Turnover of territorial Scops Owls Otus scops as estimated by spectrographic analyses of male hoots

In this study we assessed territory turnover of the European Scops Owl Otus scops, a migratory nocturnal raptor, using sonagrams of hoots recorded in the same territories in the 1993 and 1994 breeding seasons. Three structural parameters of the hoot (duration, inter-note interval and frequency) varied considerably, and a Discriminant Function Analysis classified >90% of individual calls correctly. In order to investigate yearly turnover and site-fidelity of male Scops Owls, the discriminant functions derived from the hoot sample recorded in 1993 were used to classify hoots of individuals recorded in the same territories in 1994. Apparently, turnover rate of territory owners was high and rapid, because 55–78% of territories censused in 1993 were defended by a different male in 1994. The fast decline of our Scops Owl population, probably due to winter mortality, could be responsible for such a high turnover of territorial owners.     

Pervasive low-frequency vocal modulation during territorial contests in Eurasian Scops Owls (Otus scops)

In several animal species, including birds, individuals are known to produce low-frequency vocalizations during aggressive interactions with conspecifics. In this study, I investigated territorial interactions between male Eurasian Scops Owls Otus scops that occupied territories in a densely packed area. The single-note hoot of the Scops Owl is generally thought to be highly repeatable; however, extensive recording of male–male interactions identified previously unrecognized variation in the structure of hoots. Male Scops Owls gave hoots at a frequency lower than usual when engaging in short-distance contests with neighbouring males. Within-subject analysis revealed that the caller’s average hoot frequency was positively correlated with the distance from its rival. During contests, males gradually reduced their hoot frequency as they approached one another, perhaps reflecting changes in the degree of escalation. Furthermore, there is evidence that male Scops Owls have voluntary control of their hoot frequency also on a very short time scale. Males gave deeper hoots immediately after the rival initiated countersinging, and returned to their usual frequency range at the end of the interaction. This study confirms in part the findings of other authors’ experimental work, where male owls adjusted their vocal frequency when challenged by an opponent. However, that study suggested that vocal frequency would encode information about the caller’s bodyweight. In contrast, the results of the present study cannot exclude the hypothesis that the hoot of the Scops Owl is a variable, conventional signal that reflects the willingness to escalate the conflict. The reliability of the signal could be maintained by the risk of retaliation by the opponent, usually located a few metres from the caller.     

The distribution and population of the Seychelles (Bare-legged) Scops Owl Otus insularis on Mahé: consequences for conservation

This study details the first baseline survey of the Critically Endangered Seychelles Scops Owl Otus insularis , currently only recorded from the 152-km² island of Mahé, the largest and highest island in the granitic Seychelles, western Indian Ocean. A survey was conducted in October 2000–March 2001 using playback of conspecific calls in random squares (500 m × 500 m) with midpoints being at least equal to 100 m asl. Playback was conducted in 159 of a possible 375 squares (42%). Territorial responses were detected in 60 of 159 (38%) squares, which represented an estimated 70 distinct territorial responses. Pairs responded in 51% and single birds responded in 49% of squares where a response was elicited. Scops Owls were more likely to be detected at higher altitudes, in mixed forest, and sympatrically with endemic frogs ( Tachycnemis seychellensis and Sooglossus spp.). Two models were used to predict population: one based exclusively on altitude resulted in a population estimate of 134–159 territories (268–318 individuals) within a 34-km² range, whereas a second incorporating altitude and vegetation, which excluded unsuitable habitat, resulted in an estimate of 125–142 territories (250–284 individuals) in a 31-km² range. We discuss the implications of our findings to the Owl's conservation status and management.     

RECENT BIOACOUSTIC PUBLICATIONS—from 1991 and a few from 1990 and 1989

ABSTRACT

The focus of this study was to determine whether individual vocal identification of Scops Owls Otus scops was possible and if there was a stability of the hoot-calls over a short time period in the same individuals. Spontaneous vocalizations of 13 owls were recorded in 2004 in Southern Tuscany, Italy. Visual analysis of spectrograms and quantitative multivariate analysis of six vocal features showed marked individual differences. In some owls a repertoire of two different hoot types was found. In 2005, 10 Scops owls were recorded three times in the same breeding season (2 hours and 10 days after the first session). Statistical analysis of data showed that 60% of owls did not change call features over time. However a slight but significant variability between successive vocal performances of the same owl was found in 40% of cases. This variability may decrease the recognition power by acoustic analysis. To overcome this obstacle I suggest a multi step qualitative/quantitative approach. A Difference Index (DI) was calculated to set a threshold between the slight intra-individual and the very high inter-individual variability. This method allowed the recognition of calls of each owl recorded over time in 2005.     

