Wearing helmets and bulky clothing, the intruders moved warily along the rooftop toward a plywood box, keeping a close watch overhead as the parents of two Peregrine falcon chicks circled.
"The defense of the adults is to swoop at you," explained Mary Hennen, a biologist with the Field Museum of Natural History who has been working with falcons for 25 years as part of the Chicago Peregrine Program.
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Once there were as many as 500 Peregrine pairs east of the Mississippi River. By the 1960s there were none, Hennen said, victims of the pesticide DDT. The chemical inhibits calcium production, leading to thinner egg shells that would be crushed by adults during incubation.
But with the help of environmental groups, the cliff-dwelling bird of prey has made a comeback and was removed from the federal endangered species list in 1999. More than two dozen pairs and their territories throughout Illinois are now monitored by scientists.
That's what brought Hennen to the sixth-floor roof of the retired Unit 6 of the mammoth Waukegan Generating Station overlooking Lake Michigan, where Peregrines have come to breed every year since 2002.
With Matt Gies, registrar for animal acquisitions at the Shedd Aquarium, running interference using a broom, Monday's task was to collect the two new arrivals from their man-made nest, move them to a trailer nearby for blood testing and banding, and get them back home as quickly as possible.
"The bands we put on are unique to the individual, so you can look at longevity and dispersal," Hennen said before what has become an annual mission at the plant. Three eggs were laid but only two chicks survived.
In this case, the bands as seen through binoculars allowed Stephanie Ware, also of the Field Museum, to confirm on her smartphone that mother, Fran, and father, Greg, indeed had returned to Waukegan. Fran was born on the Hoan bridge in Milwaukee in 1999 and Greg in 2005 on the Racine County courthouse in Wisconsin. Fran has had 40 chicks since she was first documented at the power plant in 2002.
In cooperation with the Field Museum, Waukegan plant workers in the early 1990s built the shelter and positioned it on the northeast corner of the building as a lure. But it took a while.
"They find it on their own," Hennen said. "They're wild birds. We don't control where they go."
Why here? "They're cliff-dwelling birds. It's a pseudo cliff to them," she said.
Peregrine pairs may or may not stay in their territory over the winter. The chicks, known as eyas, are about 3 weeks old and will stay with the parents for three or four more weeks as they are weaned and taught to fly and hunt.
"Then, essentially they're on their own," Hennen said.
But their arrival creates an annual buzz, with the movements of the young available on a real-time basis.
"People are excited," said Mark Nagel, station director. "They watch the webcam. The chicks get bigger by the day."
The chicks were identified as a male and female. The female was named Nancy after Nancy Johnson, a recently retired teacher at Greenwood School in Waukegan who this year presented an e-book project on Peregrine falcons at a tech conference in Springfield. The male was named Cesar, after Jan Cesar, another Greenwood teacher, who died of cancer.