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Breeding Performance, Apparent Survival, Nesting Location and Diet in a Local Population of the Tawny Owl Strix aluco in Central Lithuania Over the Long-Term
Abstract
In the present study, we used 37-year long dataset on Tawny Owls from the annual monitoring of nestboxes at a sample plot in Central Lithuania. We expected that Tawny Owls responded to changes in land use practices, stemming from a change in both political and economic system, which may affect prey abundance and composition, breeding performance and demography. To analyze temporal changes in monitored parameters, we divided the study period into three phases (1978-1989,1990-2001 and 2002-2014), corresponding to different socio-economic conditions. The number of nesting pairs of Tawny Owls decreased significantly in the last 13 years of the study, but the number of successful pairs fluctuated without any trend. The dutch size and number of nestlings varied without significant trends, but nesting success improved over the last 13 years. Annual apparent survival probability of the female Tawny Owls did not vary significantly over the study period (model averaged values between 0.71 and 0.73). Owls occupied nest boxes irrespective to the distance from the agricultural land during the first two study periods, but since early 2000s, owls tended to occupy nestboxes located deeper in the forest. Birds and small mammals were similarly important as prey items by biomass. Since the 1990s, the share of Microtus voles significantly decreased in the diet, while that of birds increased. In summary, changes in the diet, improved nesting success of the Tawny Owl and tendency of nesting in forest interior may indicate ongoing complex responses to the changes in environmental conditions.
... Finally, vole density affects owl behavior [46,47], migratory decisions [32], and life history via the carry-over effect (events that occur in one season but influence individual success in the following season) [48]. Most of studies on the subject have been conducted in cyclic environments of Fennoscandia, and there have been very few studies conducted in the Baltic states (see [28,36] and references therein). We are not aware of other studies focusing on the boreonemoral region during prolonged periods of depleted small mammal population dynamics. ...
... The average FNB value was slightly lower than in Lithuania [28]. We observed a temporal increase in FNB, which was similar to the observation in Lithuania, with a declining proportion of Microtus voles [36]. This species is known to be a generalist [152]. ...
... This species is known to be a generalist [152]. Its food composition can considerably vary between breeding regions within the same year and between years in the same breeding territory [28,36,[152][153][154][155][156][157][158][159][160]. Nevertheless, in the cyclic environment of Fennoscandia, a strong numerical response to vole abundance has been reported, including the timing of breeding [6,160], breeding performance [3,5,6,39,160,161], and winter survival [3,39]. ...
Strong numerical and functional responses of owls to voles in cyclic environments are well known. However, there is insufficient knowledge from the boreonemoral region in particular, with depleted populations of small mammals. In this study, we describe the dynamics of the small mammal population in Latvia from 1991 to 2016 and link them to owl population characteristics. We used food niche breadth, number of fledglings, and population trends to lay out the numerical response of six owl species to dampened small mammal population cycles. We found temporarily increasing food niche breadth in tawny and Ural owls. There were no other responses in the tawny owl, whereas the breeding performance of three forest specialist species—pygmy, Tengmalm’s, and Ural owls—corresponded to the vole crash years in Fennoscandia. Moreover, the populations of forest specialist owls decreased, and the change in the Ural owl population can be attributed to the depletion of small mammal populations. We found evidence of a carry-over effect in the eagle owl arising from a strong correlation of declining breeding performance with the small mammal abundance indices in the previous autumn. We conclude that dampening of the small mammal population cycles is an important covariate of the likely effects of habitat destruction that needs to be investigated further, with stronger responses in more specialized (to prey or habitat) species.
... The average FNB value was slightly lower than in Lithuania [28]. We observed a temporal increase in FNB, that is similar to observation in Lithuania with a declining proportion of Microtus voles [36]. Species is known to be a generalist feeding on the available prey with no particular preferences [154]. ...
