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S.R. Wilbur

Publications and source records attributed to S.R. Wilbur.

At least 19 recordsLinked to original sources
USGS1989

Turkey vulture and California condor

Populations of turkey vultures in the western states appear stable, with no evidence of recent significant changes in distribution. Turkey vultures occupy a variety of habitats, nest in diverse situations and utilize a wide variety of carrion. Consequently, no particular limiting factor is likely to have a major effect on the total population. California condor numbers, in contrast, have continued to decline. With the capture of the last wild bird in 1987, the species has been extirpated from the wild. Reestablishment will depend on production and introduction of captive-reared birds, hopefully within the next 10 years. In the 18th century, condors inhabited areas along the Pacific coast from British Columbia to Baja California. Since 1950, the range has been restricted to a six county area adjacent to the southern San Joaquin Valley of California. Within this area, there appears to be no shortage of suitable nest sites; all recently used nest sites are within federally-controlled lands. Suitable foraging grounds have continued to diminish and are now largely limited to private rangelands and some Bureau of Land Management rangelands within the foothills of the San Joaquin Valley. Only these areas continue to supply the large mammal carcasses that the California condor needs for survival. The habitat of the condor is subject to a variety of land use practices and development pressures. Excessive mortality, coupled with low reproductive potential, continues to threaten the recovery of the species. Development of management practices to reduce mortality, particularly those that are contaminant-related, and of a preserve design to insure adequate habitat for the reintroduced population are still necessary for eventual recovery of the species.

Book chapter↗
USGS1983

Environmental contaminants in tissues, foods, and feces of California condors

Two wild California Condors contained moderate to high levels of DDE in their tissues. The levels found could be high enough to cause reproductive problems in adult condors, if the assumption is made that condors are as susceptible to DDE as many other species of birds of prey. Other organochlorines occurred at low levels and probably were not high enough to cause deleterious effects. Metal residues in tissues of one bird were generally low except for copper in liver and lead in bone. Normal background levels of these metals in cathartids are unknown, making interpretation of the results difficult. Organochlorine residues in biopsy samples from a captive condor were low and probably would not have an adverse effect on reproduction if the bird were used for captive breeding. Organochlorines were not detected in food items used in the supplemental feeding program, and mercury and lead residues in these items were generally low. Information is needed on current contaminant levels in natural condor prey throughout the condor range.

Book chapter↗
USGS1983

Eggshell thickness and DDE residue levels in vlulture eggs

Post-DDT (post-1947) eggshell thickness was examined in samples of Turkey Vulture, Black Vulture, and Crested Caracara eggs from several parts of the United States. Highly significant post-DDT decreases in eggshell thickness indices of at least 10 percent were found in Turkey Vulture eggs from California, Florida, and Texas and in Black Vulture eggs from Texas and Florida. Over one-third of the Black VUlture eggs and about 30 percent of the Turkey Vulture eggs from Texas showed thinning exceeding 20 percent, a level associated with reproductive failure and population decline in other species. A strong negative correlation was found between eggshell thickness indices and DDE residues extracted from eggshell membranes in California and Texas samples of Turkey Vulture eggs and in Texas Black Vulture eggs. Crested Caracara eggs from Texas and Florida showed mean changes in eggshell thickness indices of only -5.6 and -8.2 percent, respectively, although thinning in a few eggs from both states exceeded 20 percent. Most of the post-DDT Old World vulture eggs examined appeared to be of normal thickness, with low DDE residue levels in eggshell membranes; but single eggs of Egyptian Vulture from India, Cinereous Vulture from Spain, and White-headed Vulture from Zambia showed apparent thinning. Further monitoring of vulture populations in tropical regions, where DDT use is still increasing, is recommended.

Book chapter↗
USGS1980

Estimating the size and trend of the California condor population, 1965-1978

During 1965-1978, three principal methods were used to measure the size and trend of the California Condor (Gymnogyps californianus) population. An October survey conducted by as many as 136 observers had limited value because daily activity patterns of condors were unpredictable and because analysis of results could not be standardized. Evaluation of 4,381 condor observations by cooperators, and results of comparative surveys done in 1970-1971 and 1977-1978 show declines in numbers of condor sightings, numbers of condors per sighting, and in the numbers of sightings including more than one immature-plumaged bird. The data cannot be evaluated statistically, but they suggest a marked decline in the condor population between 1965 and 1978. Future refinements of survey techniques are dependent on having individually recognizable condors in the population.

California Fish and Game↗
USGS1978

The California condor, 1966-1976: A Look at its Past and Future

The California condor (Gymnogyps californianus) was studied on about 900 field days between 1966 and 1976. In addition, some 1,000 items of literature, specimen records from 56 museums, and 3,500 reports of condor sitings by cooperators were analyzed. Distribution does not appear to have changed significantly since the 1930's, although there are some areas within the. species' range that have become unusable. Two subpopulations of condors exist, one occupying the Coast Range Mountains, and the other found in the Transverse Ranges, Tehachapi Mountains, and Sierra Nevadas. There are well-defined seasonal movements within each subpopulation area. The surviving, wild population was estimated to be 45 condors in 1976, a decline of about 20% since 1965 and probably over 50% since 1940. No reliable population estimates are available before the 1940's, but it appears that a major decline occurred between 1880 and 1920. Shooting and specimen collecting were the primary causes of the early decline, and shooting continued as a major problem into the 1960's. Recent declines are a result of inadequate production; annual surveys indicate that only 16 young have been produced since 1968. Causes of low production are unknown but inadequate food supply, environmental contaminants, and disturbance from air traffic and petroleum extraction are implicated. A recovery plan for the condor is in operation; steps have been taken to supplement food supplies, preserve nesting and roosting habitat, and protect surviving birds from man-caused mortality or disturbance. The condor's prospects of recovery in its natural habitat seem bleak; a captive propagation program is proposed to supplement wild production.

North American Fauna↗