| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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feature Peregrine Falcons Seek High-Rise Habitat We were on one of those rare landing patterns that took us south right down the middle of the Bay and over the San Francisco-Oakland Bay Bridge. I craned my neck for a view of the bridge just east of Yerba Buena Island. There, on tower E2, was the location of a peregrine falcon eyrie. It was the first week of June in 1993 and according to all calculations the three falcon chicks present in the eyrie should have fledged the previous week. I missed it, I was out of town, and now all I could do was hope that it had gone well. It had not. Right after landing I called David Gregoire, private businessman and research associate with the Santa Cruz Predatory Bird Research Group. Gregoire reported that the first chick to fledge drowned, the second had vanished entirely, and the status of the third one was still unknown. It apparently had survived its first flight from the eyrie located 150 feet above the water, but it had not been seen since a violent storm had rolled through the day after it had fledged. The next day I was at the base of the Bay Bridge, craning my neck again in the hope of spotting the fledgling amongst the girders. Only a couple of weeks earlier, at the time of banding, all three chicks were in good shape. Now the entire brood may have been lost. These fears were getting the better of me when I heard the unmistakable, high-pitched hunger scream of a fledgling. What relief, what joy! It had survived, and now it was clumsily chasing an adult who was about to hand its offspring a whole freshly caught pigeon. The nesting season was not going to be a complete bust after all. In reality, the Bay Bridge falcons were right in line with the statistics: on average, about one chick in three survives fledging from a bridge, whereas two chicks in three may survive fledging from a skyscraper. Peregrines nesting on bridges and skyscrapers are faced with many hazards unique to the urban environment. Young, inexperienced falcons fledging from buildings have been known to crash into wires and windows, fall down ventilation shafts, and land on city streets and sidewalks where the parent falcons are reluctant to come down and feed them. Bridges are even less forgiving of a fledgling's mistakes because of the pervasive traffic and the surrounding water. Peregrine chicks fledging from a natural cliff ledge tend to have a higher probability of surviving their first flight. On a cliff, falcon chicks go through a long process of exercising their wings and scrambling about the ledge before making their first flight. They may even flutter and hop across rocks from one ledge to another, effectively making many short flights before launching out into the blue unknown. When they do fly from the ledge, they usually fly down to some other location on the cliff where they may safely rest and gather their bearings for the next flight. Neither bridges nor many buildings offer this luxury. In a now quite familiar story, the production and use of DDT and related organochlorine insecticides caused worldwide declines in the peregrine falcon (Falco peregrinus) through reproductive failure. Scientists determined that a breakdown product of DDT, known as DDE, accumulates in the fatty tissues of the falcon and disrupts the bird's calcium metabolism, causing it to lay fragile, thin-shelled eggs that often break soon after laying. To combat the extirpation of the peregrine from many parts of its range, intensive management, captive propagation, and release of peregrines over the last 20 years has been undertaken with phenomenal success. Release programs have either reestablished wild breeding populations or augmented existing populations. The number of wild pairs of peregrines breeding in the eastern United States rose from zero in 1975 to 95 in 1991, and in California, where Brian Walton's Santa Cruz Predatory Bird Research Group managed eyries and released peregrines from 1977 through 1992, the number of known active peregrine eyries rose from twelve to 113. Along with the recovery of the peregrine in North America we have seen its increasing urbanization. Peregrines now breed and overwinter in most major metropolitan areas of the United States. At the 1993 Raptor Research meetings, Tom Cade of the Peregrine Fund reported that at least 75 pairs of peregrines currently nest in urban areas of North America. In fact, Cade hypothesizes that peregrines may exceed their historical numbers in some urban environments. New York City, which contains the largest population of nesting peregrines of any city in the world, is a prime example. In 1993 eight pairs of falcons successfully raised young. It is rare to find more than one pair in a 20-to-30- square- mile area. Four of these pairs nested on different bridges while the other four chose buildings, mostly in Manhattan, with the highest nest site on the 57th floor of the Met Life Building. Saul Frank, a retired businessman, closely followed peregrine population growth in the city between 1983, when the first two pairs showed up, and 1993. Of particular interest was a female that took possession of the Throgs Neck Bridge. Nicknamed Queen, she came from a cliff-nesting pair of peregrines in New Hampshire. Queen's mate, Rio, originated from a release, or "hack," site on a building in Boston. A hack site is a safe place from which captive- hatched falcons are released without benefit of parental care. The young falcons are surreptitiously supplied with food by humans until they can strike out on their own. Frank reports that Queen went through three different mates and laid a total of 40 eggs over the course of ten years as a breeding adult. One year, her mate Rio became entangled in some rigging and had to