... Species is known to be a generalist feeding on the available prey with no particular preferences [154]. The food composition can highly vary between breeding regions within the same year and between years in the same breeding territory [28,36,[154][155][156][157][158][159][160][161][162]. Nevertheless, in the cyclic environment of Fennoscandia a strong numerical response to vole abundance was found, including the timing of breeding [6,162], breeding performance ( [3,5,6,39,162] and winter survival [3,39]. ...
... (μ±SD; n=9668) in Finland, where the population is also stable [98]. And lower than in Lithuania, where an increasing trend of breeding performance (2002-2014) was observed and co-occured with decline in number of breeding pairs [36]. We consider the relatively low breeding performance in Latvia related to the high population density -estimated around 16 604 in Latvia and below 4000 in Lithuania [115]. ...
Strong numerical and functional response of owls to voles in cyclic environments is well known, but there is insufficient knowledge from boreonemoral region, in particular, with depleted populations of the small mammals. In this study we describe the dynamics of the small mammal population in Latvia from 1991 to 2016 and link them to owl population characteristics. We used food niche breadth, number of fledglings and population trends to describe the numerical response of six owl species to dampened small mammal population cycles. We found temporarily in-creasing food niche breadth in tawny and Ural owls. There were no other responses in tawny owl, whereas the breeding performance of three forest specialist species – pygmy, Tengmalm`s and Ural owls – were similar to vole crash years in Fennoscandia. Moreover, the populations of forest specialist owls are decreasing and the change in Ural owl can be attributed to the depletion of small mammal populations. We found evidence of carry-over effect in eagle owl arising from strong correlation of declining breeding performance with the small mammal abundance indices in previous autumn. We conclude that dampening of the small mammal population cycles is an important covariate to overwhelming impacts of habitat destruction with stronger response in more specialized (to prey or habitat) species.
... Moreover, the local distribution pattern of occupied territories holds for many years, despite the turnover of individual birds (Sunde & Bølstad 2004). In a previous study, we found that the number of nesting pairs fluctuated over several decades, with a marked decrease between 2003 and 2008 (Grašytė et al. 2016). In the same study, we also found some support for a longterm shift in diet during the breeding season, preference for nesting in the forest interior and improved breeding performance over time. ...
... Occupied nest boxes were checked later between April and May to record breeding performance. More detailed descriptions of the study area and field procedures are provided elsewhere (Grašytė et al. 2016). From 1995 to 2014, 30-49 (median 36) nest boxes were available for occupation, and a median of five pairs of Tawny Owls attempted to breed annually. ...
... Unfortunately, no data concerning food abundance were available for our study site. Considering the widely demonstrated numerical response of raptors to abundance fluctuations in their main prey (Newton 2003 and references therein, Sundell et al. 2004, Solonen 2005, however, we used the number of nesting pairs at our study site (Grašytė et al. 2016) as a proxy index for prey availability. We used the median number of nesting pairs during the period 1995-2014 as a threshold value for classifying years as good (≥5 nesting pairs; 11 years; mean annual number of nesting pairs -6.2) or poor (<5 nesting pairs; 9 years; mean annual number of nesting pairs -2.2). ...
Capsule: Tawny Owls Strix aluco occupying nest boxes preferred habitats which were positively associated with the probability of nesting success.
Aims: We aimed to determine whether or not: (1) Tawny Owls showed habitat preferences when occupying nest boxes; (2) nesting performance was related to the habitats around occupied nest boxes and (3) habitat availability had changed around available and occupied nest boxes between 1995–2004 and 2005–14.
Methods: Tawny Owls were studied using nest boxes erected in a commercial forest. During nest boxes checks (724 cases), data on occupancy and nesting performance (88 cases) were recorded, and habitat within a 0.4 km radius around nest boxes was analysed.
Results: Tawny Owls had preferences for clearings within forests, mature forests and grasslands but avoided young forests. We found a positive relationship between nesting success and abundance of clearings within the forest, and a negative relationship between nesting success and abundance of young forests. A change in habitat preferences over the two decades was evident, but habitat availabilities remained similar.
