3,023 research outputs found
American Oystercatcher productivity monitoring, breeding survey and banding in Virginia: 2004 report
Biologists along the east coast of the United States have recently expressed concern about the conservation status of the American Oystercatcher (Haematopus palliatus). The species has a relatively small population size and depends on coastal habitat that is under pressure from humans for development and recreation. Several studies of American Oystercatcher productivity have suggested that the species is inherently intolerant to human disturbance and experiences low productivity rates in areas where their breeding habitat is also used for human recreation. As a beach-nesting bird, their nesting attempts are also very vulnerable to destruction by mammalian and avian predators, which have been known to experience population increases because of human influenced factors. The undeveloped barrier islands and marshes of Virginia’s Eastern Shore support over 500 pairs of breeding American Oystercatchers and provide a unique opportunity to study their productivity in the relative absence of disturbance from direct human activity. Some of the islands are, however, inhabited by high numbers of common raccoons (Procyon lotor). The Nature Conservancy’s Virginia Coast Reserve (TNC), in cooperation with The Center for Conservation Biology at The College of William and Mary (CCB) and other partners, currently supports several efforts aimed at investigating the population status and breeding and wintering ecology of the species. These efforts include annual studies of oystercatcher productivity on the barrier islands and in the lagoon system, annual surveys of breeding and wintering populations and a banding program for oystercatcher adults and chicks. The primary objective of productivity monitoring has been to compare productivity in habitats managed for mammalian predators and those that are not in order to assess the effectiveness of the predator removal as a management strategy for increasing the reproductive success of beach nesting shorebirds and waterbirds. The annual breeding and wintering surveys provide consistent and precise counts of oystercatchers along the Virginia coast so that managers can track changes in population numbers and distribution. Finally, the banding program will increase the number of color-banded birds in the population so that researchers may examine questions concerning migration and dispersal, survival rates and habitat use. One hundred and fifty-three pairs of American Oystercatchers were monitored for productivity during the 2004 breeding season. Eighty-one nested on Metompkin Island, which is managed by TNC for mammalian predators. Twenty-five nested on Wreck Island Natural Area Preserve, which is not managed for mammalian predators but did not have any predator activity during the 2004 breeding season. Fortyseven nested in the marshes of the lagoon system located adjacent to Wachapreague, Virginia. American Oystercatchers experienced high productivity on Metompkin Island and Wreck Island (0.79-1.18 young fledged per pair) for the third and second consecutive years, respectively. These productivity levels continue to be well above what is typically recorded for this species in other parts of its range, and our results suggest that the absence of mammalian predators on these islands allows for the higher productivity of pairs nesting there. American Oystercatchers breeding in the marshes off of Wachapreague also experienced high productivity in 2004 – 0.85 young fledged per pair. Productivity of oystercatchers in this habitat is very vulnerable to flooding events and appears to be highly variable from year to year depending on the timing of egg-laying, spring tide events, storms and storm influenced tide events. A total of 706 adult American Oystercatchers (327 pairs and 52 single adults) were recorded during the 2004 Piping Plover, Wilson’s Plover and American Oystercatcher survey, a 26.1% increase from 2000 when oystercatchers were first systematically surveyed along the coast of Virginia. Breeding pairs were documented on every barrier island along the Eastern Shore of Virginia except for Walllops Island. Only seven pairs were documented west and south of the Chesapeake Bay. Sixty-two American Oystercatcher chicks and two adults were banded during the 2004 breeding season. At least fifty-seven of the chicks fledged. We will continue to work with other states to further develop Virginia’s banding program, upon which researchers, managers and students can build in the future
Status and distribution of colonial waterbirds in coastal Virginia: 2018 breeding season
Colonial waterbirds are highly visible components of coastal avifaunas that share the unusual characteristic of nesting in dense assemblages. One consequence of having large portions of populations nesting in few locations is that even restricted disturbance may have profound consequences on a population level. Development of conservation strategies for these sensitive species requires current status and distribution information. In the fall of 1992, a consortium of agencies and individuals agreed that a comprehensive monitoring program for the Virginia colonial waterbird community was needed and that assessments should be made on regular (initially every 10 years but reduced to 5 years in 2003) intervals for trend analyses. Systematic surveys have been conducted during the breeding seasons of 1993, 2003, 2008 and 2013. The 2018 survey reported here is the fifth in the time series. These surveys have covered colonial waterbird populations (24 species – Great Blue Herons were not included in 2008 