be rescued. He was placed in rehabilitation for two weeks to recuperate. Another male promptly replaced Rio at the bridge, but upon his release Rio was able to supplant the intruder and reclaim his mate. Frank has seen rapid replacement of mates that have disappeared at several eyries in New York City. As he points out, this is a good sign that the population is healthy--that there are enough "floaters" (unpaired adults) around to replace birds that die. Prior to the disappearance of the peregrine in the eastern United States, the 50-mile stretch of the Palisades along the lower Hudson River was a traditional nesting haunt. During the 1930s and 40s up to eight peregrine eyries could be found here. But, today, the reestablished peregrines seem to forgo the Palisades in favor of the city. Why are there no peregrines nesting along these cliffs now, especially since there are plenty of peregrines just downriver? Certainly, some of the Palisades may no longer be suitable for peregrines because of too much human pressure above and below the cliffs. Despite their attraction to urban areas, peregrines do not generally tolerate well the close proximity of people. But habitat loss cannot explain the fact that all of the Hudson River cliffs are still devoid of peregrines. The question must be put another way: why are peregrines so inexorably attracted to the urban environment? The bottom line is that peregrines like cliffs, and New York City with its skyscrapers and bridges represents a sort of superstimulus that attracts like a magnet. These surrogate cliffs offer peregrines myriad eyries and roosting sites as well as a commanding view and height advantage for launching hunting attacks at their favorite food--pigeons, and other city birds such as mourning doves, starlings, and jays. Peregrines have always been attracted to urban environments. They have a long history of isolated attempts to nest on buildings, but many of these attempts were short-lived due to the inhospitable reaction of the "landlords." For example, in 1943 peregrines nested behind a cornice on the 16th floor of the Hotel St. Regis in New York City. The chicks were destroyed when someone complained about these "killer birds" nesting in their city. The first time people intentionally provided peregrines with an ideal nesting situation was at the Sun Life Assurance Company of Canada building in Montreal in the 1930s. Under natural circumstances, peregrines do not build a nest. Rather, they just scrape out a depression for their eggs in the soft soil or sandy substrate of a cliff ledge. Most building ledges do not have enough debris present to allow peregrines to make a sufficient indentation. After initial nesting attempts by the Montreal peregrines failed because the eggs were laid on bare ledges or in rain gutters, volunteers installed wooden nest shelves containing a few inches of gravel and soil which were readily accepted by the falcons. In the course of her 16 breeding seasons, the Sun Life female had three successive mates and successfully reared 21 young. She became an admired inhabitant of the city, and her presence was welcomed by an understanding public. The use of artificial nest shelves has since become an important tool for enhancing falcon eyries throughout the world. Nearly all of the New York City peregrine eyries, whether on bridges or skyscrapers, have been outfitted with such shelves. The urbanization of the peregrine raises additional intriguing aspects of its biology. Where a falcon is raised and released may exert an influence on its subsequent choice of a nest site. For years Harrison B. Tordoff and Patrick T. Redig of the Raptor Center at the University of Minnesota have been releasing peregrines at hack sites throughout the Midwest. Originally, these sites were set up at historical nest cliffs along river bluffs, but the effort was thwarted by heavy great horned owl predation. Rather than embark on an extensive owl control program, the researchers moved the hack sites to downtown buildings. The program has been a great success. Today, of 43 successfully breeding peregrine pairs in the Midwest, 31 are located in cities. Although falcons released from cliffs will nest on man-made structures and vice versa, Tordoff and Redig found that there is a tendency for falcons released from buildings to subsequently nest on structures. There is a tenacity to the peregrine. Once a pair establishes a tradition of occupying a successful eyrie, the site will usually remain occupied for generations. British peregrine authority Derek Ratcliffe reported that several eyries noted from the sixteenth to nineteenth centuries in Great Britain are still occupied. An eyrie site in California that has had a long history of peregrine occupancy is Morro Rock on the central coast: there are peregrines on it now, there were peregrines nesting on it in 1970 when the California population was at an all-time low, and there were probably falcons on it when the first European explorer, Juan Rodriguez Cabrillo, sailed past in 1542. Peregrines are drawn to this impressive rock because it offers the dual stimuli of tenancy tradition and large cliffs. The same may now be said of cities such as New York. The converse of the peregrine's attachment to eyries is their reluctance to reinhabit abandoned sites. We have seen this along the California coastline from San Francisco north to the Oregon border. Prior to the population crash, this coastline harbored an estimated 30 eyries, but now only a few sites are active despite the fact that the northern interior Coast Ranges support the largest population of peregrines in the state--over 70 eyries. The habitat is by and large not degraded, yet some of the finest coastal nest cliffs remain empty. This is not to say that original habitat will not again be settled. Where population growth