The soaring flight of raptors, also known as birds of prey, can be observed in all its glory during the months of September and October at Hawk Hill in the Marin Headlands.
This hill, at the top of Conzelman Road, is a popular spot for viewing the many species of raptors and other migratory birds that congregate here in their travels along the Pacific Flyway, which runs the length of North and South America.
At the height of the migration, the skies above the hill see a steady stream of hawks, ospreys, turkey vultures, kestrels, and peregrine falcons, gliding along the thermal updrafts. Hawks and other raptors have a preference for flying over land, so they often linger at Hawk Hill, waiting to get a good tailwind to carry them across the open water of the Golden Gate and continue their migration southward.
I attended a talk that was held at Hawk Hill by Tom Delebo of the Golden Gate Raptor Observatory (GGRO). A group of people gathered at one of the old military platforms at the top of hill, where GGRO volunteers have laid out tape to demonstrate the average wingspan of a variety of raptors, including hawks, falcons, harriers, and a bald eagle. We looked out from the hill across the Golden Gate, where the towers of the bridge peeked out just above the receding fog. A group of red-tailed hawks were circling below us, riding the thermals of warm air that rise up above the Marin Headlands. Tom explained to us that this formation of birds is called a "kettle," with birds circling and wheeling around as if they were inside a cauldron.
Tom talked about the history of Hawk Hill, his voice occasionally drowned out by the bellowing foghorns. Dr. Laurence Binford, an ornithologist with the California Academy of Sciences in the 1970s, noticed large numbers of raptors flying past his office window in Golden Gate Park. He set out to find a point of concentration and determined that Hawk Hill, which was previously known as Point Diablo, was a raptor migration site. A banding program began in the 1980s, and volunteers began to conduct tabulations of the various different raptor species that frequent Hawk Hill.
These species are many and varied, so Tom gave us some pointers to help identify the birds that were circling high above us. First he talked about Hawks, or "buteos," which is the scientific name of the genus. There are many different kinds of hawks, including Cooper's Hawks, Swainson's Hawks, Red-tailed Hawks, and Red-shouldered Hawks. While they are all known for their sharp eyesight, they can be differentiated by size, color, shape of the tail, and specific markings such as "patagium," which is the distinctive pattern that appears on the underside of the shoulders. Then we learned about Ospreys, which are known as "fish hawks," because of their favored food. An Osprey will carry a fish in its talons, always with the fish head facing forward, because the bird is smart enough to know that air will flow over the fish scales with the least resistance that way.
As many as 30,000 raptors can pass over Hawk Hill during the annual migration, which takes place from late August until the end of October. The peak of the migration is generally the third week of September, during which time observers have counted as many as 1,200 birds in a single day.
After the talk, we were joined by GGRO Research Director Buzz Hull, who brought up a Cooper's Hawk that had just been captured and banded. Buzz told us about the banding program, and the way that birds are captured using a trap with a small bird as bait. The birds are never held in captivity more than hour, so as not to stress them out too much, just long enough to band them and release them.
Banding the birds can provide a wealth of information about the migration patterns of these birds, some of which can travel from Alaska to Argentina in a single year. Buzz kept the Cooper's Hawk, which is a very tiny bird of prey, in a long metal tube, confining it to keep it calm. He pulled it out slowly and held it gently, showing it to us for a few seconds before he released it. The bird immediately took off and headed for the sky, most certainly relieved to be out of captivity.
Hawk Hill has undergone a major transformation in recent years, with the removal of a grove of non-native Monterey pines and cypress trees and the planting of many native plants around the hillside. The pines and cypresses were brought in unintentionally by the Army in the 1940s, when the seeds were dispersed during the construction of Battery 129.
As they grew into large trees over the ensuing decades, they not only blocked the dramatic views from the hill, but also disrupted the habitat of the endangered Mission Blue butterfly. The dense canopy of trees blocked sunlight and prevented the growth of native lupines, which are favored plants for the Mission Blue. Adult butterflies lay their eggs on the lupine plants, and when caterpillars develop, they feed on the leaves. Mission Blue caterpillars will only eat silver lupine, summer lupine, or varied lupine, three species that once thrived on Hawk Hill.
Today the hill is slowly transforming back into its native coastal grassland and scrub habitat, as the lupine takes root and begins to grow along with other native plants such as California poppy, purple needle grass, yarrow, lizard tail, seaside daisy, and coast buckwheat.
One of the highest points in the Marin Headlands, Hawk Hill offers a spectacular view of San Francisco and the Golden Gate Bridge. The top of the hill is easily reached from Conzelman Road, up a short trail that goes up from the parking area. A series of viewing platforms provide a dramatic 360-degree panorama that is truly unparalleled, looking out over the bridge, the city, the Bay and the ocean, sweeping northward as well, taking in the entire profile of Mount Tamalpais and the rugged coastline going up to Point Reyes.
One thing that makes this view so postcard-perfect is that the skyline of San Francisco is perfectly centered between the two towers of the Golden Gate Bridge.
Hawk Hill is also the site of the World War II era Battery 129, which is burrowed into the side of the hill. Known by its project number as Battery Construction 129, this hilltop fortification was planned to be the highest battery in the Bay Area. Two 16-inch guns were going to be mounted in the large round emplacements that can still be seen today. Each gun had the capability of firing a 2,100 pound shell a distance of 27 miles.
The guns were delivered in 1944, but never mounted. Construction of the battery was abandoned shortly thereafter, when the Army determined that this type of battery would not be effective in deterring aircraft. Historical preservation of Battery 129 is in progress, with some parts of it closed to the public. The tunnels are still open, however, and the gun emplacements can be seen embedded in the hillside.
The Golden Gate Raptor Observatory is conducting educational talks about the raptor migration from now until the end of October. The programs take place starting at noon on Saturdays and Sundays at the top of Hawk Hill. The talk is followed by the release of a recently banded bird at 1 p.m.
GGRO is a program of the Golden Gate National Parks Conservancy, a nonprofit organization that partners with the National Parks Service. GGRO monitors the annual fall migration of birds of prey over the Marin Headlands, conducting a census of the 19 different species of raptors that are found here. They have more than 250 volunteers, who help with monitoring the raptor migration, tabulating numbers for each species, and banding birds.