Conclusions: Findings indicate adaptive habitat selection in Tawny Owls because preferred habitats were associated with higher fitness and this type of habitat became more frequently selected over time.
http://www.tandfonline.com/eprint/TZb4acqDmpIiK2rewEU2/full
... On the other hand, use of nest-boxes can be a very cost-efficient method for obtaining data on various other breeding productivity measures, including clutch size (Southern, 1970;Saurola and Francis, 2004). In a number of European countries nest-box monitoring is already established: Norway (Overskaug et al., 1999), Sweden (Ericsson et al., 2014), Finland (Saurola, 2012), Estonia (Nellis, 2012), Latvia (Reihmanis, 2012), Lithuania (Grašytė et al., 2016), Ukraine (Yatsiuk, 2010), Poland , Czech Republic (Luka and Riegert, 2018), Slovakia (Karaska, 2007), Hungary (Sasvári and Hegyi, 1998), Slovenia , Italy (Sacchi et al., 2004), Switzerland (Roulin et al., 2011), Germany (Mammen et al., 2017), France (Baudvin and Jouaire, 2003), Denmark (Jensen et al., 2012), and United Kingdom (Petty et al., 1994). While monitoring studies based on nest-boxes may not be wholly representative of local populations, they enable reasonable comparisons of population contextual parameters between countries and regions of Europe. ...
... It reflects main prey population fluctuations (Gryz and Krauze-Gryz, 2016;Luka and Riegert, 2018), therefore seasonal and annual differences in exposure to contaminants can be expected Christensen et al., 2012). In terms of population contextual data explaining contaminant levels in the tissues of Tawny Owls, it is important to differentiate seasonal/annual shifts in diet composition changes (Kirk, 1992;Jedrzejewski et al., 1994) from long-term dietary shifts as a response to environmental changes in populations of prey species (Grašytė et al., 2016). ...
Top predators are often used as sentinel species in contaminant monitoring due to their exposure and vulnerability to persistent, bioaccumulative and, in some cases, biomagnificable contaminants. Some of their ecological traits can vary in space and time, and are known to influence the contamination levels and therefore information on ecological traits should be used as contextual data for correct interpretation of large-scale contaminant spatial patterns. These traits can explain spatiotemporal variation in contaminant exposure (traits such as diet and dispersal distances) or contaminant impacts (traits such as population trend and clutch size). The aim of our research was to review the spatial variation in selected contextual parameters in the Tawny Owl (Strix aluco), a species identified by the COST Action European Raptor Biomonitoring Facility as one of the most suitable candidates for pan-European biomonitoring. A considerable variation in availability of published and unpublished contextual data across Europe was found, with diet being the most extensively studied trait. We demonstrate that the Tawny Owl is a suitable biomonitor at local scale but also that taking spatial variation of other contextual data (e.g. diet) into account is necessary. We found spatial gaps in knowledge about the species ecology and biology in Southern Europe, along with gaps in certain population parameters (e.g. population trends) in several countries. Based on our findings, we proposed a minimal recommended scheme for monitoring of population contextual data as one of the first steps towards a pan-European monitoring scheme using the Tawny Owl.
... Habitat preferences may vary between populations of the same species that breed in different geographical areas (Väli et al. 2004) or among different landscape types within a compact region (Skuja et al. 2019). There may even be shifts in habitat preferences in an expanding (Bai et al. 2009;Treinys et al. 2016) or stable (Grašytė et al., 2016;Rumbutis et al. 2017) population. ...
... Habitat preferences may vary between populations of the same species that breed in different geographical areas (Väli et al. 2004) or among different landscape types within a compact region (Skuja et al. 2019). There may even be shifts in habitat preferences in an expanding (Bai et al. 2009;Treinys et al. 2016) or stable (Grašytė et al., 2016;Rumbutis et al. 2017) population. ...
