Investigators searched the property in Pleasanton and found about 600 birds along with various bird fighting paraphernalia.
There were as many as 60 people at the party, which is a violation of the stay-at-home order.
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Law enforcement says many of them took off running when officers arrived.
Several, however, were detained and investigators are working to figure out who organized the event.
Detectives were notified and an active animal cruelty and cockfighting investigation is underway, officers said.
They added, "All of the birds have been seized by the Sheriff's Office and are safe. They will be cared for and treated for any illness or injury."
Anyone with information about this case is asked to contact ACSO at (510) 667-3600.
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