Demography of the California Condor: Implications for Reestablishment

The remnant wild population of California Condors ( Gymnogyps californianus ) of the 1980s exhibited a rapid population decline caused by high mortality rates among adult and immature birds. The most prominent mortality factor was lead poisoning resulting from ingestion of bullet fragments in carcasses. Successful captive breeding has allowed many birds to be released to the wild since 1992, based originally on an assumption that exposure to lead could be prevented by food subsidy. The mortality of released birds, however, has generally exceeded levels needed for population stability calculated from simple population models. Collision with overhead wires was the most frequent cause of death in releases before 1994. Lead poisoning again surfaced as a problem starting in 1997 as older birds began feeding on carcasses outside the subsidy program. Although poisonings have been treated successfully by chelation therapy in recaptured birds, food subsidy is proving an ineffective solution to lead exposure. The best long-term solution appears to be either the creation of large reserves where hunting is prohibited or the restriction of hunting to nontoxic ammunition in release areas. Until sources of lead contamination are effectively countered, releases cannot be expected to result in viable populations. In addition, problems involving human-oriented behavior have resulted in the permanent removal of many released birds from the wild. The most promising reduction in human-oriented behavior has been achieved in one release of aversively conditioned, parent-reared birds. Rigorous evaluation of the factors reducing attraction to humans and human structures has been hampered by confounding of techniques in releases. Behavioral problems could be more quickly overcome by adoption of a comprehensive experimental approach. Demografía del Cóndor de California: Implicaciones para su Restablecimiento Resumen: Las poblaciones silvestres remanentes del cóndor de California ( Gymnogyps californianus ) de los anõs 80 exhibieron una disminución poblacional rápida debido a altas tasas de mortalidad de individuos adultos e inmaduros. El factor de mortalidad más prominente fue el envenenamiento por plomo ocasionado por la ingestión de fragmentos de municiones en cadáveres. La reproducción exitosa en cautiverio ha permitido muchas liberaciones en ambientes silvestres desde 1992, bajo el argumento de que la exposición al plomo puede ser prevenida mediante el subsidio de alimento. Sin embargo, la mortalidad de aves liberadas ha excedido generalmente los niveles necesarios para alcanzar una estabilidad poblacional calculada a partir de modelos poblacionales simples. Las colisiones con alambres en lo alto fueron la causa más frecuente de las muertes en liberaciones anteriores a 1994. A partir de 1997, el envenenamiento con plomo surgió una vez más como un problema, puesto que las aves de edad avanzada comenzaron a alimentarse de cadáveres fuera del programa de subsidio. A pesar de que el envenenamiento ha sido tratado exitosamente mediante terapia de quelación de las aves recapturadas, el subsidio de alimento ha probado ser una solución ineficaz contra la exposición al plomo. Las mejores soluciones de largo plazo aparentan ser la creación Paper submitted February 26, 1999; revised manuscript accepted December 24, 1999. 958 Condor Demography and Reestablishment Meretsky et al. Conservation Biology Volume 14, No. 4, August 2000 Introduction The decline of the wild California Condor ( Gymnogyps californianus ) population was documented by Koford (1953), Miller et al. (1965), and Wilbur (1978). In the early 1800s, the species ranged along the Pacific Coast from British Columbia to Baja California. By the late 1970s, this range had shrunk to a limited region surrounding the southern San Joaquin Valley of California, and the population had dropped to about 30 birds (Wilbur 1980). In 1980, concern over the decline led to a new conservation program involving intensive field research and captive breeding (Ricklefs 1978; Verner 1978; Snyder 1986). Establishment of a captive flock began in 1982, at first largely through multiple clutching of wild pairs and artificial incubation of their eggs (Snyder & Hamber 1985) but later through capture of free-flying birds when it became clear that the wild population was beyond rescue (Snyder & Snyder 1989). By 1987, when the last wild condor was captured, the captive population consisted of 27 individuals (14 females and 13 males). First reproduction in captivity occurred in 1988 and was followed by near-exponential growth of the captive population (Kuehler 1996). By mid-1998 total numbers of condors exceeded 150. Reintroductions started in 1992; by early 1999, 88 birds had been released in 16 attempts. To evaluate the potential for condor reestablishment, we present (1) a simple demographic model of the historic wild population to derive benchmark mortality rates that may allow populations to persist in the wild and (2) an analysis of releases based mainly on demographic and behavioral considerations. Demographic Characteristics of the Historic Wild Condor Population Reproductive Parameters Although few data exist on the age of first breeding among wild California Condors, adult coloration is normally achieved at 6 years, and no subadults have been documented breeding (Koford 1953; Snyder & Snyder 1989). In captivity, both males and females have usually begun breeding at 6–8 years of age (Kuehler 1996). For modeling purposes, we assumed that this range also applies to the wild population. A 32-year-old male is the oldest California Condor of known age breeding in captivity, but other captive breeders of unknown age may be much older. One female of unknown age, who has ceased egg laying in the past 4 years despite consistent earlier production, appears from surgical examination to be post-reproductive (P. Ensley, personal communication). A male Andean Condor ( Vultur gryphus ) at the National Zoo (Washington, D.C.) successfully fertilized an egg at age 55 (S. Derrickson, personal communication). In modeling efforts, we explored ages of reproductive senescence ranging from 50 to 100 years and found that senescence had little effect on model outcomes, mainly because few individuals lived beyond 50 years in most scenarios. All data on wild and captive birds indicate a clutch size of one egg (Koford 1953; Snyder & Hamber 1985; Kuehler 1996). Snyder and Hamber (1985) documented that breeding pairs normally lay each year they do not continue to care for juveniles produced in the previous breeding season. Pairs that fledge young in September, however, often breed again late in the following laying season, whereas pairs that fledge young in October and November generally forego breeding the following year if their fledglings survive. Thus, successful pairs are likely to breed in 2 out of 3 years, and unsuccessful pairs can be expected to lay every year. In addition, pairs failing early in breeding often lay replacement eggs in the same laying season (Snyder & Hamber 1985). Four of seven natural nesting failures (57%) in the 1980s occurred early enough to allow replacement eggs. Because this sample size is too small to give an accurate frequency of natural replacement clutching, we assume a wide range of values (25–75%) for chances of a second egg after loss of a first (i.e., double clutching). No natural cases of triple clutching have been observed in the wild, although induced triple clutches occurred occasionally, usually when both first and second eggs were taken rapidly for artificial incubation. Because natural triple-clutching appears to be rare, we assume double clutching only. de reservas grandes donde la caza sea prohibida o se restrinja la caza a municiones no tóxicas en las áreas de liberación. Solo una vez que la contaminación por plomo sea contrarrestada efectivamente, no se podrá esperar que las liberaciones resulten en poblaciones viables. Además, los problemas de conductas orientadas hacia humanos ha resultado en la remoción permanente de muchas aves liberadas de zonas silvestres. La reducción más prometedora de conductas orientadas hacia humanos ha sido obtenida en una liberación de aves criadas por sus padres y condicionadas adversamente. La evaluación rigurosa de los factores que reducen la atracción hacia humanos y estructuras de humanos ha sido obstaculizada por la confusión de técnicas en las liberaciones. Los problemas de conducta podrían ser superados más rápidamente mediante la adopción de una estrategia experimental comprensiva.