Quantifying Vocal Activity and Detection Probability to Inform Survey Methods for Barred Owls (Strix varia)

Species occurring at low density can be difficult to detect and if not properly accounted for, imperfect detection will lead to inaccurate estimates of occupancy. Understanding sources of variation in detection probability and how they can be managed is a key part of monitoring. We used sightings data of a low-density and elusive raptor (white-headed vulture Trigonoceps occipitalis ) in areas of known occupancy (breeding territories) in a likelihood-based modelling approach to calculate detection probability and the factors affecting it. Because occupancy was known a priori to be 100%, we fixed the model occupancy parameter to 1.0 and focused on identifying sources of variation in detection probability. Using detection histories from 359 territory visits, we assessed nine covariates in 29 candidate models. The model with the highest support indicated that observer speed during a survey, combined with temporal covariates such as time of year and length of time within a territory, had the highest influence on the detection probability. Averaged detection probability was 0.207 (s.e. 0.033) and based on this the mean number of visits required to determine within 95% confidence that white-headed vultures are absent from a breeding area is 13 (95% CI: 9–20). Topographical and habitat covariates contributed little to the best models and had little effect on detection probability. We highlight that low detection probabilities of some species means that emphasizing habitat covariates could lead to spurious results in occupancy models that do not also incorporate temporal components. While variation in detection probability is complex and influenced by effects at both temporal and spatial scales, temporal covariates can and should be controlled as part of robust survey methods. Our results emphasize the importance of accounting for detection probability in occupancy studies, particularly during presence/absence studies for species such as raptors that are widespread and occur at low densities.

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Using Detection Dogs to Conduct Simultaneous Surveys of Northern Spotted (Strix occidentalis caurina) and Barred Owls (Strix varia)

PLoS ONE ◽

10.1371/journal.pone.0042892 ◽

2012 ◽

Vol 7 (8) ◽

pp. e42892 ◽

Cited By ~ 14

Author(s):

Samuel K. Wasser ◽

Lisa S. Hayward ◽

Jennifer Hartman ◽

Rebecca K. Booth ◽

Kristin Broms ◽

...

Keyword(s):

Strix Occidentalis ◽

Strix Varia ◽

Strix Occidentalis Caurina ◽

Barred Owls

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Does The Presence of Barred Owls Suppress the Calling Behavior of Spotted Owls?

The Condor ◽

10.1093/condor/108.4.760 ◽

2006 ◽

Vol 108 (4) ◽

pp. 760-769 ◽

Cited By ~ 4

Author(s):

Michelle L. Crozier ◽

Mark E. Seamans ◽

R. J. GutiÉRrez ◽

Peter J. Loschl ◽

Robert B. Horn ◽

...

Keyword(s):

Calling Behavior ◽

Field Observations ◽

Spotted Owl ◽

Strix Occidentalis ◽

Response Data ◽

Strix Varia ◽

Spotted Owls ◽

Strix Occidentalis Caurina ◽

Northern Spotted Owls ◽

Barred Owls

Abstract Abstract Barred Owls (Strix varia) have expanded their range throughout the ranges of Northern (Strix occidentalis caurina) and California Spotted Owls (S. o. occidentalis). Field observations have suggested that Barred Owls may be behaviorally dominant to Spotted Owls. Therefore, we conducted a test of behavioral dominance by assessing responsiveness of Spotted Owls to conspecific calls when they were in the simulated presence (i.e., imitation of Barred Owl vocalizations) of a Barred Owl. We hypothesized that Spotted Owls would be less likely to respond to conspecific calls in areas where Barred Owls were common. We used a binary 2 × 2 crossover experimental design to examine male Spotted Owl responses at 10 territories randomly selected within two study areas that differed in abundance of Barred Owls. We also conducted a quasi experiment at four study areas using response data from any Spotted Owl (male or female) detected following exposure to Barred Owl calls. We inferred from the crossover experiment that the simulated presence of a Barred Owl might negatively affect Spotted Owl responsiveness. Both subspecies of Spotted Owl responded less to Spotted Owl calls after exposure to Barred Owl calls, Northern Spotted Owls responded less frequently in areas having higher numbers of Barred Owls, and California Spotted Owls responded less frequently than Northern Spotted Owls overall.

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Are Barred Owls Displacing Spotted Owls?

