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At least 217 records · Page 12Linked to original sources
PubMed2005

Genetic structure, introgression, and a narrow hybrid zone between northern and California spotted owls (Strix occidentalis).

The northern spotted owl (Strix occidentalis caurina) is a threatened subspecies and the California spotted owl (Strix occidentalis occidentalis) is a subspecies of special concern in the western United States. Concern for their continued viability has arisen because of habitat loss caused by timber harvesting. The taxonomic status of the northern subspecies has been the subject of continuing controversy. We investigated the phylogeographical and population genetic structure of northern and California spotted owls with special reference to their region of contact. Mitochondrial DNA (mtDNA) control region sequences confirmed the existence of two well-differentiated lineages connected by a narrow hybrid zone in a region of low population density in north central California. Maximum-likelihood estimates indicated bidirectional gene flow between the lineages but limited introgression outside the region of contact. The lengths of both the mtDNA hybrid zone and the reduced density patch were similar and slightly exceeded estimates of natal dispersal distances. This suggests that the two subspecies were in secondary contact in a hybrid zone trapped by a population density trough. Consequently, the zone of interaction is expected to be geographically stable. We discovered a third, rare clade of haplotypes, which we interpreted to be a result of incomplete lineage sorting; those haplotypes result in a paraphyletic northern spotted owl with respect to the California spotted owl. A congeneric species, the barred owl (Strix varia), occasionally hybridizes with spotted owls; our results indicated an upper bound for the frequency of barred owl mtDNA haplotypes in northern spotted owl populations of 3%.

Animals↗
PubMed2005

The influence of Pleistocene glacial refugia on tawny owl genetic diversity and phylogeography in western Europe.

The glacial refugia hypothesis indicates that during the height of the Pleistocene glaciations the temperate species that are today widespread in western Europe must have survived in small and climatically favourable areas located in the southern peninsulas of Iberia, Italy and Balkans. One such species is the tawny owl, a relatively sedentary, nonmigratory bird presently distributed throughout Europe. It is a tree-nesting species closely associated with deciduous and mixed coniferous woodlands. In this study I used control region mtDNA sequences from 187 individuals distributed among 14 populations to determine whether current genetic patterns in tawny owl populations were consistent with postglacial expansion from peninsular refugia. European, North African and Asian tawny owls were found to represent three distinct lineages, where North Africa is the sister clade to all European owls. Within Europe, I found three well-supported clades that correspond to each of the three allopatric refugia. Expansion patterns indicate that owls from the Balkan refugium repopulated most of northern Europe, while expansion out of Iberia and Italy had only regional effects leading to admixture in France. Estimates of population divergence times between refugia populations are roughly similar, but one order of magnitude smaller between Greece and northern Europe. Based on a wide range of mutation rates and generation times, divergence between refugia appears to date to the Pleistocene.

Africa, Northern↗
PubMed2004

Error rate of axons at the owl's optic chiasm.

BACKGROUND: In owls, the visual pathways from the retina are totally crossed. Attempts to find ganglion cells with uncrossed axons have failed consistently, when retrograde labeling with HRP is used for their identification. In the present investigation we have used retrograde fluorescent tracers of complementary colour in each optic tectum to demonstrate a tiny population of ipsilaterally-projecting retinal ganglion cells in the owl. METHODS: We studied two species, one from each of the two important owl families: the Southern boobook owl, Ninox boobook: (Family Strigidae); and the Barn Owl, Tyto alba: (Family Tytonidae). RESULTS: The small numbers, random distribution and heterogeneity of the mis-projecting ganglion cells, taken together, argue against a functional role for them. Instead, they appear to be the result of developmental errors in the specification of laterality. At a number of different eccentricities and ganglion cell densities, the error rate was roughly a constant fraction of the neurons involved, at around 10-4 for the tytonid owl (lacking a fovea) and around 10-5 for the strigid owl (which has a fovea and a higher overall density of ganglion cells). CONCLUSIONS: These values are close to the error rates of replicating enzymes, such as nucleic acid polymerases. The evolution of a higher retinal ganglion cell density and a fovea in the Strigidae appears to be accompanied by an improvement in the error rate.

Animals↗
PubMedDate not supplied

Primary closure of the corneas of two Great Horned owls after resection of nonhealing ulcers.