Playback survey trial for the Little Owl Athene noctua in the UK

Capsule: Playback is effective at improving detectability of Little Owls.

Aims: To trial a playback survey methodology for Little Owls in the UK to determine detectability and practicalities which could inform potential future national monitoring efforts.

Methods: Nocturnal playback surveys were carried out in an open landscape in the east of England in spring 2015. Twelve 2?×?2?km survey squares were each visited four times. Little Owl calls were broadcast from three locations within each square along a linear transect and spaced 500 m apart. The responses of owls of any species were recorded. Little Owl occupancy was later confirmed through existing nest monitoring efforts.

Results: Little Owls were detected responding to playback in all survey squares and were confirmed to be breeding in 8 of the 12. Detection and response rates were greater when combining data from multiple broadcast locations. The likelihood of Little Owls responding decreased with greater distance of broadcast location to known nest site, but no relationships were identified with time of evening or the presence of other owl species.

Conclusion: Playback methodology proved to be effective at increasing detectability of Little Owls and this study provides an estimated response rate to inform future efforts.     

Altitudinal segregation between Ural Owl Strix uralensis and Tawny Owl S. aluco: evidence for competitive exclusion in raptorial birds

Capsule The Owls were significantly segregated in space with the most important factor being altitude.

Aims To establish if the segregation between Ural and Tawny Owl on the level of habitat selection is due to different habitat requirements of the species or a consequence of competitive exclusion.

Methods Seven variables were recorded for habitat of Ural Owls, Tawny Owls that live in sympatry with Ural Owls and Tawny Owls that live in allopatry with Ural Owls. Data were gathered in five mountain areas covered with similar continuous montane forest inside and outside known Ural Owl distribution in Slovenia. Owl territories were surveyed in 2001 using playback method. Evidence for segregation was searched for using discriminant function analysis.

Results The altitudinal distribution of Tawny Owls sympatric to Ural Owls was restricted to low elevations with Ural Owls at high elevations. Where Ural Owls were absent, Tawny Owls widened the altitudinal part of their ecological niche to the mountaintop.

Conclusion Segregation between Tawny and Ural Owls is due to competitive exclusion, with the less competitive Tawny Owl being out-competed by the superior Ural Owl. The forests at foothills are influenced by human presence and therefore avoided by Ural Owls. In areas where both species live in sympatry, these areas act as refugia for Tawny Owls.     

Conservation of Scops Owl Otus scops in the Alps: relationships with grassland management, predation risk and wider biodiversity

The Scops Owl Otus scops is the most endangered and least studied owl in Europe, with widespread declines reported throughout Europe. The species is qualitatively associated with two threats: changes in agricultural practices and predation by Tawny Owls Strix aluco . We investigated these two threats to a population in the Alps, where land abandonment is causing widespread woodland expansion with unknown consequences. Predation risk and environmental indicators of the degree of agricultural change both predicted Scops Owl distribution. Furthermore, agricultural change also affected broader biodiversity, as estimated by the richness and diversity of bird and diurnal butterfly species. This resulted in a strong association between Scops Owl presence and wider biodiversity, which held at different spatial scales and justified a conservation focus on a single, threatened species. The persistence of Scops Owls in the Alps could be promoted through subsidies halting land abandonment and promoting extensive grassland management practices. Such interventions would have the additional advantage of yielding broader biodiversity benefits.     

Sex-specific territorial responses in Tawny Owls Strix aluco

A territorial intruder often poses more of a threat to the territory holder of the same sex as itself. As territorial aggression is associated with costs, aggression shown by birds towards territorial intruders of the opposite sex deserves investigation. This behaviour could arise due to the reproductive value of a mate or through mutualism between members of a pair. We investigated these hypotheses by presenting mated pairs of Tawny Owls Strix aluco with playback of male calls, female calls and a male and female duetting, and recording the number and intensity of responses by the male and female territory holders. Females responded significantly more often to female than to male calls. Males responded equally often to male and female playback. Males which had previously bred successfully with their mate were significantly more likely to respond to female playback in the spring, which suggested males were responding to female playback due to the reproductive value of their mate. There was no evidence of mutualism between members of a pair.     

