365,880 signatures within 180 days in order to have the measure placed on the statewide ballot in November 2018.

Egg production is a $1-billion industry in California, with some 15.5 million egg-laying hens producing nearly 5 billion eggs annually, mostly on family-owned farms.

The Assn. of California Egg Farmers said it opposes the initiative as currently written because it requires cage-free compliance by 2022, even though the Humane Society had persuaded many retail and restaurant customers to phase in the exclusive use of cage-free eggs by 2025.

“With this new initiative now calling for full compliance by the end of 2021, HSUS is reneging on the original agreement and this expedited timeline may result in supply disruptions, price spikes and a shortage of eggs for sale,” the group said in a statement.