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At least 163 records · Page 9Linked to original sources
USGS1948

Application of Brianchon's theorem to construction of geologic profiles

Brianchon's theorem states that the three diagonals joining opposite vertices of a hexagon circumscribed about a conic are concurrent. A corollary of this theorem applies to a pentagon so that the points of tangency of an inscribed conic may be located. Any five non-concurrent straight lines in a plane, no three of which are parallel, will ordinarily form some kind of a pentagon; and if considered as tangents to a conic, they will define its shape and position. If these five lines are also normals to the traces of parallel stratigraphic surfaces having a constant strike, the derived conic may be regarded as an evolute, from which a set of involutes can be drawn that will constitute a structural profile. A method is thus afforded for constructing a profile normal to the strike of the rocks, and for measuring stratigraphic thickness, by the utilization of five observations of dip along a suitable linear traverse. Graphical methods are also given for constructing a parabolic evolute from four observations and a circular evolute from three observations. Additional points on the conic evolutes are obtained by the application of Pascal's theorem. A mathematical analysis is presented of the relationship between a conic and five of its tangents; and the conic evolute, rather than its involutes, is recommended as a satisfactory record of the structure of a parallel fold. To obtain the equation of this evolute, the equations of the five tangents are first derived, using trilinear coordinates. Thereafter the tangential and trilinear equations of the general conic are deduced. Criteria are given for classifying the conic as a hyperbola, ellipse, parabola, or circle. These graphical and analytical methods are adaptations of the general method of evolute and involutes. They are offered, not as substitutes for the general method, but as quicker, though somewhat less accurate, means of obtaining similar results, where structural conditions justify their use. © 1948, The Geological Society of America, Inc.

Geological Society of America Bulletin↗
USGS1966

Crustal study of a continental strip from the Atlantic Ocean to the Rocky Mountains

Twenty aeromagnetic profiles over a 100-mile-wide strip along the arc of a great circle passing through Denver, Colorado, and Washington, D. C, reveal large anomalies of major crustal significance. Contoured data disclose several areas of distinct magnetic patterns reflecting basement lithology and structure. The mafic rocks of the Blue Ridge and Piedmont and the Keweenawan mafic belt in Iowa and Nebraska give rise to strong linear trends. Areas with a more random pattern of closely spaced magnetic anomalies appear in central Ohio, eastern Iowa, and central and western Nebraska. Except in the Blue Ridge and Piedmont areas, crystalline basement rocks are covered by a thick blanket of virtually nonmagnetic sedimentary rocks, and lithology must be inferred from correlations of the magnetic data with scattered drill-hole data and regional gravity data. The area of highly magnetic rocks in central Ohio has a sharp western boundary that coincides with the western limit of metamorphism associated with the probable extension into Ohio of the Grenville province of Canada. This area and a similar one in eastern Iowa are linked by an arcuate, nearly continuous belt of positive gravity anomalies that extends north into Wisconsin and northern Michigan and then swings southeast across central Michigan. This horseshoe-shaped feature is associated with lithologically diverse but highly magnetic basement rocks. A group of linear magnetic anomalies in western Iowa and eastern Nebraska correlates with the well-known midcontinent gravity high. In Nebraska the magnetic data provide a basis for grouping the extremely complex drill-hole data into three over all lithologic terranes. An analysis of the long-wavelength (>40 miles) variations of the profiles shows that they form a number of large coherent anomalies, many of which show little relation to the major tectonic trends and lithologic patterns of the basement surface. The very broad and less numerous anomalies in the east, which have more or less north-south trends, are significantly different from the more numerous anomalies in the western part of the strip , which tend to trend east-west. One linear anomaly extends for nearly 500 miles across Nebraska and Iowa and may mark a zone of rifting. Heat-flow data show that rocks at the Curie point, which determines the depth below which rock magnetization cannot occur, may be deep enough, at least in shield and other stable parts of the continent, to include a part of the upper mantle. The concentration of the large magnetic features in Iowa and Nebraska may indicate that the thickness of magnetized rock is greater in this area and that perhaps some of these features originate in the upper mantle.

Geological Society of America Bulletin↗
USGS1969

Aeromagnetic investigation of crustal structure for a strip across the western United States

This report represents part of a larger study undertaken to interpret the gross features of the earth's crust by aeromagnetic methods. The larger survey covers a 100-mile-wide strip along a great circle arc from Washington, D.C., to San Francisco, California. The area considered extends from about 200 miles east of the Rocky Mountains to 150 miles west of the coast of the Pacific Ocean. Inferences on gross structure and lithology are made from the magnetic patterns and trends shown on the map. At the continental margin, the magnetic data strongly suggest that the oceanic crust becomes much more deeply buried as the continent is approached. A large magnetic anomaly extending northwestward along the center of the Great Valley is probably caused by mafic intrusive rocks.

