The baby falcon began hatching Monday night, said Glenn Stewart, a biologist at the University of California at Santa Cruz.
The gender of the falcon chick will be known on April 28, when Stewart climbs to the nest on a City Hall ledge 18 stories above the street to band the bird. The banding procedure allows scientists to track them and collect data.
The falcon's parents are Clara and Esteban Colbert. This is Clara's fifth nesting season at City Hall and her third with mate Esteban Colbert.
Three more of Clara's eggs have not yet hatched. Stewart said she is in "terrific" condition.
"She's been lying in one place for 33 days," he said. "She'll continue to brood the babies for two more weeks."
The remaining three eggs are expected to hatch in the next few days, Stewart said.