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R.J. Camp

Publications and source records attributed to R.J. Camp.

7 recordsLinked to original sources
USGS2009

Recent status and trends of the land bird avifauna on Saipan, Mariana Islands, with emphasis on the endangered Nightingale Reed-warbler Acrocephalus luscinia

The avifauna of the Mariana Islands, an archipelago in the western Pacific, faces the threats of rapid economic development and the spread of non-native species, particularly a devastating predator, Brown Tree Snake Boiga irregularis. In this paper, we examine the status and trends of the land bird fauna of Saipan Island based on three island-wide surveys conducted in 1982, 1997, and 2007. During this period, the human population on Saipan increased more than four-fold and much of the island has been developed. The surveys employed standard point-transect methods based on Distance Sampling. Remarkably, we found nearly all species of land birds - 11 native species and three introduced species - to be common or abundant. The exception was the Micronesian Megapode Megapodius laperouse, a historically rare species that was not observed on the 2007 survey, although it does persist on Saipan and other Mariana islands. A comparison of species densities among the three surveys showed that seven species, mainly fruit and seed-eaters, had increased and three species of insectivorous birds had decreased - Rufous Fantail Rhipidura rufifrons, Nightingale Reed-warbler Acrocephalus luscinia, and Golden White-eye Cleptornis marchei. Of these three, Nightingale Reed-warbler is listed as Endangered on the IUCN Red List and as an Endangered Species by the U.S. Fish and Wildlife Service. Reed-warbler densities on Saipan decreased by more than half between 1982 and 2007. Although point transect sampling worked well for this species, density estimates and trends assessment could be improved by reallocating sampling stations among habitats and by more frequent sampling. ?? BirdLife International 2009.

Bird Conservation International↗
USGS2009

A spatial model of bird abundance as adjusted for detection probability

Modeling the spatial distribution of animals can be complicated by spatial and temporal effects (i.e. spatial autocorrelation and trends in abundance over time) and other factors such as imperfect detection probabilities and observation-related nuisance variables. Recent advances in modeling have demonstrated various approaches that handle most of these factors but which require a degree of sampling effort (e.g. replication) not available to many field studies. We present a two-step approach that addresses these challenges to spatially model species abundance. Habitat, spatial and temporal variables were handled with a Bayesian approach which facilitated modeling hierarchically structured data. Predicted abundance was subsequently adjusted to account for imperfect detection and the area effectively sampled for each species. We provide examples of our modeling approach for two endemic Hawaiian nectarivorous honeycreepers: 'i'iwi Vestiaria coccinea and 'apapane Himatione sanguinea. ?? 2009 Ecography.

Ecography↗
USGS2006

Ecology of a population of subsidized predators: Common ravens in the central Mojave Desert, California

Human subsidies have resulted in the rapid growth of populations of common ravens (Corvus corax) in the Mojave Desert. This is a management concern because ravens prey on threatened desert tortoises (Gopherus agassizii). We conducted weekly counts for 29 months at 10 sites on the US Army's National Training Center, Fort Irwin, California to evaluate factors affecting the distribution of ravens. Raven abundance varied seasonally, diurnally, and with human abundance. It was greatest near resource subsidies, specifically the landfill and sewage ponds. Although other studies have documented heavy use of landfills by ravens, the use of sewage ponds had not been previously reported in the published literature. We suggest that raven management should focus on reducing access to anthropogenic resources. ?? 2006.

