(CNN) The only word Stephen Maciejewski can use to describe the event is traumatic.
It should have been a routine morning for Maciejewski, the 71-year-old Audubon Pennsylvania volunteer.
He expected to find some dead or injured birds that need to be collected, and tallied. It was, after all, peak migration time.
But on the morning of Sunday, October 2, he quickly realized there were far more birds than he was used to retrieving. Hundreds more.
"I've never seen anything like this," Maciejewski told CNN on Thursday. "There were birds everywhere, and they were all dead."
Maciejewski had begun collecting the fallen birds at 5:30 a.m. Two hours later, he realized he needed backup. He called Keith Russell, the program manager for urban conservation for Audubon Pennsylvania.
"I immediately jumped out of bed," Russell told CNN. "This was a big deal."
Maciejewski, Russell and other volunteers were able to collect 400 birds, most of which were dead, others injured. But they weren't able to reach every building they needed to that day.
So, they estimate the birds they found represents just one-third of the deaths -- meaning, the total could be anywhere between 1,000 to 1,500.
"I just can't stop thinking about it," Maciejewski told CNN.
Audubon Pennsylvania suspects that the mass deaths of hundreds of birds that collided with several buildings in Center City, Philadelphia, started the previous night. Peak migration season, which typically begins in the fall, combined with poor weather conditions, may have led to the incident, it said.
"Philadelphia is along the Atlantic Flyway, so the birds are migrating through the city in gigantic numbers," Russell told CNN. "With lots of clouds and rain, and the bright lights that come from buildings, they get disoriented and they gravitate towards towers or buildings that are nearby."
Some of the species found that day include warblers, thrushes, vireos and sparrows.
As many as a billion birds die each year from colliding with buildings in the US.
According to Russell, mass killings, of a much lesser magnitude, have been documented before.
"Birds have been known to strike buildings since the 19th century," Russell told CNN.
Other recorded events similar to last week's have occurred in 1905, 1915 and 1948. Last fall, there were similar mass killings at a NASCAR building in Charlotte, North Carolina, and in Galveston, Texas, in 2017 during a storm.
Audubon Pennsylvania recommends that residents practice turning out their lights and pulling down shades or curtains each night to avoid attracting birds.
Those collected by Audubon Pennsylvania will be donated to the Academy of Natural Sciences for research purposes.