The Condor ◽

10.1093/condor/105.1.45 ◽

2003 ◽

Vol 105 (1) ◽

pp. 45-53 ◽

Cited By ~ 9

Author(s):

Elizabeth G. Kelly ◽

Eric D. Forsman ◽

Robert G. Anthony

Keyword(s):

Pacific Northwest ◽

Current Status ◽

Spotted Owl ◽

Strix Occidentalis ◽

Strix Varia ◽

The Pacific ◽

Spotted Owls ◽

Before And After ◽

Strix Occidentalis Caurina ◽

Barred Owls

Abstract Barred Owls (Strix varia) have expanded their range into the Pacific Northwest, and anecdotal evidence suggests that they may be displacing the federally threatened Northern Spotted Owl (Strix occidentalis caurina). Our objectives were to describe the current status of Barred Owls in Oregon and compare occupancy of Spotted Owls in historic Spotted Owl territories before and after Barred Owls were first detected in those territories. Between 1974 and 1998, we estimated that 706 different Barred Owl territories were located in Oregon. From 1989–1998 an average of 60 new Barred Owl territories were located in Oregon each year. In Spotted Owl demographic study areas in Oregon and Washington, Barred Owl detections increased at Spotted Owl territories from 1987–1999. Occupancy of Spotted Owl territories declined after Barred Owls were detected within 0.80 km of the territory center. When Barred Owls were detected 0.81–2.40 km from Spotted Owl territory centers, occupancy of Spotted Owls was only marginally less than at territories without Barred Owls. This suggests that the frequency and intensity of interactions between the two species is negatively associated with distance between them. Our results suggest that land managers and regulatory agencies should regard Barred Owls as a threat to Spotted Owls, particularly if Barred Owls continue to increase in number as they have during the past 25 years. ¿Está Strix varia Desplazando a Strix occidentalis caurina? Resumen. Desde su expansión hacia el Pacífico Noroeste, existe evidencia anecdótica de que Strix varia podría estar desplazando a S. occidentalis caurina. Nuestros objetivos fueron describir el estatus actual de S. varia en Oregon y comparar la ocurrencia de S. occidentalis caurina en sus territorios históricos antes y después de que S. varia se detectó por primera vez en dichos territorios. Entre 1974 y 1998, estimamos que se confirmaron 706 territorios diferentes de S. varia en Oregon. Entre 1989 y 1998, se localizaron en promedio 60 nuevos territorios de S. varia anualmente. En áreas con estudios demográficos de S. occidentalis caurina en Oregon y Washington, las detecciones de S. varia en territorios de S. occidentalis caurina se incrementaron entre 1987 y 1999. En comparación con territorios sin S. varia, la ocupación de territorios de S. occidentalis caurina disminuyó luego de que se detectaron individuos de S. varia a menos de 0.80 km del centro del territorio. Cuando se detectaron individuos de S. varia entre 0.81 y 2.40 km del centro de los territorios, la ocupación de éstos fue sólo marginalmente menor que en territorios sin S. varia. Esto sugiere que la frecuencia e intensidad de la interacción entre las dos especies está asociada con la distancia entre ellas. Nuestros resultados sugieren que las autoridades ambientales y de regulación deben considerar a S. varia como una amenaza para S. occidentalis caurina, particularmente si los números de S. varia se siguen incrementando como en los últimos 25 años.

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RECENT RECORDS OF HYBRIDIZATION BETWEEN BARRED OWLS (STRIX VARIA) AND NORTHERN SPOTTED OWLS (S. OCCIDENTALIS CAURINA)

The Auk ◽

10.1642/0004-8038(2004)121[0806:rrohbb]2.0.co;2 ◽

2004 ◽

Vol 121 (3) ◽

pp. 806 ◽

Cited By ~ 20

Author(s):

Elizabeth G. Kelly ◽

Eric D. Forsman

Keyword(s):

Strix Varia ◽

Spotted Owls ◽

Northern Spotted Owls ◽

Barred Owls

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Trichomonosis due to Trichomonas gallinae infection in barn owls (Tyto alba) and barred owls (Strix varia) from the eastern United States

Veterinary Parasitology Regional Studies and Reports ◽

10.1016/j.vprsr.2019.100281 ◽

2019 ◽

Vol 16 ◽

pp. 100281

Author(s):

Kevin D. Niedringhaus ◽

Holly J. Burchfield ◽

Elizabeth J. Elsmo ◽

Christopher A. Cleveland ◽

Heather Fenton ◽

...

