Live blind snakes (Leptotyphlops dulcis) in eastern screech owl (Otus asio) nests: a novel commensalism

Summary Eastern screech owls bring live blind snakes to their nestlings, whereas all other prey are delivered dead. Some of the snakes are eaten but most live in nest debris, where they eat soft-bodied insect larvae from the decomposer community in fecal matter, pellets, and uneaten prey. Consumption of larvae may reduce larval parasitism on owl nestlings or larval competition with nestlings for food stored in the nest, because nestlings with live-in blind snakes grow faster and experience lower mortality than same-season broods lacking snakes. We propose a commensalistic association in which the screech owl benefits reproductively and the live-in blind snake is not affected.     

Missing piece of top predator-based conservation: Demographic analysis of an owl population on a remote subtropical island

Top predators are frequently the target of conservation programs. Owls are such predators. However, previous studies of owls are biased to species occurring in temperate regions, whereas most owl species occur in tropical or subtropical regions and are understudied. Furthermore, owls are often endemic to islands and of unknown conservation status. Demographic data for such species are especially scarce although they are essential for initiating and promoting their conservation. As a case study of demographic analysis of owls in a tropical or subtropical area and on islands, we applied an integrated population model to 7-year monitoring data (2012–2018) of the Ryukyu Scops Owl population on Minami-daito Island, Japan. We used survival history data from 903 individuals, reproduction and sex ratio data from 213 broods, and count data of 2,526 individuals in total. Long-term averages of annual survival rates were 0.73 for adult females and 0.74 for adult males, although the sexual difference was not significant. Sex ratio estimates fluctuated annually and long-term averages were slightly skewed to males: 0.51 among fledglings, 0.54 among yearlings, and 0.52 among adults. Long-term averages of population size were estimated to be 273.4 females and 296.8 males. The long-term average of population growth rate was 0.98, suggesting a slightly declining trend. It was fortunate to recognize the declining trend during the early phase. Considering the general lack of fundamental ecological data on owls of tropical or subtropical areas and on islands, it seems likely that many endangered owl populations await conservation efforts.     

The demography of an increasing insular Eurasian Scops Owl (Otus scops) population in western France

Rapid population declines of many long-distance Afro-Palaearctic migratory bird species are ongoing across Europe but the demographic drivers are often poorly understood, thereby limiting the development of appropriate conservation actions. Using long-term population monitoring (39 years), capture–mark–recapture data and a matrix model, we estimated demographic parameters and the effect of climate variables on adult survival, and modelled the dynamics of an increasing population of Eurasian Scops Owls Otus scops in a landscape with agricultural abandonment in western France. The observed mean annual population growth rate was 1.055 (from 68 to 523 territorial males between 1981 and 2019). Over the study period, clutch size and hatching success were stable, but fledging success and breeding success showed slight negative trends, probably due to density-dependence. Survival varied with age, with an increase during early life and evidence for rapid senescence from 4 years old. Adult survival remained stable and was positively linked to the amount of autumn rainfall in the Sahel and to the winter North Atlantic Oscillation. Survival of younger age-classes made the largest contribution to the variance of the population growth rate, followed by clutch size, fledging success and survival of older birds. Such a long-term population increase in a landscape where intensive agriculture has decreased by 64.6% sheds some new light on the causes of the decline of European Scops Owl and other Afro-Palaearctic bird populations. We infer some of the possible causes of this large-scale decline, in particular food shortage, and discuss conservation measures that could be applicable to reverse this trend.     

Preliminary estimates of the population density of the Sokoke scops owl Otus ireneae Ripley in the East Usambara lowlands, Tanzania

Preliminary population density estimates are presented for a recently discovered population of the Sokoke scops owl Otus ireneae in the lowlands of the East Usambara mountains, Tanzania. Calling birds were mapped at two sites totalling 6·3 km². Approximate densities were 3–4 territories/km² in Kwamgumi Forest Reserve and <1·5 territories/km² in Manga Forest Reserve, a much more heavily logged site. Densities in prime habitat in Arabuko‐Sokoke forest, Kenya, the only other locality for the species, exceed seven territories/km². The total population in the East Usambaras cannot be calculated, but it is probably markedly smaller than in Arabuko‐Sokoke. Suggestions for more precise surveys are made.     

