Falcon drama in downtown San Francisco

By Glenn Stewart

At this moment, thousands of people are nervously watching the falcon nest camera atop the PG&E headquarters building at 77 Beale Street in downtown San Francisco. The female Peregrine Falcon nesting there since 2008 has disappeared and a new female has moved in to the territory.

All Peregrine Falcons look similar but not exactly alike. Variations in plumage on the head and neck make it possible to differentiate among adults. For example, some have a full black hood while others have more discernible malar stripes over their eyes. Markings on the upper chest area vary as well.

I will provide some background before reviewing the drama unfolding at this nest.

After we at the Santa Cruz Predatory Bird Research Group were involved in the Peregrine Falcon recovery from the mid-1970s to mid-1990s, we began watching the Bay Area population as a manageable sample of the statewide population. Today, we try to monitor about thirty Peregrine Falcon nests for occupancy and productivity from Marin to Monterey counties, and east to Contra Costa and Santa Clara counties. We enter nests to band about 25 nestling Peregrine Falcons each year.

During 2012, we followed 12 pairs nesting on traditional cliffs and another 12 pairs that we refer to as urban pairs because they use buildings, bridges and cranes as nest structures. It was interesting that the urban birds were more productive, producing an average 2.25 young per site while those on natural cliffs produced an average of 1.66 young.

The first post-DDT-era Peregrine Falcons seen in the Bay Area appeared on the Bay Bridge in 1983. The female was found shot to death a few months later near the duck blinds adjacent to the Bay Bridge Toll Plaza. Peregrines began using sites on both the east and west spans of the Bay Bridge in following years with little success. The bridge proved advantageous for staging hunting forays but was lethal for fledging youngsters, due to the long distance to land and paucity of perches below the roadbed for clumsy fledglings.

When we found peregrines hunting downtown, we put a nest tray filled with gravel on an unused 33rd story balcony of the PG&E building. The building was a favorite perch in the late 1980s but the nest tray was not used until 2003.

Today, we are witnessing events at Peregrine Falcon nest territories unknown to most living peregrine watchers. The population of floating (non-breeding) adults appears be robust because we are observing challenges by intruder adults with increasing frequency. Last year, we observed challenges at four nest territories. At the San Jose City Hall eyrie, an intruder displaced the breeding male and stayed to help feed his new mate during incubation and then share in incubation duties. He hunted for birds to feed the growing young and in every way became a parent to progeny that were not his.

Such behavior indicates the lengths a Peregrine Falcon will go to in order to gain ownership of a breeding territory.

Over the past several days, those closely watching the PG&E nest camera soon realized that the male—known as Dapper Dan—was doing all the incubating. Dan and his partner, Diamond Lil, have very similar markings and at times it is difficult to differentiate between them even though Lil (as in most raptors) is about one third larger than her male counterpart.

Lil just disappeared. But what was striking to us was that a new female appeared almost immediately. We watched as over a period of about three days, Dan became the sole incubator of the eggs and the new female appeared. At one point on March 12, the new female approached the nest and Dan stood above the eggs and bowed forward in an apparent nest ledge display that is part of peregrine courtship.

Dan has left the nest tray for brief rests and to quickly capture a bird and return to the PG&E balcony to eat before returning to his incubation duties. The new female has walked up into the gravel-filled nest tray (peregrines do not build nests but scrape a depression in sand or gravel for an egg cup), laid down briefly as if trying out the gravel, but has not laid upon or incubated the eggs.

Typically, Peregrine Falcons share incubation duties. The female incubates all night and the male does all the hunting and covers the eggs while the female eats and takes a break in the sunshine. Eggs are fairly resilient. They can withstand periods of exposure — sometimes for hours — if incubation has otherwise been normal and consistent. Erratic incubation can be harmful.

This set of eggs has had 20 days of incubation. The normal incubation period is about 33 days. We are all waiting to see the outcome at a Peregrine Falcon territory that has been watched closely for nine years, thanks to a nest camera. Will Dapper Dan accept the new female as a mate? Will the new female help incubate the eggs of another falcon? Will they initiate a new nesting attempt at the current nest box?

You’re invited to join us in watching this drama unfold. To see the PG&E falcon nest camera, go to www.scpbrg.org. To join the online PG&E falcon forum and receive up-to-the-minute reports on the nesting pair, join the Yahoo! Group: SF_PGE_Falcons.

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Glenn Stewart is director of the UC Santa Cruz Predatory Bird Research Group, which was a leader in the work to recover the Pacific States Peregrine Falcon population from near extinction. Today, it operates a university based conservation education program that includes: Falcon Nest Cameras, School Assemblies, Citizen Science Participation, University Internships, and Peregrine Falcon Research