Two Great Horned owls were presented to the Colorado State University Veterinary Teaching Hospital for evaluation of corneal lesions. Each bird had a corneal ulcer and bullous keratopathy. Following unsuccessful attempts at medical and surgical management, the corneal defects in each bird were treated with penetrating keratoplasties and conjunctival pedicle grafts. In each bird the cornea healed well and regained approximately its normal shape. Both birds fully recovered and have been released into the wild.

Animals↗
PubMed2006

Conservation of the northern spotted owl under the Northwest Forest Plan.

Development of the Northwest Forest Plan (NWFP) was motivated by concerns about the over-harvest of late-seral forests and the effects of intensive forest management on the long-term viability of the Northern Spotted Owl (Strix occidentalis caurina). Following several years of intense political and legal debates, the final NWFP was approved in 1994. Even though the plan evolved with a broad ecosystem perspective, it remained anchored in the Spotted Owl reserve design proposed in 1990. Based on a criterion of stable or increasing populations, a decade later it remains unclear whether the enactment of the NWFP has improved the conservation status of Spotted Owls. The results of intensive monitoring of several Spotted Owl populations for over a decade suggest a continuing range-wide decline even though rates of timber harvest have declined dramatically on federal lands. The cause of the decline is difficult to determine because the research needed to establish cause and effect relations has not been done. One plausible hypothesis is that the owl's life history greatly constrains its rate of population growth even when habitat is no longer limiting. Since enactment of the NWFP, new threats have arisen, including the movement of Barred Owls (S. varia) into the range of the Spotted Owl, political pressure to increase levels of timber harvest, and recent changes to forest laws that eliminate the requirement to assess the viability of wildlife populations on U.S. Department of Agriculture Forest Service lands. At this time is appears that Spotted Owl conservation rests critically on continued implementation of the protections afforded by the NWFP and the U.S. Endangered Species Act.

Animals↗
PubMed2002

Seroprevalence of avian paramyxovirus 1, 2, and 3 in captive and free-living birds of prey in Spain (preliminary results): implications for management of wild and captive populations.

Since December 1997, 700 blood plasma samples from 31 different species of captive and free-living birds of prey from Spain were analyzed by hemagglutination inhibition (HI) test for the presence of antibodies to avian paramyxovirus (aPMV) 1,2, and 3. Out of 700 birds, 120 tested positive for aPMV-1, 10 birds had antibodies to aPMV-2, and 4 birds tested positive against aPMV-3. Prevalence of antibodies against aPMV-1 was significantly higher in captive than in free-living birds of prey and in Falconiformes than in Strigidae and Accipitridae. Infection or exposure in captive birds may be due to the use of avian-derived food in rehabilitation and captive-breeding centers. This may be of concern at the time of reintroduction of these birds into free-living populations.

Animals↗
PubMed2005

Creating a sense of auditory space.

Determining the location of a sound source requires the use of binaural hearing--information about a sound at the two ears converges onto neurones in the auditory brainstem to create a binaural representation. The main binaural cue used by many mammals to locate a sound source is the interaural time difference, or ITD. For over 50 years a single model has dominated thinking on how ITDs are processed. The Jeffress model consists of an array of coincidence detectors--binaural neurones that respond maximally to simultaneous input from each ear--innervated by a series of delay lines--axons of varying length from the two ears. The purpose of this arrangement is to create a topographic map of ITD, and hence spatial position in the horizontal plane, from the relative timing of a sound at the two ears. This model appears to be realized in the brain of the barn owl, an auditory specialist, and has been assumed to hold for mammals also. Recent investigations, however, indicate that both the means by which neural tuning for preferred ITD, and the coding strategy used by mammals to determine the location of a sound source, may be very different to barn owls and to the model proposed by Jeffress.

Acoustic Stimulation↗
PubMed2003

Auditory spatial discrimination by barn owls in simulated echoic conditions.

In humans, directional hearing in reverberant conditions is characterized by a "precedence effect," whereby directional information conveyed by leading sounds dominates perceived location, and listeners are relatively insensitive to directional information conveyed by lagging sounds. Behavioral studies provide evidence of precedence phenomena in a wide range of species. The present study employs a discrimination paradigm, based on habituation and recovery of the pupillary dilation response, to provide quantitative measures of precedence phenomena in the barn owl. As in humans, the owl's ability to discriminate changes in the location of lagging sources is impaired relative to that for single sources. Spatial discrimination of lead sources is also impaired, but to a lesser extent than discrimination of lagging sources. Results of a control experiment indicate that sensitivity to monaural cues cannot account for discrimination of lag source location. Thus, impairment of discrimination ability in the two-source conditions most likely reflects a reduction in sensitivity to binaural directional information. These results demonstrate a similarity of precedence effect phenomena in barn owls and humans, and provide a basis for quantitative comparison with neuronal data from the same species.