The objectives of this thesis were to analyse the breeding population trends and distribution of the Common Cranes in Estonia and the availability of potential breeding habitat (I, IV), to describe nesting habitats in Estonia and to explore the relationships between habitat and nesting success and nesting phenology (II), to investigate the relation between the numbers and distribution of autumn staging cranes and agricultural land use (III), to examine the stability of the habitat network used by cranes (IV) and to study the long-distance migration pattern of cranes (V). The population of the Common Crane started to increase in Estonia since 1970s (I). In parallel, the mean density of pairs increased, and distribution widened all over the country (I). First time breeders started to occupy new suitable habitats for nesting (I). The extended distribution of the crane has been favoured by the good availability of potential nesting sites (IV). The Common Crane nesting sites are different wetlands in Estonia have similar water regimes, plant communities and microrelief. The preferred breeding habitats are mires (II). The breeding success is associated with quality of breeding habitat and distance between neighbouring nests. The beginning of egg laying has a significant trend of advancement over time. Human activity had a significantly negative effect on the breeding success of cranes (II). The Common Crane numbers in staging sites are positively correlated with cropping area of cereals and negatively with the extent of potato fields. Changes in the local numbers and distribution of cranes when staging during their migration depend on changes in agricultural land use in staging areas (III). Migration routes used by cranes form a widespread ecological network (IV). Despite the modest contribution of nature conservation to the population increase of cranes (IV), the conservation efforts to protect peatlands play key roles in conservation of the Common Crane in the longer term. The climate change scenario modelling suggests minimal impact on cranes in future (IV). Common Crane’s long-distance autumn migration strategies differed between northern and southern sub-populations by the density and location of stopovers, daily flight distances, and the total migration duration. Cranes used during migration both, time- and energy-minimization strategies (V).
... Significant decrease in the Common Buzzard abundance due to reduction of hedgerows, woodlots and grasslands areas, as well as with the decrease in prey abundance was indicated in Western France (Butet et al. 2010). Recently, it was discovered that in forests of central Lithuania, Tawny Owls Strix aluco moved to nest deeper in the forest interior of since the 2000s, which was considered as a response to ongoing changes in agricultural areas (Grašytė et al. 2016). Notably, changes in the nest sites preferences could be expected in any raptor species because of ongoing habitat changes either in forests, agricultural areas or in both of them. ...
Environmental changes are expected in Europe due to ongoing timber harvesting in forests and changes in agriculture practices in cultivated areas. This study aimed to determine whether the nest site characteristics of the Common Buzzard Buteo buteo – a generalist raptor – have changed over time due to ongoing changes in forests and agricultural areas that are highly important for its breeding. A comparison of Common Buzzard nest sites occupied in 2002–2004 with nest sites occupied in 2017–2018 in commercially managed forests indicated certain changes. Common Buzzards preferred to nest in more mature stands with the higher proportion of deciduous trees in composition of the first tree layer. The location of stands in regard to agricultural areas did not shape habitat choice. The oak was most important nests tree. The nest sites of the Common Buzzard remained similar in terms of location within the landscape, however, age of stands used for nest significantly increased. In summary, these results suggest that Common Buzzard nest site selection pattern was driven by stand level decisions, but were not shaped by the landscape features. These findings indicate that behavioural plasticity typically assumed for this ubiquitous raptor may not necessarily act at the all levels of nest site selection process, which may further indicate species potential sensitivity to the changes in forest utilisation intensity. Keywords: habitat selection, raptor, forestry impact
... For example, habitat preferences may vary between populations of the same species that breed in different geographical areas (Väli et al., 2004) or among different landscape types within a compact region (Skuja et al., in press). There may even be shifts in habitat preferences in an expanding (Bai et al., 2009;Treinys et al., 2016) or stable (Grašytė et al., 2016;Rumbutis et al., 2017) population. Thus, rigorous ecological knowledge on species nesting ecology is required to ensure cost-effective conservation. ...