Isolation of a recent Korean epizootic strain of Newcastle disease virus from Eurasian Scops Owls affected with severe diarrhea

Velogenic Newcastle disease virus (NDV) was recovered from two dead Eurasian Scops Owls (Otus scops) from a wildlife rescue center in Korea during 2005. Phylogenetic analysis based on the sequence of the partial fusion (F) protein revealed that the isolates had the highest level of homology to recent Korean NDV strains from poultry.     

Effects of the risk of competition and predation on large secondary cavity breeders

The effects of competition and risk of predation on secondary cavity breeders were examined between the 2008 and 2009 breeding seasons using an experimental design manipulating two nest entrance sizes (large entrances allowed Barn Owls (Tyto alba) to enter, while the small entrances excluded them). During the 2009 breeding season, the entrance sizes of nest boxes were exchanged, so that if during one year a nest box in a particular location had a small entrance, the second year it had a large entrance and vice versa. Barn Owls and Eurasian Kestrels (Falco tinnunculus) occupied 67.3 and 17.3%, respectively, of large entrance nest boxes. Significantly more Jackdaws (Corvus monedula), House Sparrows (Passer domesticus) and Scops Owls (Otus scops) bred in nest boxes with small than with large entrances. After nest box entrance sizes were exchanged, Barn Owls and smaller species did not breed in the same nest boxes with the new entrance size. Jackdaws probably did not breed in large entrance nest boxes due both to exploitation competition (Barn Owls and Eurasian Kestrels occupied the majority of large entrance nest boxes), and may also have avoided empty nest boxes because of the risk of interference competition; whereas smaller species may have also avoided large entrance nest boxes due to risk of predation.     

Food and habitats requirements of the Eurasian Scops Owl (Otus scops) in Switzerland revealed by very high-resolution multi-scale models

In Europe, agricultural practices have progressively evolved towards high productivity leading either to the intensification of productive and accessible areas or to the abandonment of less profitable sites. Both processes have led to the degradation of semi-natural habitats like extensive grasslands, threatening species such as the Eurasian Scops Owl Otus scops that rely on extensively managed agricultural landscapes. In this work, we aimed to assess the habitat preferences of the Scops Owl using habitat suitability models combined with a multi-scale approach. We generated a set of multi-scale predictors, considering both biotic and abiotic variables, built on two newly developed vegetation management and orthopteran abundance models. To select the variables to incorporate in a ‘best multi-scale model’, we chose the best spatial scale for each variable using univariate models and by calculating their relative importance through multi-model inference. Next, we built ensembles of small models (ESMs) at 10 different scales from 50 to 1000 m, and an additional model with each variable at its best scale (‘best multi-scale model’). The latter performed better than most of the other ESMs and allowed the creation of a high-resolution habitat suitability map for the species. Scops Owls showed a preference for dry sites with extensive and well-structured habitats with 30–40% bush cover, and relied strongly on semi-extensive grasslands covering at least 30% of the surface within 300 m of the territory centre and with high orthopteran availability near the centre (50-m radius), revealing a need for good foraging grounds near the nest. At a larger spatial scale within a radius of 1000 m, the habitat suitability of Scops Owls was negatively related to forest cover. The resulting ESM predictions provide valuable tools for conservation planning, highlighting sites in need of particular conservation efforts together with offering estimates of the percentage of habitat types and necessary prey abundance that could be used as targets in future management plans to ensure the persistence of the population.     

SOME NOTES ON THE SPOTTED FOREST THRUSH

Brown, C. J., Riekert, B. R. &; Morsbach R. J. 1987. The breeding biology of the African Scops owl. Ostrich 58: 58–64.

The incubation and nestling periods of two pairs of African Scops Owls Otus senegalensis breeding in nesting boxes in the Daan Viljoen Game Park near Windhoek were studied. The incubation and nesting periods were about 22 ± 2 days and 25–28 days respectively. Incubation and brooding was by the female. The male provided all the food during the incubation period, but by the end of the nestling period 30% of food was brought by the female. The growth of the nestlings, parental behaviour and foraging methods are describe2 Of 100 food items brought to the nests, 93% (by number) consisted of arthropods, 6% reptiles and 1% small mammals.     

Home range and movement patterns of Sokoke Scops Owl Otus ireneae

Virani, M. 2000. Home range and movement patterns of Sokoke Scops Owl Otus ireneae. Ostrich 71 (1 & 2): 139–142.