California, Colorado, Kansas, Nebraska, Nevada, Ut↗
USGS1973

Potassium-argon ages and paleomagnetism of the Waianae and Koolau Volcanic Series, Oahu, Hawaii

Paleomagnetic and potassium-argon measurements on 786 oriented cores from 99 volcanic units at 18 sites in the Waianae and Koolau Ranges, Oahu, when combined with data from previous studies, show that the sub-aerial Waianae Volcano was active only from about 3.6 to 2.4 m.y. ago and the subaerial Koolau Volcano from about 2.6 to 1.8 m.y. ago. There is some evidence that Waianae Volcano was still active when Koolau Volcano emerged from the sea. The predominantly tholeiitic lower and middle members of the Waianae Volcanic Series are approximately contemporaneous and were extruded during the late Gilbert and early Gauss geomagnetic polarity epochs. They were followed within less than 0.2 m.y. by the alkalic lavas of the upper member, which were probably extruded largely during the later part of the Gauss normal polarity epoch. The Koolau Volcanic Series was extruded entirely during the early Matuyama reversed polarity epoch. Data from three thick stratigraphic sections in the Waianae and Koolau Volcanic Series indicate that stacks of lava flows as much as 470 m thick can be formed in less than 0.25 m.y. and that the maximum average period between superimposed lava flows is on the order of 103 yrs. Additional data on the hawaiite flow that led to the discovery of the Kaena reversed event indicate that this reversed flow is 2.85 ± 0.05 m.y. old. Angular dispersion of virtual geomagnetic poles (VGP) in the Hawaiian Islands appears to have decreased during the past 5 m.y. This may be caused by a decrease in dipole wobble, a decrease in the nondipole component of the Earth' magnetic field, or the accumulated effects of weathering, tectonism, and geomorphic processes in older rocks. The mean Waianae and Koolau VGPs are slightly on the side of the Earth's rotation axis away from Oahu. This supports, but does not prove, the hypothesis that the axial dipole is displaced slightly northward from the Earth's center. Three VGP “excursions” were recorded in sections of lava in the Waianae and Koolau ranges. During these excursions, the VGPs appear to have traveled away from or toward the geographic axis along great circle paths, suggesting they may be related to the dipole rather than the nondipole field and may record aborted reversals in polarity or rapid and infrequent dipole tilts.

Hawai'i↗
USGS2011

Geology and petroleum potential of the Timan-Pechora Basin Province, Russia

The Timan–Pechora Basin Province represents the northeastern-most cratonic block of Eastern European Russia. More than 16 billion barrels of oil (BBO) and 40 trillion cubic feet of gas (TCFG) have been discovered in this basin. Three geological assessment units (AU) were defined for assessing the potential for undiscovered oil and gas resources. The NW Izhma Depression AU encompasses all potential structures and reservoirs in the northwestern part of the Izhma–Pechora Depression, but this part of the basin contains little source and reservoir rocks and so was not assessed quantitatively. The Main Basin Platform AU includes all structures and reservoirs that developed in the central part of the basin where the tectonic evolution and development of petroleum systems were complex. The Foredeep Basins AU includes all potential reservoirs within the thick sedimentary section of the foredeep basins developed during the Permo-Triassic Uralian Orogeny. For the Timan–Pechora Basin Province, the estimated means of undiscovered resources are 3.3 BBO, 17 TCFG and 0.3 billion barrels of natural gas liquids (BBNGL). For the areas of the AUs north of the Arctic Circle in the Timan–Pechora Basin Province, the estimated means of undiscovered resources are 1.7 BBO, 9.0 TCFG and 0.2 BBNGL. The Pechora Sea was assessed with the South Barents Sea Province and is not included in this assessment.