Forest-dwelling raptors nest in the same close, mature forest stands over many years. As mature stands are targets for timber harvesting, the conservation of nest sites should be integrated into commercial forestry practices. Ecological data supporting conservation decisions are essential for ensuring effective conservation, and minimising costs and conflicts among different stakeholders. Here, we analysed nest site selection and nest site turnover patterns in a typical mature-forest-dwelling raptor in the core area of its global population. Our aim
was to provide a basis for nesting habitat conservation in the context of commercial forestry. The lesser spotted eagle Clanga pomarina was found to prefer mature stands, located close to the forest’s edge, for nesting. Pine stands were largely avoided by nesting pairs, but the composition of other tree species was similar to stands located in surrounding forests. The lesser spotted eagle occupied the nest for an average of three years, and the number of used nests within a territory increased progressively with the longevity of its occupation. Within a
territory, the pair moved between alternate nests mostly up to 600 m. The results of this study suggest that longterm conservation approaches for mature-forest-dwelling raptors should use breeding territory, which contains several nest sites (or suitable stands) spaced at certain distances and covered by temporal buffer, as a target unit
in conservation-supporting forestry practices.
... More frequently it is conducted by examination of installed artificial nest-boxes, which in most circumstances have low influence on overall breeding density in the Tawny Owl due to its high adaptability in selection of breeding sites (Southern 1970, Petty et al. 1994, Avotinš 2004). In addition to assessment of breeding success, nest-boxes provide an opportunity for monitoring other ecological aspects such as diet, survival, colour variation, as well as prevalence of diseases, parasites and environmental contaminants (Appleby et al. 1999a, Ahrens et al. 2011, Grašytė et al. 2016, 2017, Saurola & Francis 2018. On the other hand, nest monitoring is labour-intensive when conducted at a large scale, especially in remote and inaccessible areas. ...
Capsule: Territory monitoring using playback of calls is a reliable approach for assessing population trends of Tawny Owls Strix aluco, particularly when human resources are limited or survey areas are difficult to access.
Aims: To explore whether response calls of Tawny Owls towards broadcast conspecific and heterospecific male owl playback calls would provide similar estimates of population size and trends over time as nest-box monitoring.
Methods: Between 1998 and 2017, Tawny Owls were monitored in a predominantly forested area of central Slovenia. Throughout the year, territories were monitored using a playback protocol comprising silent listening during five minutes before and after ten minutes of broadcasting male Tawny Owl. Seasonal variation in response rate was examined and results from the playback method were compared to data on occupancy rate of nest-boxes.
Results: Territory monitoring using playback calls showed a similar direction of population trends as nest-box monitoring but a different population dynamics pattern. The overall response rate in occupied territories to conspecific playback calls at first visits was 70%. This was significantly higher than for heterospecific playback calls and the frequency of spontaneous vocalizations. The response rate to conspecific playback calls when including two visits rose to nearly 90%. There was no difference in response rate between seasons. The average time to respond to conspecific playback calls was five minutes.
Conclusions: Compared to nest-box monitoring of breeding pairs, territory monitoring of breeding and non-breeding Tawny Owls using playback provides a robust and cost-effective method for monitoring. We recommend conducting territory monitoring between January and May during the breeding season, with two visits to each site using conspecific playback of territorial male hoot calls using the 5 + 10 + 5 minutes protocol.
