SAN FRANCISCO – Thousands of critically endangered baby toads are en route to Puerto Rico from the Oakland Zoo.

Thursday’s flight marks the second year the Oakland Zoo has taken part in a breeding program to re-establish the green, brown, red and yellow Puerto Rico crested toad on its home island. Zoological manager Adam Fink says the zoo uses wine fridges and rain chambers to simulate conditions that adult toads need to breed.

Fink and others spent Wednesday netting, bagging and counting 4,069 week-old tadpoles for their flight in an airline cargo hold. Wildlife officials expect to receive the tadpoles later Thursday in Puerto Rico. They’ll be released in protected ponds on the island.

The Puerto Rico crested toad had been thought extinct until biologists found a few survivors in 1984.