Pechora Basin Province↗
USGS2011

Geology and petroleum potential of the east Barents Sea Basins and Admiralty Arch

The US Geological Survey (USGS) recently assessed the potential for undiscovered oil and gas resources of the East Barents Basins and Novaya Zemlya Basins and Admiralty Arch Provinces as part of the USGS Circum-Arctic Resource Appraisal. These two provinces are located NE of Scandinavia and the northwestern Russian Federation, on the Barents Sea Shelf between Novaya Zemlya to the east and the Barents Platform to the west. Three assessment units (AUs) were defined in the East Barents Basins for this study – Kolguyev Terrace Assessment Unit (AU), South Barents Basin and Ludlov Saddle AU, and North Barents Basin AU. A fourth, defined as Novaya Zemlya Basins and Admiralty Arch AU, is coincident with the Novaya Zemlya basins and Admiralty Arch Province. These AUs, all lying north of the Arctic Circle, were assessed for undiscovered, technically recoverable resources resulting in total estimated mean volumes of approximately 7.4 billion barrels of crude oil, 318 trillion cubic feet of natural gas and 1.4 billion barrels of natural gas liquids.

Geological Society Memoir↗
USGS2011

Geology and petroleum potential of the West Greenland-East Canada Province

The US Geological Survey (USGS) assessed the potential for undiscovered oil and gas resources of the West Greenland–East Canada Province as part of the USGS Circum-Arctic Resource Appraisal programme. The province lies in the offshore area between western Greenland and eastern Canada and includes Baffin Bay, Davis Strait, Lancaster Sound and Nares Strait west of and including part of Kane Basin. A series of major tectonic events led to the formation of several distinct structural domains that are the geological basis for defining five assessment units (AU) in the province, all of which are within the Mesozoic–Cenozoic Composite Petroleum System. Potential petroleum source rocks include strata of Ordovician, Lower and Upper Cretaceous, and Palaeogene ages. The five AUs defined for this study – the Eurekan Structures AU, NW Greenland Rifted Margin AU, NE Canada Rifted Margin AU, Baffin Bay Basin AU and the Greater Ungava Fault Zone AU – encompass the entire province and were assessed for undiscovered technically recoverable resources. The mean volumes of undiscovered resources for the West Greenland–East Canada Province are 10.7×10 9 barrels of oil, 75×10 12 cubic feet of gas, and 1.7×10 9 barrels of natural gas liquids. For the part of the province that is north of the Arctic Circle, the estimated mean volumes of these undiscovered resources are 7.3×10 9 barrels of oil, 52×10 12 cubic feet of natural gas, and 1.1×10 9 barrels of natural gas liquids.

Geological Society Memoir↗
USGS2024

Seasonal mortality of Wild Atlantic Menhaden (Brevoortia tyrannus) is caused by a virulent clone of Vibrio (Listonella) anguillarum; Implications for biosecurity along the Atlantic Coastal United States

Atlantic menhaden are a highly migratory marine species in the Eastern United States that suffer from seasonal chronic mortality. Affected fish show neurologic signs referred to as spinning disease, including circling at the surface and erratic corkscrew swimming before death. We investigated three similar menhaden mortality events consistent with spinning disease in coastal New Jersey and New York between 2020 and 2021 to understand the cause. A unique strain of Vibrio (Listonella) anguillarum (serogroup O3) was detected regularly in high loads, particularly in the brains of moribund fish, by both metagenomics and bacterial isolation. The most common histopathological changes in moribund fish were hemorrhagic meningitis, encephalitis, pyknosis, and karyorrhexis of hematopoietic tissues in the kidney and spleen. Whole genome sequencing of isolates from moribund fish representing a wide spatial and temporal range showed that they were nearly identical clones, suggesting it to be a pathogenic strain circulating in the population. Though V. anguillarum is believed to be the main pathogen associated with spinning disease and mortality, Yersinia ruckeri (serotype O1) was isolated from smaller numbers of fish. Considering the highly migratory nature of Atlantic menhaden throughout the eastern United States and their use as bait for other fisheries, these findings identify potential biosecurity challenges that should be considered in Atlantic salmon aquaculture, fisheries, and emerging marine aquaculture in the region.

Transboundary and Emerging Diseases↗
USGS2010

Microbial oxidation of arsenite in a subarctic environment: diversity of arsenite oxidase genes and identification of a psychrotolerant arsenite oxidiser