A radio-telemetric study of the endangered Sokoke Scops Owl Otus ireneae was conducted between July and December 1993. The aims were to determine the species' home range and movement patterns with respect to time and moon phases. Information gathered would then be used to formulate a rational conservation plan to ensure their survival. Six adult owls (from three adjacent pairs) were monitored during the study period. Pairs held exclusive home ranges with little (2.6%) or no overlap with their neighbours. Mean home range size (MCP) of individual owls was 8.8 ha while mean home range size of each pair was 11.0 ha. Peak owl activity took place immediately after dusk and prior to dawn. Periods of lulls occurred between 23h00 and 03h00. Movement per unit time was negatively correlated with distance from roost. The owls tended to forage further from their roosts on nights with decreased illumination from the moon. Maximum owl distances from roosts were attained between 00h00 and 02h00.     

Variable effect of playback of chickadee mobbing calls on detection probability of boreal forest birds

Modification of the point count survey method to include playback of songbird mobbing calls in an attempt to increase detection probabilities has met with mixed success. We compared detection probabilities for boreal forest songbirds using traditional point count methods and counts using broadcasts of the mobbing calls of Black‐capped Chickadees (Poecile atricapillus) in an attempt to increase detection probability. We conducted 594 point counts during the 2010 breeding season in Newfoundland, Canada. Each point count consisted of an 8‐min silent observation period followed by an 8‐min broadcast of Black‐capped Chickadee mobbing calls. Occupancy model results showed that response to playback broadcast varied across species, with detection probabilities higher for seven of 17 species during the silent portions of point counts and three species more likely to be detected during playback intervals. For all species, the number of visual detections increased during periods of playback and, averaged across species, individuals were >6 times more likely to be seen during the playback period than during the silent period. Differences in detection probability among observers were apparent during both silent and playback periods. We suggest that using playback of chickadee mobbing calls during point count surveys of common boreal forest songbird species may be most beneficial when visual detection is important. However, playback may also be useful for species‐specific surveys during periods when birds are less likely to be vocal or for studies of less common species with chronically low detection probabilities. A combined silent and playback approach could also be useful, although observer and species differences should be accounted for if comparing data across species or studies.     

Interspecific responses to distress calls in bats (Chiroptera: Vespertilionidae): a function for convergence in call design?

Distress calls were recorded from three sympatric species of pipistrelle bat (Pipistrellus nathusii, P. pipistrellus and P. pygmaeus) in England and Northern Ireland. At foraging sites, we conducted playback experiments, consisting of experimental distress call sequences from each species and control sequences of random noise and sound recorded with no bats present. We measured response by simultaneously recording ultrasound during playbacks and counting the echolocation pulses above a predetermined threshold which were then identified to species. All three species responded to each other's calls. The number of recorded echolocation pulses of all species increased eight-fold, on average, during the playback of distress call sequences compared with the playback of ultrasonic noise, and four-fold compared with the playback of silence. In a separate playback experiment, the number of echolocation pulses of P. pygmaeus increased 14-fold during the playback of distress calls of four endemic species of bat from Madagascar (Emballonura atrata, Myotis goudoti, Miniopterus majori and M. manavi) compared with the playback of silence. This increased response might have been caused by the high calling rates of the Malagasy species. Distress calls of P. nathusii, P. pipistrellus and P. pygmaeus were structurally convergent, consisting of a series of downward-sweeping, frequency-modulated elements of short duration and high intensity with a relatively strong harmonic content. Selection may favour convergence in the structure of distress calls among bat species, if attracting heterospecifics increases the chance of repelling predators by mobbing.     

Using dual-sex calls improves the playback census method for a nocturnal burrow-nesting seabird,the Manx Shearwater Puffinus puffinus

Capsule: Playback with dual-sex calls increases the response rate of Manx Shearwaters Puffinus puffinus by 40–60% and reduces its daily variability, which would improve the precision of population estimates.

Aims: To test whether playing male and female calls would elicit more responses to playback than male-only calls, reduce daily variability and the length of response rate calibration trials, and give more precise estimates. We also measured response times to playback and assessed the reliability of visual signs of occupancy at burrow entrances.

Methods: Responses to four playback call-types (male-only and three variants of dual-sex calls) were compared in repeated trials at two colonies (Ramsey and Rum, UK) during May–June 2014.

Results: Dual-sex calls gave higher response rates with lower variance than male-only calls, because females frequently replied to female calls but rarely to male calls. In simulated 3–5-day calibration trials, response rates and correction factors were up to 50% more precise with dual-sex calls. Visual signs of burrow occupancy were unreliable.

