A second batch of birds that have recovered after getting covered in a mysterious gray substance will be released back onto San Francisco Bay Area's shorelines.
The International Bird Rescue was releasing 20 to 30 rehabilitated birds at Middle Harbor Shoreline Park on Wednesday.
About 170 birds died after getting covered in the odorless substance, which sapped the seabirds' ability to insulate themselves against the cold.
State investigators haven't yet determined what was in the gunk that looked and felt like rubber cement, or where it came from.
About 80 birds previously were released in the Bay Area.
Another 323 live birds have been taken to the International Bird Rescue to be cleaned since mid-January, when the substance first showed up on San Francisco Bay Area shorelines.
No new goo-covered birds have been found since Jan. 22.
Explore further: Tests results pending on dead birds in Northern California