The structure and diversity of small mammal (SM) communities over the long term may show the influences of climate change, landscape changes and local disturbances. We review published data regarding SM trapping and owl pellet analysis from Lithuania (the most southerly of the three Baltic States, Northern Europe), covering the period 1975–2021. Over decades, we analysed trends in the diversity of SM communities and the proportions of species and proportions of trophic groups. The large increase in granivores, from 6.9% in 1975–1980 to 45.4% in 2011–2020 and 54.7% in 2021, coincided with a decrease in omnivores and insectivores. The proportion of herbivores increased less notably. At the species level, significant decreases in the proportions of M. arvalis, C. glareolus and S. araneus were accompanied by notable increases in the proportions of A. flavicollis and A. agrarius, the latter from 1.0% in 1975–1980 to 25.3% in 2021. Concluding, two periods were identified, specifically before the 1990s and subsequently. In the second period, in the aftermath of land-use changes within the country in 1990, diversity increased, and dominance decreased, a situation that has not subsequently changed. Not excluding the concomitant effects of climate change, we relate these patterns to the alterations in habitat and anthropogenic impact.
The demand for timber, bioenergy feedstock and other forest products, leading to intensified forest harvesting, is expected to increase in the coming decades in European Union. A reduction in the delivery of forest ecosystem services and, specifically, biodiversity, including the provision of habitats for mature-forest-dwelling raptors, has been attributed to the intensification of forest exploitation. Therefore, in order to adopt adequate conservation measures to create a timely buffer against the consequences of increased harvesting, it is critically important to understand how the availability of nesting habitats for protected species will fluctuate in the future landscape. In this study, using the LandSim tool, we modelled the dynamics of the forests and nesting habitat availability for the forest-nesting raptor, the lesser spotted eagle Clanga pomarina, for the next 50 years in eastern Lithuania, Central Europe. Our findings indicate that the share of forests available for final harvesting is expected to increase rapidly in the coming decades due to a large amount of forest stands reaching a mature age, if current forest management practices, despite them being considered as relatively conservative, are continued. As a consequence, the availability of nesting habitats will constantly decrease in nesting territories, as well as elsewhere in the landscape, in the coming decades. We suggest that species conservation strategies should not only incorporate directly targeted measures to protect nest sites from destruction and disturbance, but also, at the very least, preserve a sufficient amount of nesting habitats in areas inhabited by eagle pairs and, at best, at the landscape scale.
Our aim was to determine dynamics in a population of tawny owls Strix aluco over 15-year period, in relation to year-to-year variation in environmental conditions. The research was carried out in a habitat mosaic of fields and forest in central Poland, over the 2004−2018 period. Numbers of pairs (territories) were established by the standard playback survey technique supplemented by searches for nest sites. The selected environmental factors studied in parallel were the acorn production, density changes in field and forest rodents, meteorological conditions in winter and density of martens (Martes spp.). At the start of the study period 20 nest boxes designed for tawny owls were placed out in the study area. This number was enlarged by additional 27 nest boxes placed in 2012. The number of owls in the area remained stable-in the range of 26−29 pairs, despite changes in nesting sites availability. However, moderate influence of rodent density and winter conditions on population abundance was detected. Also, peak in the rodent population coincided with greater clutch size and numbers of young owls reared. Densities of martens remained relatively stable throughout the study period, and there were no reported cases of these carnivores killing tawny owls, despite the former taking shelter in the owl-boxes. ARTICLE INFO Regular research paper Pol.
key words tawny owls Strix aluco onset of laying eggs number of fledglings urban heat island artificial light prey availability ABSTRACT An increasing urbanization of habitats in which birds live influences their populations by modifying behaviour, dietary composition , causes of mortality and variables related to breeding. This study aimed to compare breeding periods and productivity among tawny owls, Strix aluco, inhabiting either an urban or an extra-urban area. Studies were conducted in parallel in two locations: central zone of Warsaw-the Polish capital-and the Rogów Forest-ca 70 km west of Warsaw. Number of fledglings and the beginning of the breeding period was assessed by direct observations and listening to calls of juveniles within tawny owl territories. Birds in the urban area were found to start breeding significantly earlier (from 5 th February) than those in the extra-urban area (from 17 th March). However, the two populations proved to be similarly productive, with 3.2 or 3.4 young per nest on average. The phenomenon of early breeding in city dwelling birds was confirmed in other tawny owl populations as well as other bird species. It can be potentially driven by factors such as higher temperature, artificial light and rich and stable food base.