Animals↗
PubMed2005

Auditory brainstem responses in the Eastern Screech Owl: an estimate of auditory thresholds.

The auditory brainstem response (ABR), a measure of neural synchrony, was used to estimate auditory sensitivity in the eastern screech owl (Megascops asio). The typical screech owl ABR waveform showed two to three prominent peaks occurring within 5 ms of stimulus onset. As sound pressure levels increased, the ABR peak amplitude increased and latency decreased. With an increasing stimulus presentation rate, ABR peak amplitude decreased and latency increased. Generally, changes in the ABR waveform to stimulus intensity and repetition rate are consistent with the pattern found in several avian families. The ABR audiogram shows that screech owls hear best between 1.5 and 6.4 kHz with the most acute sensitivity between 4-5.7 kHz. The shape of the average screech owl ABR audiogram is similar to the shape of the behaviorally measured audiogram of the barn owl, except at the highest frequencies. Our data also show differences in overall auditory sensitivity between the color morphs of screech owls.

Acoustic Stimulation↗
PubMed2001

Auditory spatial receptive fields created by multiplication.

Examples of multiplication by neurons or neural circuits are scarce, although many computational models use this basic operation. The owl's auditory system computes interaural time (ITD) and level (ILD) differences to create a two-dimensional map of auditory space. Space-specific neurons are selective for combinations of ITD and ILD, which define, respectively, the horizontal and vertical dimensions of their receptive fields. A multiplication of separate postsynaptic potentials tuned to ITD and ILD, rather than an addition, can account for the subthreshold responses of these neurons to ITD-ILD pairs. Other nonlinear processes improve the spatial tuning of the spike output and reduce the fit to the multiplicative model.

Acoustic Stimulation↗
PubMed2002

Gated visual input to the central auditory system.

The central auditory system translates sound localization cues into a map of space guided, in part, by visual experience. In barn owls, this process takes place in the external nucleus of the inferior colliculus (ICX). However, to date, no trace of visual activity has been observed in this auditory nucleus. Here we show that strong visual responses, which are appropriate to guide auditory plasticity, appear in the ICX when inhibition is blocked in the optic tectum. Thus, visual spatial information is gated into the auditory system by an inhibitory mechanism that operates at a higher level in the brain.

Acoustic Stimulation↗
PubMed2003

Cyclic dynamics in a simple vertebrate predator-prey community.

The collared lemming in the high-Arctic tundra in Greenland is preyed upon by four species of predators that show marked differences in the numbers of lemmings each consumes and in the dependence of their dynamics on lemming density. A predator prey model based on the field-estimated predator responses robustly predicts 4-year periodicity in lemming dynamics, in agreement with long-term empirical data. There is no indication in the field that food or space limits lemming population growth, nor is there need in the model to consider those factors. The cyclic dynamics are driven by a 1-year delay in the numerical response of the stoat and stabilized by strongly density-dependent predation by the arctic fox, the snowy owl, and the long-tailed skua.

Animals↗
PubMed2005

Allometry of alarm calls: black-capped chickadees encode information about predator size.

Many animals produce alarm signals when they detect a potential predator, but we still know little about the information contained in these signals. Using presentations of 15 species of live predators, we show that acoustic features of the mobbing calls of black-capped chickadees (Poecile atricapilla) vary with the size of the predator. Companion playback experiments revealed that chickadees detect this information and that the intensity of mobbing behavior is related to the size and threat of the potential predator. This study demonstrates an unsuspected level of complexity and sophistication in avian alarm calls.

Animals↗
PubMed1999

Functional selection of adaptive auditory space map by GABAA-mediated inhibition.

The external nucleus of the inferior colliculus in the barn owl contains an auditory map of space that is based on the tuning of neurons for interaural differences in the timing of sound. In juvenile owls, this region of the brain can acquire alternative maps of interaural time difference as a result of abnormal experience. It has been found that, in an external nucleus that is expressing a learned, abnormal map, the circuitry underlying the normal map still exists but is functionally inactivated by inhibition mediated by gamma-aminobutyric acid type A (GABAA) receptors. This inactivation results from disproportionately strong inhibition of specific input channels to the network. Thus, experience-driven changes in patterns of inhibition, as well as adjustments in patterns of excitation, can contribute critically to adaptive plasticity in the central nervous system.

Acoustic Stimulation↗