has been phenomenal, such as in Arizona or Great Britain, previously deserted cliffs are settled quickly. As in New York City, but on a much smaller scale, the San Francisco Bay Area has seen the appearance of urban peregrines. Historically, peregrines bred on inland cliffs, sea cliffs, and a few islands in the Bay, and even made a few isolated attempts to nest on man-made structures. But they disappeared from the region as breeding birds by 1970. Between 1977 and 1992 the recovery of the peregrine in California was greatly aided by the captive breeding efforts of the Santa Cruz Predatory Bird Research Group (SCPBRG). To release peregrines, the SCPBRG relied on hacking, fostering, and cross-fostering. In fostering, chicks are placed in the nests of wild peregrine falcons; in cross-fostering, they are placed in the nest of other raptors, typically prairie falcons (Falco mexicanus). The first peregrine pair to settle in the Bay Area showed up on the cantilever section of the Bay Bridge east of Yerba Buena Island in 1983. Both adult falcons wore blue anodized aluminum bands-- indicating that they had been released by the SCPBRG. The male had been fostered as a chick into a peregrine eyrie in Napa County, while the female was from a prairie falcon cross-foster site in San Luis Obispo County. Unfortunately, the female was soon found shot. It was another five years before a blue-banded pair was again spotted in the vicinity of tower E2. A check of their nest site confirmed the presence of three eggs, but they broke before hatching. Since 1988 a pair of falcons has been resident at this site, thus establishing what we expect to be a new tradition of occupation. In 1992 a new, unbanded male replaced the previous one. A second pair of blue-banded peregrines appeared in downtown San Francisco in the late summer of 1986. The female, which had originated from a foster site on the Big Sur coast, soon disappeared. A new, blue-banded female paired with the resident male, and breeding was first observed in 1988 when a five-week-old chick was recovered alive from a pier on the San Francisco waterfront below the Bay Bridge. The chick had apparently fallen from a nest site on one of the bridge towers. This West Bay pair produced many clutches through 1991, but the female has since ceased to lay eggs although the pair continues to make nest scrapes each spring. A third pair of peregrines uses the Golden Gate Bridge as a hunting lookout, while for nesting they prefer natural ledges as well as an old raven's nest on nearby sea cliffs. The habits of this near-urban pair are slightly different from those of the Bay Bridge pairs. The Golden Gate falcons have been seen hunting from the bridge throughout the winter, taking advantage of the relatively steady stream of migrant birds crossing the Golden Gate, whereas the Bay Bridge pairs nest on the bridge in the spring and early summer months and then move into "their" respective cities of Oakland and San Francisco during the fall and winter. The West Bay pair is particularly fond of San Francisco's Financial District and its pigeons, while the East Bay pair is often seen in downtown Oakland as well as on buildings adjacent to the marshes of the Emeryville Crescent, where they have been seen chasing shorebirds along the mudflats. Surprisingly, the bridge peregrines are bothered neither by the constant traffic and noise nor by bridge workers, as long as they keep their distance. But if something out of the ordinary occurs, the falcons take notice. Peregrines on the Bay Bridge will get upset when an orange Caltrans truck stops on the road above their nest sites, probably because they know that a biologist is about to climb down and check on their progress. The East Bay female seems to be particularly fearless; she has strafed us on numerous occasions while we inspected her eyrie. In contrast, peregrines in wilderness areas tend to be more fearful of humans and maintain a respectful distance. Because of the risk of poor fledging success from the Bay Bridge, the SCPBRG removed all eggs from the falcons each spring for incubation and subsequent release of chicks elsewhere. This greatly increased the productivity of the birds, because after each clutch was taken the pairs would usually lay a second clutch--or in one instance, a third--in the same season. In spite of the best efforts of the SCPBRG however, not all eggs from these falcons hatched. Some thin-shelled eggs broke under incubating females before they could be collected, while others never hatched. Clearly, eggshell thinning is still a problem. Lloyd Kiff, of the Western Foundation of Vertebrate Zoology, found that eggshell thinning in the East Bay female averaged 15.2 percent, nearing a critically high value, whereas the West Bay female averaged 7.2 percent. Differing food habits partly explain these different levels. Some prey species, such as European starlings and various shorebirds, exhibit relatively high levels of DDE contamination. In the 1980s, over ten years after the use of DDT had been banned, Robert Risebrough of The Bodega Bay Institute found that every tenth killdeer in California still had enough DDE in it to potentially knock out a female peregrine's reproduction. The East Bay peregrines appear to take more shorebirds than the other pairs, and this is refelected in their higher eggshell thinning. Urban falcons are generally less contaminated with DDE and other toxics than their rural counterparts because, ironically, their urban prey is relatively uncontaminated. Pigeons, of all things, are some of the cleanest prey, provided they have not been poisoned by control programs. In contrast, one of the most pristine-looking sections of the California coast, Big Sur, harbors peregrines that lay eggs with shells so thin that they crush under the weight of the incubating