Capsule In the areas studied, Tawny Owls showed extreme flexibility and opportunism, occupying most available vegetation types and elevations below the tree-line. Aims To assess habitat relationships for the species so as to forecast the potential impact of habitat changes. Methods We censused a total of 321 territories in 12 study areas covering the regional variation in elevation and the associated heterogeneity in vegetation types and landscape configurations. Results The species occupied virtually all vegetation types and landscape configurations below the tree-line. Nests were mostly located on Cliffs, but also in tree cavities, buildings, and stick nests of other raptors. Density varied from 12 to 60 territories/100 km(2) and mean nearest-neighbour distance from 0.7 to 1.8 km, depending on the availability of broadleaved woodland. Compared to availability, nests were nearer to cliffs and had higher richness of habitat edges and vegetation types, which positively affected chick production. Diet composition varied widely according to habitat and was mostly dominated by Edible Dormouse Glis glis, voles and mice. Conclusion The conservation status of the species was satisfactory. Future habitat changes may favour an increase in density. Silvicultural practices encouraging more old trees with large cavities would further favour this opportunistic species.
Since Viral Haemorrhagic Pneumonia (VHP) reached the study area (Upper Ebro Valley, Spain) in 1989, thus reducing the wild Oryctolagus cuniculus population, a 15% decline in the proportion of rabbit in the diet and a 50% decrease in breeding success of golden eagles was noticed. The breeding parameter most affected by the shortage of winter food was the proportion of pairs laying eggs. The number of fledglings per successful pair did not change, probably because of the increased accessibility of sick rabbits in spring (the season when VHP virus is most virulent). -from Author
We studied population size and productivity of White Storks Ciconia ciconia in 1983-2000 in 417 km2 of the Obra river valley in western Poland and its response to Common Vole Microtus arvalis density. The 33 to 60 nesting pairs fluctuated in close accordance with vole density. Similarly, the total number of fledglings produced in the population was strongly correlated with vole density. We suggest that voles are important prey because of their high calorific value. Storks arriving from their Africa wintering quarters probably evaluate the study area with respect to available prey (mainly voles) and subsequently decide whether to breed in the Obra river valley.
Tawny Owl diet was studied in the primeval deciduous forests of Białowieża National Park (eastern Poland), where the Tawny Owl was the most numerous predator and the forest rodents were non-cyclic with recurrent waves of outbreaks-crashes (triggered by heavy seed crops at 6-9-year intervals). The study (1985-1992) covered both noncyclic and outbreak-crash years. Analysis of 1,116 pellets showed that Tawny Owls were specialised on forest rodents, mainly the yellow-necked mouse Apodemus flavicollis, but that they also exploited secondary resources: birds (especially Turdus, Parus and Carduelis), insectivores, and frogs. Between-year changes in the Tawny Owls' diet were mainly influenced by the abundance of yellow-necked mice, which thus determined the proportion of bank voles Clethrionomys glareolus taken and of alternative prey such as birds, amphibians, and shrews. Snow cover decreased the efficiency with which owls hunted for bank voles but not for mice and shrews. The alternative prey hypothesis, proposed to explain the linked cyclicity of microtines, shrews, and small game in northern Europe, may also be applied to animal population fluctuations in the temperate oak-dominated forests of Europe. During rodent crash years, predation by Tawny Owls may heavily affect the population dynamics of other small animals.
A countrywide White Stork (Ciconia ciconia) nest census was carried out in Lithuania in 2009 and 2010 by over 20 observers using standardized equipment and methods. The census revealed that the White Stork population has doubled since the previous census in 1994, possibly due to favourable changes in farming practices, with birds increasingly settling on the overhead electricity line poles and gradually abandoning nests in trees. In total, 21,192 White Stork nests were recorded, of which 18,782 (81.4%) were occupied by
breeding birds, compared to 9400 occupied nests recorded in 1994. The mean density of occupied nests in the country was 29 nests/100 km2. Electricity line poles were the most common nest sites (49% of all nests), followed by trees (21%) and specially erected poles with nesting platforms (11%). Mean breeding success of White Storks was rather high, with 2.7 young raised per successful pair, suggesting a favourable conservation status for this species in Lithuania.