Journal of Arid Environments↗
USGS2005

Population trends of Mariana Crow Corvus kubaryi on Rota, Commonwealth of the Northern Mariana Islands

Endemic to the islands of Guam and Rota in the Mariana Islands, Mariana Crow Corvus kubaryi is the only corvid in Micronesia. Currently, it survives on Guam only because of translocation of individuals from Rota (1999-2003). Island-wide surveys in 1982 and 1995 on Rota yielded population estimates of 1,348 and 592 respectively, indicating a 56% decrease in only 13 years. A sharp decline in the only viable Mariana Crow population has serious implications for conservation efforts on Rota and for efforts to re-establish the Guam population. However, the validity of the apparent decline has been debated among scientists and government management agencies. We augmented the 1982 and 1995 island-wide VCP surveys with (1) an additional island-wide survey conducted in 1998, and (2) roadside surveys conducted during 1991-1993 and again during 1999-2002. We also outline historical changes in Rota's limestone forest based on aerial photographs and historical information. Data from all surveys indicate a significant decline in the Mariana Crow population. Declines occurred especially along the north-central coast and in the area east of the airport known as As Dudo in the 1990s, but the data indicate an island-wide decline over the entire span of the surveys. introduced predators, human persecution, and habitat loss and degradation by anthropogenic and natural causes have all contributed to the decline. Long-term preservation of this species will require effective brown treesnake Boiga irregularis control, habitat protection, continued monitoring and research, and increased public education and awareness of Rota's rare and endangered species. ?? BirdLife International 2005.

Bird Conservation International↗
USGS2005

Problems with sampling desert tortoises: A simulation analysis based on field data

The desert tortoise (Gopherus agassizii) was listed as a U.S. threatened species in 1990 based largely on population declines inferred from mark-recapture surveys of 2.59-km2 (1-mi2) plots. Since then, several census methods have been proposed and tested, but all methods still pose logistical or statistical difficulties. We conducted computer simulations using actual tortoise location data from 2 1-mi2 plot surveys in southern California, USA, to identify strengths and weaknesses of current sampling strategies. We considered tortoise population estimates based on these plots as "truth" and then tested various sampling methods based on sampling smaller plots or transect lines passing through the mile squares. Data were analyzed using Schnabel's mark-recapture estimate and program CAPTURE. Experimental subsampling with replacement of the 1-mi2 data using 1-km2 and 0.25-km2 plot boundaries produced data sets of smaller plot sizes, which we compared to estimates from the 1-mi 2 plots. We also tested distance sampling by saturating a 1-mi 2 site with computer simulated transect lines, once again evaluating bias in density estimates. Subsampling estimates from 1-km2 plots did not differ significantly from the estimates derived at 1-mi2. The 0.25-km2 subsamples significantly overestimated population sizes, chiefly because too few recaptures were made. Distance sampling simulations were biased 80% of the time and had high coefficient of variation to density ratios. Furthermore, a prospective power analysis suggested limited ability to detect population declines as high as 50%. We concluded that poor performance and bias of both sampling procedures was driven by insufficient sample size, suggesting that all efforts must be directed to increasing numbers found in order to produce reliable results. Our results suggest that present methods may not be capable of accurately estimating desert tortoise populations.

California↗
USGS1999

Predatory bird populations in the east Mojave Desert, California

We surveyed 7 species of predatory birds weekly during a 12-month period (December 1992 through November 1993) in the east Mojave Desert, California. The Common Raven (Corvus corax) was the most frequently observed species with an average of 6.9 sightings per 100 km. Turkey Vultures (Cathartes aura), Red-tailed Hawks (Buteo jamaicensis), Loggerhead Shrikes (Lanius ludovicianus), American Kestrels (Falco sparverius), Golden Eagles (Aquila chrysaetos), and Prairie Falcons (Falco mexicanus) were seen in decreasing order of frequency of observation through the study period. Ravens, Red-tailed Hawks, Loggerhead Shrikes, American Kestrels, and Prairie Falcons were seen throughout the year. Turkey Vultures were not present during winter months, while Golden Eagles were seen only during November and December. Turkey Vultures, Red-tailed Hawks, and ravens were most numerous on agricultural lands, while Loggerhead Shrikes were most Common at urban areas. Raven numbers increased with increasing number of linear rights-of-way parallel to the survey route. Perching was the most common behavior type, although Turkey Vultures and ravens were often observed soaring, flying, or standing on the ground near highways. Transmission powerline towers and telephone poles were used as perch sites disproportionately to availability.

Great Basin Naturalist↗