and 2018 due to budgetary constraints) throughout the Coastal Plain province of Virginia. We surveyed 270 waterbird colonies during the breeding season of 2018. Colonies supported an estimated 43,159 breeding pairs of 23 species. Gulls were the most abundant group with more than 19,700 breeding pairs. Terns and waders accounted for 7,129 and 6,386 pairs respectively. Although they have declined dramatically, Laughing Gulls continue to be the most abundant species and were three times more abundant than any other species, accounting for nearly 40% of the total waterbird community. The barrier island/lagoon system of the Eastern Shore was the most important region for the majority of colonial species encountered. In 2018, this region supported 22 of the 23 species evaluated. The Eastern Shore accounted for 50.5% and 46.6% of all breeding pairs and colonies respectively. For 17 of the 23 species, the region supported more than 50% of the known coastal population. The colonial waterbird community as a whole in coastal Virginia has declined dramatically since 1993 (2018 survey did not include Great Blue Herons or all Great Egrets). Population estimates for 15 (68%) of the 22 species assessed declined between 1993 and 2018. Declines varied considerably between species with 14 species declining more than 40% and 9 species declining more than 60%. Cattle Egrets showed the highest loss rate (-96.7%), declining from an estimated 1,459 to only 48 pairs. Little Blue Herons declined by 83% from 374 to only 64 pairs. Seven species increased between 1993 and 2018. Dramatic expansions were documented for White Ibis, Double-crested Cormorant, and Brown Pelican. Over the past 25 years, two major forces appear to be shaping the colonial waterbird community in Virginia. These include 1) regional shifts in population centers that are driving population increases in Virginia and 2) habitat degradation related to sea-level rise. With the exception of Great Egrets, all species that have increased over the past 20 years have experienced ongoing range expansions and are riding a population wave that is progressing through Virginia. This includes Great Black-backed Gull, Double-crested Cormorant, Brown Pelican, and White Ibis. Most of the decline in medium-sized waders is being driven by habitat loss related to erosion of islands. This erosion results from sea-level rise, is ongoing and represents a significant threat to these populations. Several ground-nesting seabirds are likely more directly impacted by the loss of viable habitat and demographic impacts related to frequent flooding. The most notable example is the Laughing Gull that has experienced a catastrophic decline in both population and distribution
On the Sets of Real Numbers Recognized by Finite Automata in Multiple Bases
This article studies the expressive power of finite automata recognizing sets
of real numbers encoded in positional notation. We consider Muller automata as
well as the restricted class of weak deterministic automata, used as symbolic
set representations in actual applications. In previous work, it has been
established that the sets of numbers that are recognizable by weak
deterministic automata in two bases that do not share the same set of prime
factors are exactly those that are definable in the first order additive theory
of real and integer numbers. This result extends Cobham's theorem, which
characterizes the sets of integer numbers that are recognizable by finite
automata in multiple bases.
In this article, we first generalize this result to multiplicatively
independent bases, which brings it closer to the original statement of Cobham's
theorem. Then, we study the sets of reals recognizable by Muller automata in
two bases. We show with a counterexample that, in this setting, Cobham's
theorem does not generalize to multiplicatively independent bases. Finally, we
prove that the sets of reals that are recognizable by Muller automata in two
bases that do not share the same set of prime factors are exactly those
definable in the first order additive theory of real and integer numbers. These
sets are thus also recognizable by weak deterministic automata. This result
leads to a precise characterization of the sets of real numbers that are
recognizable in multiple bases, and provides a theoretical justification to the
use of weak automata as symbolic representations of sets.Comment: 17 page
Potential Energy Surface for H_2 Dissociation over Pd(100)
The potential energy surface (PES) of dissociative adsorption of H_2 on
Pd(100) is investigated using density functional theory and the full-potential
linear augmented plane wave (FP-LAPW) method. Several dissociation pathways are
identified which have a vanishing energy barrier. A pronounced dependence of
the potential energy on ``cartwheel'' rotations of the molecular axis is found.
The calculated PES shows no indication of the presence of a precursor state in
front of the surface. Both results indicate that steering effects determine the
observed decrease of the sticking coefficient at low energies of the H_2
molecules. We show that the topology of the PES is related to the dependence of
the covalent H(s)-Pd(d) interactions on the orientation of the H_2 molecule.Comment: RevTeX, 8 pages, 5 figures in uufiles forma
Nanoscale grains, high irreversibility field, and large critical current density as a function of high energy ball milling time in C-doped magnesium diboride
Magnesium diboride (MgB2) powder was mechanically alloyed by high energy ball
milling with C to a composition of Mg(B0.95C0.05)2 and then sintered at 1000 C
in a hot isostatic press. Milling times varied from 1 minute to 3000 minutes.