Keyword(s):

United States ◽

Eastern United States ◽

Tyto Alba ◽

Trichomonas Gallinae ◽

Strix Varia ◽

Barn Owls ◽

Barred Owls

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Estimating Detection Probability and Density from Point-Count Surveys: A Combination of Distance and Double-Observer Sampling

The Auk ◽

10.1093/auk/123.3.735 ◽

2006 ◽

Vol 123 (3) ◽

pp. 735-752 ◽

Cited By ~ 8

Author(s):

Michelle L. Kissling ◽

Edward O. Garton

Keyword(s):

Detection Probability ◽

Survey Methods ◽

Effective Area ◽

Survey Method ◽

Density Estimates ◽

Point Count ◽

Detection Probabilities ◽

Fixed Radius ◽

Count Surveys ◽

Double Observer

Abstract Point counts are the method most commonly used to estimate abundance of birds, but they often fail to account properly for incomplete and variable detection probabilities. We developed a technique that combines distance and double-observer sampling to estimate detection probabilities and effective area surveyed. We applied this paired-observer, variable circular-plot (POVCP) technique to point-count surveys (n = 753) conducted in closed-canopy forests of southeast Alaska. Distance data were analyzed for each species to model a detection probability for each observer and calculate an estimate of density. We then multiplied each observer's density estimates by a correction factor to adjust for detection probabilities <1 at plot center. We compared analytical results from four survey methods: single-observer fixed-radius (50-m) plot; single-observer, variable circular-plot (SOVCP); double-observer fixed-radius (50-m) plot; and POVCP. We examined differences in detection probabilities at plot center, effective area surveyed, and densities for five bird species: Pacific-slope Flycatcher (Empidonax difficilis), Winter Wren (Troglodytes troglodytes), Golden-crowned Kinglet (Regulus satrapa), Hermit Thrush (Catharus guttatus), and Townsend's Warbler (Dendroica townsendi). Average detection probabilities for paired observers increased ≈8% (SE = 2.9) for all species once estimates were corrected for birds missed at plot center. Density estimators of fixed-radius survey methods were likely negatively biased, because the key assumption of perfect detection was not met. Density estimates generated using SOVCP and POVCP were similar, but standard errors were much lower for the POVCP survey method. We recommend using POVCP when study objectives require precise estimates of density. Failure to account for differences in detection probabilities and effective area surveyed results in biased population estimators and, therefore, faulty inferences about the population in question. Estimaciones de la Densidad y de las Probabilidades de Detección a Partir de Muestreos Utilizando Conteos en Puntos: Una Combinación de Muestreos de Distancia y de Doble Observador

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Influence of moonlight on nightjars’ vocal activity: a guideline for nightjar surveys in Europe

Biologia ◽

10.1515/biolog-2015-0099 ◽

2015 ◽

Vol 70 (7) ◽

Cited By ~ 3

Author(s):

Luís Reino ◽

Miguel Porto ◽

Joana Santana ◽

Tomasz S. Osiejuk

Keyword(s):

Survey Methods ◽

Mediterranean Area ◽

Lunar Cycle ◽

Early Summer ◽

Factors Affecting ◽

Moon Phases ◽

Vocal Behaviour ◽

Bird Survey ◽

Southern Portugal ◽

Vocal Activity

AbstractStrictly nocturnal or crepuscular habits on birds are restricted to a very limited number of species such as nightjars. Physiological skills of these species combined with ecological and behavioural adaptations provide them the ability to colonise a less competitive niche. Nightjars’ nocturnal ecological adaptations have been largely neglected on bird survey methods, which can bias the obtained results. Here, we studied the factors affecting vocal activity of two nightjar species, red-necked nightjar Caprimulgus ruficollis and European nightjar C. europaeus in a Mediterranean area of southern Portugal by modelling observed vocal activity of both species as a function of astronomic variables, particularly moon phase and day of the year. Red-necked nightjar vocal behaviour was positively associated with brighter moon phases and springtime, whereas European nightjar was only influenced by the season, being more active in the summer. This reveals the importance of light conditions on vocal activity of the former species. We suggest that specifically designed field surveys for these species should be preferentially conducted during brighter nights, when the lunar cycle is close to full moon. Furthermore, positive contacts with birds are maximized from late spring to early summer, at least for the red-necked nightjar.

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