Arsenic is toxic to most living cells. The two soluble inorganic forms of arsenic are arsenite (+3) and arsenate (+5), with arsenite the more toxic. Prokaryotic metabolism of arsenic has been reported in both thermal and moderate environments and has been shown to be involved in the redox cycling of arsenic. No arsenic metabolism (either dissimilatory arsenate reduction or arsenite oxidation) has ever been reported in cold environments (i.e. < 10°C). Results : Our study site is located 512 kilometres south of the Arctic Circle in the Northwest Territories, Canada in an inactive gold mine which contains mine waste water in excess of 50 mM arsenic. Several thousand tonnes of arsenic trioxide dust are stored in underground chambers and microbial biofilms grow on the chamber walls below seepage points rich in arsenite-containing solutions. We compared the arsenite oxidisers in two subsamples (which differed in arsenite concentration) collected from one biofilm. 'Species' (sequence) richness did not differ between subsamples, but the relative importance of the three identifiable clades did. An arsenite-oxidising bacterium (designated GM1) was isolated, and was shown to oxidise arsenite in the early exponential growth phase and to grow at a broad range of temperatures (4-25°C). Its arsenite oxidase was constitutively expressed and functioned over a broad temperature range. Conclusions : The diversity of arsenite oxidisers does not significantly differ from two subsamples of a microbial biofilm that vary in arsenite concentrations. GM1 is the first psychrotolerant arsenite oxidiser to be isolated with the ability to grow below 10°C. This ability to grow at low temperatures could be harnessed for arsenic bioremediation in moderate to cold climates.

BMC Microbiology↗
USGS1960

A comprehensive system of automatic computation in magnetic and gravity interpretation

In the interpretation of magnetic and gravity anomalies, downward continuation of fields and calculation of first and second vertical derivatives of fields have been recognized as effective means for bringing into focus the latent diagnostic features of the data. A comprehensive system has been devised for the calculation of any or all of these derived fields on modern electronic digital computing equipment. The integral for analytic continuation above the plane is used with a Lagrange extrapolation polynomial to derive a general determinantal expression from which the field at depth and the various derivatives on the surface and at depth can be obtained. It is shown that the general formula includes as special cases some of the formulas appearing in the literature. The process involves a "once for all depths" summing of grid values on a system of concentric circles about each point followed by application of the appropriate one or more of the 19 sets of coefficients derived for the purpose. Theoretical and observed multilevel data are used to illustrate the processes and to discuss the errors. The coefficients can be used for less extensive computations on a desk calculator.

Geophysics↗
USGS1968

Disposable syringe techniques for obtaining small quantities of pore water from unconsolidated sediments

Disposable plastic syringes, fitted with screen discs and circles of filter paper, can be used to extract small amounts of pore water from unconsolidated sediments. A wooden screw frame or large C clamp supplies pressure for field use. Supplementary techniques enable small volumes of fluid to be recovered and handled easily. The Goldberg compensating refractometer provides a useful companion tool for field studies. It can determine, in less than a minute, total dissolved solids in brackish to salt waters to an accuracy of 0.5 parts per thousand on a drop of fluid.

Journal of Sedimentary Research↗
USGS2013

New equal-area map projections for noncircular regions

A series of new equal-area map projections has been devised. Called Oblated Equal-Area, its lines of constant distortion follow approximately oval or rectangular paths instead of the circles of the Lambert Azimuthal Equal-Area projection or the straight lines of the Cylindrical Equal-Area projection. The projection series permits design of equal-area maps of oblong regions with less overall distortion of shape and scale than equal-area maps on other projections.

American Cartographer↗
USGS2002

Breeding behavior and reproductive life history of the Neosho Madtom, Noturus placidus (Teleostei: Ictaluridae)

The Neosho madtom, Noturus placidus , is a small catfish listed by the U. S. Fish and Wildlife Service as threatened. Little is known of its breeding biology and behavior because high turbidity and flow during its spawning season prevent direct observation in the field, and captive propagation has met with limited success. We held Neosho madtoms in laboratory aquaria in 1996 and 1998 to study sexual dimorphism during breeding season, courtship and nesting behavior, egg and clutch size, and embryological and larval development. We also attempted to induce spawning. Courtship behaviors were recorded on videotape, including “carousel” and “tail curl” displays in which the fish spun in circles, head to tail, then quivered, with the male's tail wrapped around the female's head. Three clutches were observed, all in nest cavities that had been excavated by the fish under a structure; one clutch (1996) consisted of approximately 60 eggs, with a mean chorion diameter of 3.1 mm, and two (1998) consisted of approximately 30 eggs, with mean diameter of 3.7 mm. In all situations, eggs hatched after eight or nine days, and yolk-sacs were fully depleted seven days later. One spawn (1998) occurred after two days of injection with synthetic hormone. Male parental care of eggs and larvae was observed in 1996. Larvae remained in the nest until yolk-sacs were absorbed, after which they dispersed throughout the tank. Dissection of two females that laid clutches in this study revealed previtellogenic eggs in the lumen of ovaries, with a mean chorion diameter of 0.9 mm. Swollen lips of males, distended abdomen of females, and differences in head shape, premaxillary tooth patch coloring, and genital papillae of breeding males and females were documented during spawning periods.