Evaluating the existence and benefit of major histocompatibility complex‐based mate choice in an isolated owl population

How mate preferences evolve in the first place has been a major conundrum for sexual selection. Some hypotheses explaining this assume fitness benefit derived from subsequent generations. Major histocompatibility complex (MHC)‐based mate choice is a representative example of the mate choice that is associated with such trans‐generational mechanisms. To provide evidences for fitness benefit of MHC‐based mate choice, previous studies assessed the association between own MHC genotype and own fitness components. However, the association between MHC‐based mate choice in the parental generation and fitness components in the resultant offspring generation has only rarely been measured in wild populations. Focusing on the isolated population of the monogamous Ryukyu Scops Owl (Otus elegans interpositus) on Minami‐daito Island, Japan, we found evidence of MHC‐based mate choice. However, we found no evidence of MHC‐based mate choice increasing own reproductive success or offspring survival. This is a rare case study that directly examines the existence of the trans‐generational indirect benefit of MHC‐based mate choice for genetic compatibility from trans‐generational data in a wild bird population. By investigating the fitness benefits of mate choice, this study serves to facilitate our understanding of the evolution of MHC‐based mate choice.     

Probability of detection of Flammulated Owls using nocturnal broadcast surveys

ABSTRACT Broadcasts of conspecific vocalizations are used to increase detection rates on surveys of secretive bird species, yet the assessment of detectability necessary to fully interpret such survey data is frequently lacking. We used radiotelemetry to evaluate the probability of detection of 17 radio‐tagged male Flammulated Owls (Otus flammeolus) using conspecific broadcast surveys in Idaho during 2005 and 2006. Probability of detection among the 11 paired, five unpaired, and one unknown pairing status owls was 100% during the pair‐bonding and incubation periods of the breeding season, after which it declined to less than 15% during the postfledging period. Paired males showed a different pattern than unpaired males. Following hatching of eggs, detectability of paired males declined gradually over a 6‐week period, whereas detectability of unpaired males dropped sharply for a 4‐week period before increasing during the postfledging period. We suggest that surveys for Flammulated Owls be conducted during the pair‐bonding and incubation periods when high detectability permits stronger inference concerning the presence or absence of owls based on broadcast survey detections.     

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.     

Vicariance and dispersal in the differentiation of vocalization in the Ryukyu Scops Owl Otus elegans

The distribution of species and species diversity can be affected by vicariance or dispersal. To understand their role in shaping species distribution and population structure these two processes must be estimated within and among populations. I analysed large‐scale variation in the call structure of the Ryukyu Scops Owl Otus elegans. This owl is distributed over a 1200‐km range, and only inhabits islands. Within this range, I studied this species across 22 continental islands of the Ryukyu Archipelago and two oceanic islands. The study aimed to assess whether there is variation in the acoustic structure of Owl hoot calls within islands, among major groups of islands and across a large area comprising a major biogeographical barrier (the Kerama Gap). The acoustic structure of calls was homogeneous within islands and among major island‐groups. Acoustic differentiation, however, increased over longer geographical distances of up to about 1200 km. The acoustic structure of hoots of the Ryukyu Scops Owl populations was clearly divided into two groups, north and south of the Kerama Gap. It is suggested that the Kerama Gap acted as a biogeographical barrier and contributed to the differentiation between the two major island‐groups. It is likely that this difference developed during the fragmentation of a widespread ancestral population by vicariant isolating events. There was also evidence of an effect of dispersal on vocal differentiation in subspecies inhabiting the two oceanic islands.     

赛罕乌拉国家级自然保护区红角鸮(Otus scops)的繁殖生态观察

为了解红角鸮(Otus scops)的繁殖状态,2009年5～8月,在内蒙古赛罕乌拉国家级自然保护区,以直接观察法对红角鸮的繁殖生态进行了研究。结果表明,所观察的4个巢址全部是以柳树为主干的树洞巢,巢内铺垫物较少或无铺垫,巢距地面的平均高度为91.3cm,巢内径平均为13.5cm,巢外径平均为15.0cm,巢深平均32.0cm(n=4)。平均窝卵数为3.75,对其中一窝进行测量,4枚卵平均量度是28.9mm×24.6mm,重量9.5g。雏鸟孵出后,雌性亲鸟承担主要育雏任务,雄性亲鸟日间停歇于离巢不远的树上负责警戒工作,夜间与雌鸟共同哺育雏鸟。小型鼬科动物的捕食可能是干扰红角鸮繁殖成功率的主要因素。