On Wednesday, wildlife and game officials tackled the task of banding the falcon chicks to keep track of them.
The nest on City Hall is one of the earliest building nests known in the world.
"They were documented nesting here in 1946 and 1949 and suspected of nesting here earlier than that," said Dr. Arthur McMorris of the Pennsylvania Game Commission.
The number of falcons in Pennsylvania today is not yet known. However, last year, there were only 29 pairs nesting in the state.
"But we were thrilled because it was an improvement over previous years," McMorris said. "In 1961 the number was zero."
Not only were there no Peregrine Falcons in Pennsylvania - there were none in all of the eastern United States. The birds were wiped out by DDT and other pesticides.
So, the game commission began raising falcons in captivity and turning them loose into the wild.
On Wednesday, officials took care to place tracking bands on the four nestlings.
"This is our important tool for identifying the birds and monitoring their population," said McMorris.
The chicks are about 23 days old. They are fully grown, but not yet able to fly.
As experts come to band the birds, their parents show their disdain for the intrusion and the chicks were frightened when taken from the security of their nests.
Once out, they got a medical exam.
"I'm just looking for anything that might be wrong," said McMorris.
Once given a clean bill of health, the banding process begins. The bands will help officials keep track of the young and determine the origins of the adult birds.
"Because of banding, we know the female bird nesting here came from a nest in Tukahoe River, New Jersey," said McMorris.
The birds nested atop the Wachovia Bank building last year, but are not expected to move outside the city.
"As far as they're concerned they chose a different cliff ledge on the same cliff, and their cliff is Center City Philadelphia," McMorris said.
Peregrine Falcon nests have also been found in Ohio and in Canada. Their nests are gradually becoming more common and wildlife officials are determined to ensure their survival.