Transactions of the Kansas Academy of Science↗
USGS2008

Dynamic stresses, Coulomb failure, and remote triggering

Dynamic stresses associated with crustal surface waves with 15-30-sec periods and peak amplitudes < 1 MPa are capable of triggering seismicity at sites remote from the generating mainshock under appropriate conditions. Coulomb failure models based on a frictional strength threshold offer one explanation for instances of rapid-onset triggered seismicity that develop during the surface-wave peak dynamic stressing. Evaluation of the triggering potential of surface-wave dynamic stresses acting on critically stressed faults using a Mohr's circle representation together with the Coulomb failure criteria indicates that Love waves should have a higher triggering potential than Rayleigh waves when incident on vertical, strike-slip faults. That (1) the onset of triggered seismicity often appears to begin during the Rayleigh wave rather than the earher-arriving Love wave, (2) vertical strike-slip faults pervade the crust in most tectonic regimes, and (3) Love-wave amplitudes typically exceed those for Rayleigh waves suggests that the explanation for rapid-onset dynamic triggering may not reside solely with a simple static-threshold friction mode. The results also indicate that thrust faults should be more susceptible to dynamic triggering by Rayleigh-wave stresses than normal faults in the shallow seismogenic crust (< 5 km), while the reverse should be true in the lower seismogenic crust (> 5 km). The latter is consistent with the observation that extensional or transtensional tectonic regimes are more susceptible to remote triggering by Rayleigh-wave dynamic stresses than compressional or transpressional regimes. Locally elevated pore pressures may have a role in the observed prevalence of dynamic triggering in extensional regimes and geothermal/volcanic systems.

Bulletin of the Seismological Society of America↗
USGS2012

Dynamic stresses, coulomb failure, and remote triggering: corrected

Dynamic stresses associated with crustal surface waves with 15–30 s periods and peak amplitudes <1 MPa are capable of triggering seismicity at sites remote from the generating mainshock under appropriate conditions. Coulomb failure models based on a frictional strength threshold offer one explanation for instances of rapid‐onset triggered seismicity that develop during the surface‐wave peak dynamic stressing. Evaluation of the triggering potential of surface‐wave dynamic stresses acting on critically stressed faults using a Mohr’s circle representation together with the Coulomb failure criteria indicates that Love waves should have a higher triggering potential than Rayleigh waves for most fault orientations and wave incidence angles. That (1) the onset of triggered seismicity often appears to begin during the Rayleigh wave rather than the earlier arriving Love wave, and (2) Love‐wave amplitudes typically exceed those for Rayleigh waves suggests that the explanation for rapid‐onset dynamic triggering may not reside solely with a simple static‐threshold friction mode. The results also indicate that normal faults should be more susceptible to dynamic triggering by 20‐s Rayleigh‐wave stresses than thrust faults in the shallow seismogenic crust (<10 km) while the advantage tips in favor of reverse faults greater depths. This transition depth scales with wavelength and coincides roughly with the transition from retrograde‐to‐prograde particle motion. Locally elevated pore pressures may have a role in the observed prevalence of dynamic triggering in extensional regimes and geothermal/volcanic systems. The result is consistent with the apparent elevated susceptibility of extensional or transtensional tectonic regimes to remote triggering by Rayleigh‐wave dynamic stresses than compressional or transpressional regimes.

Bulletin of the Seismological Society of America↗
USGS1999

The strain in the array is mainly in the plane (waves below ~1 Hz)