female. Reproduction in these falcons is so impaired that a population-modelling study done by J. Timothy Wootten and myself indicated that the south and central coast peregrines could not sustain themselves without immigration from other areas, such as northern California's interior Coast Ranges. However, it remains uncertain whether nearby populations are producing a surfeit of potential immigrants. In a collaborative effort between the SCPBRG and the California Academy of Sciences, we are currently searching for dna markers that can be used to measure interchange between the northern and southern populations to allow more accurate predictions of their fate. How is it that peregrines here are still accumulating organochlorines, 20 years after the use of DDT was banned? The standard reply is to blame countries south of the border for continued DDT use--DDT manufactured by subsidiaries of U.S. corporations. The fact is, organochlorines are still in California's environment. The problem is serious enough that, according to Roger C. Helms, chief of the Natural Resources Damage Assessment Branch of Region 1 of the U.S. Fish and Wildlife Service, several government agencies are currently in litigation with the Montrose Chemical Corporation, the largest manufacturer of DDT in the world. Between 1947 and 1970, millions of pounds of DDT were discharged into the ocean off southern California. DDT and its breakdown compound DDE are still entering the food chain from sediment. Yet another source of DDE is still legally in use in the United States. A commercial pesticide known as dicofol is being sprayed annually on millions of acres of American farmlands and vineyards. Dicofol is a close chemical relative of DDT. Up until 1989 the industrial production of dicofol was fraught with high impurity rates such that up to 15 percent of it was contaminated with pure DDT/DDE. In 1989 the EPA obtained an injunction against the manufacturers requiring them to clean up the manufacturing process. Now dicofol contains less than one percent DDT- related compounds. Despite the high contamination of the East Bay female, she had been able to hatch three of her four eggs in 1993. Thanks to the efforts of two volunteers with the Golden Gate Raptor Observatory, David and Lynn Jesus, we know what happened to the first fledgling. It succeeded in landing on the base of the E2 tower but was soon forced off by a nesting western gull. With several gulls in hot pursuit, the confused fledgling attempted to fly east on shaky wings to the next tower but didnUt make it. It crashed into the water and then tried to swim the quarter mile back to Yerba Buena Island. A U.S. Coast Guard Cutter was scrambled, but it arrived too late. Only the third chick survived all the hazards of fledging, including numerous gull attacks. The mother located her fledgling and never missed an opportunity to dive upon any gull attempting to hassle it. To improve the chance of successful fledgings from the Bay Bridge, David Gregoire and I have been working closely with Caltrans to enhance the nest ledges used by the falcons. Our first proposal was to install simple plywood extensions to the current eyrie sites so that the falcon chicks would have more space to exercise their wings before making their first flight. Personnel at the California Academy of Sciences were ready to donate their time, but, due to liability problems, Caltrans mandated that the ledges be constructed as integral structures of the bridge made of thick bridge steel, and permanently bolted into place. With much consultation and several trips to the eyries for measurements, Caltrans engineers drew up plans for the nest shelves. Everything was set: all we needed was the approval of the California Department of Fish and Game's endangered species staff. The approval never came. The staff is concerned that peregrines from the Bay Bridge might disturb a colony of endangered least terns nesting at the Alameda Naval Air Station. There is evidence, based on least tern-peregrine interactions in southern California, that a peregrine eyrie in the wrong location can impact tern colonies. Even if the peregrine does not attack the terns, its mere appearance can cause the birds to fly up in alarm, an action which allows other predators such as crows and ravens to sneak in and grab tern eggs and chicks. But the Bay Area is not southern California. The Bay Bridge falcons appear to be sufficiently distant from the least tern colony. Furthermore, the least tern colony at the Air Station has been growing each year and is now one of the largest in the state. Installing nest shelves on the Bay Bridge will not cause a population explosion of peregrines, but the shelves may save a few young peregrines from drowning. The survivors will still be faced with all the risks of life in an urban environment. It would be easy to continue arguing about potential least tern-peregrine conflicts, but the real issue appears to be a philosophical one. Should we not try to give the peregrines a better chance as well as find a way to improve least tern nesting habitat? Certainly, the urban environment and its supporting infrastructure creates not just human-wildlife conflicts, but also wildlife-wildlife conflicts. Our job is to find solutions that benefit both species. It is comforting to know that at least some wild creatures can take advantage of our expanding urban environments. The peregrine falcon stands as a shining example of the successes that can accrue from giving nature a hand. It has adapted to our cities; all it needed was proper management and human tolerance.
Douglas A. Bell is the Tilton Postdoctoral Fellow at the California Academy of Sciences. He is studying the population genetics and molecular systematics of peregrine falcons. |
Spring 1994
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