By Glenn Stewart, UC Santa Cruz Predatory Bird Group
SAN FRANCISCO — Falcon watchers from all over the world witnessed a Darwinian turn of events in the nest atop PG&E’s headquarters this year. First, resident falcon “Lil” was driven from the nest by a new falcon. Humans mourned the loss of Lil, but the Bay Area peregrine falcon population was served when a new female, “Cher,” took her place.
Challenges to resident falcon pairs by new individuals indicate a healthy and expanding population.
Each year since 2004, those in downtown San Francisco and a wide audience of online nest camera watchers have thrilled to the courtship, breeding, and falcon chick rearing activities of a pair of peregrines on a 33rd floor balcony overlooking the Bay Bridge. Five different peregrine falcons have been pair members at the scenic location in 10 nesting seasons.
This year, nesting already was under way when the new female arrived. The peregrines, named Dan and Lil, were incubating four eggs. Lil’s ouster made Dan the sole incubator of the eggs. Cher was eager to gain ownership of a nest territory but had no interest in incubating the eggs of another.
In the process of gearing up for nesting, Cher kicked one of Lil’s eggs from the nest. Soon, she added three of her own eggs to the nest and began to incubate all six. Thirty-three days of incubation for Lil’s eggs came and went and we learned that Dan’s absences from incubation to feed himself were too frequent and too long in duration for the growing embryos to survive. When 33 days of incubation for Cher’s eggs also passed, nest watchers who had been following the drama since February knew that none of the eggs would hatch.
The pair continued to incubate. Dan shared incubation duties with two females for an unbelievable 128 days. With the passage of Summer Solstice and the diminishing daylight that accompanies the season-changing event, we collected the eggs while still intact for routine contaminant analysis.
Peregrine falcons are the fastest animals in the world, diving to capture birds in the air at 200 to 300 miles per hour. The peregrine population shrank to near extinction between World War II and 1970 due to the eggshell-thinning effects of persistent toxic chemicals in the environment, principally DDT. By then, peregrines were extinct east of the Mississippi River, just a handful existed in the Rocky Mountains, and just two pairs could be found in California. DDT was banned, the Endangered Species Act was passed, and a massive cooperative effort got under way to avoid total extinction.
Biologists at UC Santa Cruz and Cornell University worked with state and federal wildlife officials to successfully remove the peregrine falcon from the list of Endangered Species by 1999. Today, there are an estimated 250 to 300 pairs of peregrine falcons in California.
Ninety-five percent of the peregrine falcon nest territories established since the population recovery have remained occupied in succeeding years so it is likely that Dan and Cher will be back for another try next spring.
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Glenn Stewart wrote this story for Currents. He is director of the UC Santa Cruz Predatory Bird Group and author of the new book, “Eye to Eye with Eagles Hawks and Falcons.”