We compare geodetic and single-station methods of measuring dynamic deformations and characterize their causes in the frequency bands 0.5-1.0 Hz and 4.0-8.0 Hz. The geodetic approach utilizes data from small-aperture seismic arrays, applying techniques from geodesy. It requires relatively few assumptions and a priori information. The single-station method uses ground velocities recorded at isolated or single stations and assumes all the deformation is due to plane-wave propagation. It also requires knowledge of the azimuth and horizontal velocity of waves arriving at the recording station. Data employed come from a small-aperture, dense seismic array deployed in Geyokcha, Turkmenistan, and include seismograms recorded by broadband STS2 and short-period L28 sensors. Poor agreement between geodetic and single-station estimates in the 4.0-8.0 Hz passband indicates that the displacement field may vary nonlinearly with distance over distances of ∼50 m. STS2 geodetic estimates provide a robust standard in the 0.5-1.0 Hz passband because they appear to be computationally stable and require fewer assumptions than single-station estimates. The agreement between STS2 geodetic estimates and single-station L28 estimates is surprisingly good for the S -wave and early surface waves, suggesting that the single-station analysis should be useful with commonly available data. These results indicate that, in the 0.5 to 1.0 Hz passband, the primary source of dynamic deformation is plane-wave propagation along great-circle source-receiver paths. For later arriving energy, the effects of scattering become important. The local structure beneath the array exerts a strong control on the geometry of the dynamic deformation, implying that it may be difficult to infer source characteristics of modern or paleoearthquakes from indicators of dynamic deformations. However, strong site control also suggests that the dynamic deformations may be predictable, which would be useful for engineering seismically resistant structures.

Bulletin of the Seismological Society of America↗
USGS2009

Methods for assessing movement path recursion with application to African buffalo in South Africa

Recent developments of automated methods for monitoring animal movement, e.g., global positioning systems (GPS) technology, yield high-resolution spatiotemporal data. To gain insights into the processes creating movement patterns, we present two new techniques for extracting information from these data on repeated visits to a particular site or patch ("recursions"). Identification of such patches and quantification of recursion pathways, when combined with patch-related ecological data, should contribute to our understanding of the habitat requirements of large herbivores, of factors governing their space-use patterns, and their interactions with the ecosystem. We begin by presenting output from a simple spatial model that simulates movements of large-herbivore groups based on minimal parameters: resource availability and rates of resource recovery after a local depletion. We then present the details of our new techniques of analyses (recursion analysis and circle analysis) and apply them to data generated by our model, as well as two sets of empirical data on movements of African buffalo (Syncerus coffer): the first collected in Klaserie Private Nature Reserve and the second in Kruger National Park, South Africa. Our recursion analyses of model outputs provide us with a basis for inferring aspects of the processes governing the production of buffalo recursion patterns, particularly the potential influence of resource recovery rate. Although the focus of our simulations was a comparison of movement patterns produced by different resource recovery rates, we conclude our paper with a comprehensive discussion of how recursion analyses can be used when appropriate ecological data are available to elucidate various factors influencing movement. Inter alia, these include the various limiting and preferred resources, parasites, and topographical and landscape factors. ?? 2009 by the Ecological Society of America.

Ecology↗
USGS2012

Hierarchical distance-sampling models to estimate population size and habitat-specific abundance of an island endemic

Population size and habitat-specific abundance estimates are essential for conservation management. A major impediment to obtaining such estimates is that few statistical models are able to simultaneously account for both spatial variation in abundance and heterogeneity in detection probability, and still be amenable to large-scale applications. The hierarchical distance-sampling model of J. A. Royle, D. K. Dawson, and S. Bates provides a practical solution. Here, we extend this model to estimate habitat-specific abundance and rangewide population size of a bird species of management concern, the Island Scrub-Jay ( Aphelocoma insularis ), which occurs solely on Santa Cruz Island, California, USA. We surveyed 307 randomly selected, 300 m diameter, point locations throughout the 250-km 2 island during October 2008 and April 2009. Population size was estimated to be 2267 (95% CI 1613-3007) and 1705 (1212-2369) during the fall and spring respectively, considerably lower than a previously published but statistically problematic estimate of 12 500. This large discrepancy emphasizes the importance of proper survey design and analysis for obtaining reliable information for management decisions. Jays were most abundant in low-elevation chaparral habitat; the detection function depended primarily on the percent cover of chaparral and forest within count circles. Vegetation change on the island has been dramatic in recent decades, due to release from herbivory following the eradication of feral sheep ( Ovis aries ) from the majority of the island in the mid-1980s. We applied best-fit fall and spring models of habitat-specific jay abundance to a vegetation map from 1985, and estimated the population size of A. insularis was 1400-1500 at that time. The 20-30% increase in the jay population suggests that the species has benefited from the recovery of native vegetation since sheep removal. Nevertheless, this jay's tiny range and small population size make it vulnerable to natural disasters and to habitat alteration related to climate change. Our results demonstrate that hierarchical distance-sampling models hold promise for estimating population size and spatial density variation at large scales. Our statistical methods have been incorporated into the R package unmarked to facilitate their use by animal ecologists, and we provide annotated code in the Supplement.

California↗