City life troublesome for peregrine falcons
Peregrine falcons don't come with airbags.
Historically that was never a problem for the birds, even though they can cruise at 60 miles per hour in level flight and hit speeds of 200 miles per hour when in a predatory, vertical dive called a stoop. But these are different times.
Whereas once Pennsylvania's falcons nested exclusively on cliffs along river edges, today most use manmade structures. The Cathedral of Learning famously has a nest, as does the Gulf Tower. Others can be found across the state on buildings, bridges and even smoke stacks.
That's been partly by design.
Peregrines disappeared from Pennsylvania in 1959 and from the rest of the East Coast two years later. That's long been blamed on the effects of the pesticide DDT.
When efforts to reintroduce them took place as recently as the 1990s, biologists sited the birds on manmade structures rather than cliffs so as to avoid their main predator, the great horned owl, said Patti Barber, a wildlife biologist with the Pennsylvania Game Commission.
But it turns out such nest sites are “an ecological trap,” she added.
Adult peregrines, which feed primarily on other birds, can thrive in urban environs with their abundant pigeons and songbirds. But their young often struggle mightily. In the process of learning to fly and hunt, they often crash into the large, reflective windows of office buildings, collide with vehicles or fall into traffic. Two-thirds die or require human rescue each year.
“Moving to these manmade structures comes with a high cost,” Barber said.
The commission's goal remains to restore falcons to the point that they can come off the state's endangered species list. A management plan spelling out how to do that is nearing completion now.
It will say the species has recovered when populations are self-sustaining, Barber said. That would mean having at least 22 nesting pairs — half as many as were known to exist at Pennsylvania's peak — with each fledging at least half their young, to a rate of about 1.5 fledglings per nest, on average.
The state isn't close to that yet. Peregrines have been spotted at about 40 places, but fewer than a dozen are known to have produced nests, and only four of those have occurred on cliffs.
Getting that last number up is key.
While manmade sites account for the majority of nests, they contribute only 25 percent of the birds added to the population each year, Barber said. Cliff sites are better for the birds and more productive.
But most of the state's falcons have been content to nest around people, where — for the adults at least — the living is easy.
If that continues, and just one-third of urban peregrine fledglings survive each year, the species won't be able to sustain itself long-term, Barber said. And well-intentioned efforts to put the birds in the city will partly be to blame.
“An unintended consequence of that was the birds realized, from a predator standpoint, those manmade structures were a great place to nest. But after that, there are a lot of other problems to deal with,” Barber said.