Intrigued by the bird since fourth grade, and all but consumed by it since 2009, Greenberg has written its sad history - and the lessons to be learned from it - in A Feathered River Across the Sky: The Passenger Pigeon's Flight to Extinction, published this month.
He'll speak at the Wagner Institute of Free Science on Thursday, and at John James Audubon's home, Mill Grove, in Montgomery County on Friday.
Other events are being planned at area institutions. At the Academy of Natural Sciences of Drexel University, visitors can see two dozen passenger pigeons preserved through taxidermy in one of the museum's permanent dioramas - possibly the largest number on display anywhere.
For many who study birds - definitely for Greenberg - the passenger pigeon was in a class by itself. Descriptions of its magnitude almost defy belief. Nesting birds were so numerous, their weight broke tree branches. Their "screaming noise" could be heard for miles.
A migrating flock along the Ohio River, Audubon wrote, blocked the sun for three days.
"The pigeon was no mere bird," the conservationist Aldo Leopold once observed, "he was a biological storm."
Passenger pigeons were much larger than today's urban rock pigeons and mourning doves. Their trim, muscular bodies were built to fly - at up to 60 m.p.h., scientists figured.
Perhaps inevitably, given passenger pigeons' abundance, their lives would become intertwined with humans', and even find their way into the lexicon. Supposedly, the Philadelphia neighborhood name Moyamensing is the Native American word for "pigeon droppings."
Beds and pillows were stuffed with their feathers; their fat went into shortening and soap. They were used in medicines, and celebrated in literature.
But their history - and much of Greenberg's book - is also a study in carnage. People killed them for food. People killed them for fun. The birds were shot, trapped, and netted. Hunters set fires under trees where they roosted or fed the birds alcohol-soaked grains, then scooped them up.
When huge flocks of passenger pigeons passed over Philadelphia, people opened fire from their balconies and rooftops, said Keith Russell of Audubon Pennsylvania, who has researched numerous historical accounts from this region.
The expansion of railroads opened up a national market for pigeon meat. Birds could be packed 300 to a barrel and shipped to major metropolitan areas - cheap food for a burgeoning urban population. And the expansion of telegraph lines meant that wherever the pigeons showed up, operators could send out alerts.
Greenberg could find only two photos of live wild birds. One, taken in 1870 in Bucks County, shows Solebury Township pigeon trapper Albert Cooper with three "blind decoys," their eyelids sewn shut to temporarily blind them.
For the species, the end came quickly. A population "bewilderingly vast" in 1860 was virtually gone by 1900, Greenberg writes. There was little outcry.
He ultimately found some reason for optimism in what was otherwise a depressing book project.
The passenger pigeon's demise spawned the country's first environmental movement, he said, and a federal law regulating the killing of migrant wildlife.
Greenberg and others have formed Project Passenger Pigeon, to ensure that no one forgets the bird, and to foster conservation for other species.
After all, "we're still losing things," Greenberg said. The dusky seaside sparrow - a bird that Greenberg once saw - went extinct in 1987. The world's only four survivors were taken from a Florida marsh to Walt Disney World, where they died.
Greenberg sees parallels to the passenger pigeon slaughter in pelagic fishing techniques, where factory boats with massive nets scoop up everything in their path.
To Audubon Pennsylvania executive director Phil Wallis, the bird is "a stark example of what could happen if we're not vigilant, if we're not good stewards, if we don't balance the needs of nature and our own wants and desires."
He need only look above the mantelpiece at Mill Grove, the state organization's base. There, in iridescent glory, is a pair of preserved passenger pigeons.
"They're beautiful," Wallis said. "They just make my heart sing and cry at the same time."
Author Joel Greenberg will give two talks this week. Both are free.
Thursday, 6 p.m., Wagner Free Institute of Science, 1700 W. Montgomery Ave. Registration requested at https://passengerpigeon.eventbrite.com Information: 215-763-6529.
Friday, 6:30 p.m., John James Audubon Center at Mill Grove, 1201 Pawlings Road, Audubon, Montgomery County. Registration requested by e-mail: JJAC_education@audubon.org. Information: 610-666-5593.