Full C incorporation required only 30-60 min of milling. Grain size of sintered
samples decreased with increased milling time to less than 30 nm for 20-50 hrs
of milling. Milling had a weak detrimental effect on connectivity. Strong
irreversibility field (H*) increase (from 13.3 T to 17.2 T at 4.2 K) due to
increased milling time was observed and correlated linearly with inverse grain
size (1/d). As a result, high field Jc benefited greatly from lengthy powder
milling. Jc(8 T, 4.2 K) peaked at > 80,000 A/cm2 with 1200 min of milling
compared with only ~ 26,000 A/cm2 for 60 min of milling. This non-compositional
performance increase is attributed to grain refinement of the unsintered powder
by milling, and to the probable suppression of grain growth by milling-induced
MgO nano-dispersions.Comment: 12 pages, 11 figure
Potential, core-level and d band shifts at transition metal surfaces
We have extended the validity of the correlation between the surface
3d-core-level shift (SCLS) and the surface d band shift (SDBS) to the entire 4d
transition metal series and to the neighboring elements Sr and Ag via accurate
first-principles calculations. We find that the correlation is quasilinear and
robust with respect to the differencies both between initial and final-state
calculations of the SCLS's and two distinct measures of the SDBS's. We show
that despite the complex spatial dependence of the surface potential shift
(SPS) and the location of the 3d and 4d orbitals in different regions of space,
the correlation exists because the sampling of the SPS by the 3d and 4d
orbitals remains similar. We show further that the sign change of the SCLS's
across the transition series does indeed arise from the d band-narrowing
mechanism previously proposed. However, while in the heavier transition metals
the predicted increase of d electrons in the surface layer relative to the bulk
arises primarily from transfers from s and p states to d states within the
surface layer, in the lighter transition metals the predicted decrease of
surface d electrons arises primarily from flow out into the vacuum.Comment: RevTex, 22 pages, 5 figures in uufiles form, to appear in Phys.Rev.
Comment on "Antilocalization in a 2D Electron Gas in a Random Magnetic Field"
In a recent Letter, Taras-Semchuk and Efetov reconsider the problem of
electron localization in a random magnetic field in two dimensions. They claim
that due to the long-range nature of the vector potential correlations an
additional term appears in the effective field theory (-model) of the
problem, leading to delocalization at the one-loop level. This calls into
question the results of earlier analytical studies, where the random magnetic
field problem was mapped onto the conventional unitary-class -model,
implying that the leading quantum correction is of two-loop order and of a
localizing nature. We show in this Comment, however, that the new term in fact
does not exist and was erroneously obtained by Taras-Semchuk and Efetov because
of an inconsistent treatment violating gauge invariance.Comment: 1 page, 2 figure
Magnetic Photon Splitting: Computations of Proper-time Rates and Spectra
The splitting of photons in the presence of an intense magnetic field has
recently found astrophysical applications in polar cap models of gamma-ray
pulsars and in magnetar scenarios for soft gamma repeaters. Numerical
computation of the polarization-dependent rates of this third order QED process
for arbitrary field strengths and energies below pair creation threshold is
difficult: thus early analyses focused on analytic developments and simpler
asymptotic forms. The recent astrophysical interest spurred the use of the
S-matrix approach by Mentzel, Berg and Wunner to determine splitting rates. In
this paper, we present numerical computations of a full proper-time expression
for the rate of splitting that was obtained by Stoneham, and is exact up to the
pair creation threshold. While the numerical results derived here are in accord
with the earlier asymptotic forms due to Adler, our computed rates still differ
by as much as factors of 3 from the S-matrix re-evaluation of Wilke and Wunner,
reflecting the extreme difficulty of generating accurate S-matrix numerics for
fields below about \teq{4.4\times 10^{13}}Gauss. We find that our proper-time
rates appear very accurate, and exceed Adler's asymptotic specializations
significantly only for photon energies just below pair threshold and for
supercritical fields, but always by less than a factor of around 2.6. We also
provide a useful analytic series expansion for the scattering amplitude valid
at low energies.Comment: 13 pages, AASTeX format, including 3 eps figures, ApJ in pres
Anisotropic magnetoresistance in a 2DEG in a quasi-random magnetic field
We present magnetotransport results for a 2D electron gas (2DEG) subject to
the quasi-random magnetic field produced by randomly positioned sub-micron Co
dots deposited onto the surface of a GaAs/AlGaAs heterostructure. We observe
strong local and non-local anisotropic magnetoresistance for external magnetic
fields in the plane of the 2DEG. Monte-Carlo calculations confirm that this is
due to the changing topology of the quasi-random magnetic field in which
electrons are guided predominantly along contours of zero magnetic field.Comment: 4 pages, 6 figures, submitted to Phys. Rev.
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