Around 700 black-headed munias died while being transported to Panvel.
The dead birds were found at Panvel bus depot on Friday evening.
The police are looking for the person who was transporting the birds reportedly brought from Karnataka in two cages.
Passengers saw the dead birds when the cages were being unloaded from the ST bus. The man who had loaded the cages fled, when they raised an alarm.
Senior Police Inspector of the Panvel police station Suresh Bhosale said: "Around 700 birds were brought to the bus depot. Hardly 17 to 18 have survived. We have handed them over to the forest department."
"We are looking for the man who had brought the birds. It seems he got scared after seeing them dead and fled," added Bhonsale.
Mahadeo Athavle, a bird lover, said: "These birds are found all over the state.
They cannot survive in rain and that could have caused their death."
Forest department officials Pawar said the surviving birds would be released in the nearby Karnala Bird Sanctuary.