Conclusion: Playback for Manx Shearwaters should use a 25 seconds recording of male and female calls intermixed, with 10 seconds listening time for delayed responses. Census-specific calibration trials are essential for accurate estimates of daily response rates.     

Neighbor Call Amplitude Influences Aggressive Behavior and Intermale Spacing in Choruses of the Pacific Treefrog (Hyla regilla)

Male Pacific treefrogs (Hyla regilla) aggregate in choruses during the breeding season. Within these choruses frogs distribute themselves nonrandomly. This study tested the hypothesis that the amplitude of neighbors' calls serves as a proximate cue in regulating the spacing of males in choruses, and that this is mediated by the incidence of aggressive-encounter calls by resident males. The amplitude of neighbors' calls showed little interindividual variation. Advertisement calls were played to males at three amplitudes that spanned the range of neighbor-call amplitudes measured between pairs of frogs. At playback amplitudes corresponding to the minimum neighbor-call amplitudes observed in choruses, frogs gave predominantly advertisement calls and few aggressive-encounter calls in response. As the playback amplitude was increased, subjects progressively decreased the number of advertisement calls and increased the number of encounter calls that they produced in response. The total number of calls (advertisement + encounter) given in response did not vary with playback amplitude. Intruders were likely to move away when a resident male gave encounter calls. In this way neighbor-call amplitude regulates intermale spacing in choruses.     

Territoriality and the significance of calling in the Lanyu Scops Owl Otus elegans botelensis

Territories of Lanyu Scops Owls Otus elegans botelensis overlap in both breeding and non-breeding seasons. Results of radiotelemetry showed that neighbouring owls do not use the shared areas of their territories at the same time. Frequent countersinging apparently permitted individuals to avoid potentially costly encounters with neighbours. Non-territorial owls can forage and rest in occupied territories. Experiments using decoys and playbacks showed that intruding owls were tolerated within either core or peripheral territories in all seasons if they remained silent, while calling intruders almost always incited threats or attacks even in autumn. Tolerating silent owls that are not competitors for mates or for nest sites appears to be an energy-saving territorial strategy.     

Female House Wrens may Use a Low‐Amplitude Call as an Aggressive Signal

The majority of work on aggressive signaling has focused on male–male contests. Although females in many species compete over important resources, female signals are understudied. In house wrens (Troglodytes aedon), females compete with other females to protect nesting cavities and eggs. We suggest that a high‐pitched, low‐amplitude call, hereafter abbreviated as ‘HI’, may be an aggressive signal used by female house wrens. Using playback of simulated female intruders, we found that females used HI calls significantly more often during playback of female house wrens compared to playback of heterospecifics. Additionally, HI calls were given significantly more often in the minute preceding physical attack. In comparison, song rates did not predict future attacks. Finally, we present pilot data that suggest listening females may respond more aggressively to female playback containing HI calls compared to playback containing only songs. This suggests the reliability of HI calls could be maintained by a receiver retaliation rule. HI calls bear a striking resemblance to the low‐amplitude songs of many male songbirds, in terms of both acoustic structure and social context. This is one of the few reports of a putative low‐amplitude aggressive signal in a female songbird. However, the nature of female competition may make low‐amplitude signaling an underappreciated signaling form in female animals.     

The impact of vegetation characteristics and prey availability on breeding habitat use and diet of Little Owls Athene noctua in Central European farmland

Capsule Foraging habitats are determined by vegetation characteristics rather than the availability of insect prey.

Aims To determine the diet composition of Little Owls in relation to the availability of insects at foraging sites, and to elucidate the main factors determining the owls’ habitat choice.

Methods The feeding ecology of Little Owls was studied during the 2002 breeding season in the agricultural landscape of western Bohemia (Czech Republic), where its population is in decline. Diet composition was determined by pellet analysis. Insect availability was studied using pitfall traps in the two most important Little Owl foraging habitats. For both habitats, we assessed the main vegetative characteristics (average and maximum vegetation height, vegetation density).

Results Based on number, insects were the most dominant prey, followed by small mammals; based on weight, insects comprised only a minor part of the diet. Among insect prey, Carabidae beetles were the most abundant. The proportion of insect numbers was strongly positively correlated with advancing day of the season and negatively correlated with the proportion of vertebrates. Although the highest densities of Carabidae were found in cornfields Little Owls significantly preferred grassland habitats, probably because of the lower vegetation cover.

Conclusions The availability of short sward vegetation in grassland habitats during the breeding season may play a key role in the conservation of Little Owls in central European farmland.