I examined the contents of Tawny Owl (Strix aluco sylvatica) pellets, between April 1977 and February 1978, in mixed woodland and gardens in northeast Suffolk, England. Six mammal, 14
bird and 5 invertebrate species were recorded in a sample of 105 pellets. Overall, the Wood Mouse (Apodemusylvaticus) was the most frequently taken mammal prey and the House Sparrow (Passer
domesticus) was the most frequently identified bird prey. Two types of seasonal diet change were found; first, a shift from mammal prey in winter to bird prey in the breeding season, and second, a shift from
small prey in the winter to medium-sized (> 30 g) prey in the breeding season. Contrary to some findings elsewhere in England, birds, rather than mammals, contributed significantly to Tawny Owl diet during
the breeding season.
Maximum likelihood or restricted maximum likelihood (REML) estimates of the
parameters in linear mixed-effects models can be determined using the lmer
function in the lme4 package for R. As for most model-fitting functions in R,
the model is described in an lmer call by a formula, in this case including
both fixed- and random-effects terms. The formula and data together determine a
numerical representation of the model from which the profiled deviance or the
profiled REML criterion can be evaluated as a function of some of the model
parameters. The appropriate criterion is optimized, using one of the
constrained optimization functions in R, to provide the parameter estimates. We
describe the structure of the model, the steps in evaluating the profiled
deviance or REML criterion, and the structure of classes or types that
represents such a model. Sufficient detail is included to allow specialization
of these structures by users who wish to write functions to fit specialized
linear mixed models, such as models incorporating pedigrees or smoothing
splines, that are not easily expressible in the formula language used by lmer.
Understanding how survival is affected by the environment is essential to gain insight into population dynamics and the evolution of life‐history traits as well as to identify environmental selection pressures. However, we still have little understanding of the relative effect of different environmental factors and their interactions on demographic traits and population dynamics. Here we used two long‐term, individual‐based datasets on Tawny Owl Strix aluco (1981–2010) and Ural Owl S. uralensis (1986–2010) to undertake capture‐mark‐recapture analysis of annual survival of adult females in response to three biologically meaningful environmental variables and their two‐way interactions. Despite the similar ecology of these two species, their survival was associated with different and uncorrelated environmental drivers. The main correlate of Tawny Owl survival was an inverse association with snow depth (winter severity). For Ural Owl, high food (vole) abundance improved survival during years with deep snow, but was less important during years with little snow. In addition, Ural Owl survival was strongly density‐dependent, whereas Tawny Owl survival was not. Our findings advise caution in extrapolating demographic inferences from one species to another, even when they are very closely related and ecologically similar. Analyses including only one or few potential environmental drivers of a species' survival may lead to incomplete conclusions because survival may be affected by several factors and their interactions.
Despite the key role of raptors (including birds of prey Falconiformes and owls Strigiformes) in ecosystems and their sensitivity to environmental change, a well coordinated, Europe-wide monitoring of raptors is lacking. EURAPMON, a Research Networking Programme of the European Science Foundation, was launched with the aim of establishing a sustainable Europewide network for monitoring of raptors. An overview of current monitoring schemes for raptor populations in 28 European countries, as reported by EURAPMON National Coordinators at the workshop in Murcia (Spain) in 2012, showed existing monitoring schemes to be limited to a restricted number of species (mostly diurnal and rare raptor species). The most widely monitored species are the Golden Eagle Aquila chrysaetos amongst diurnal raptors and the Eagle Owl Bubo bubo amongst owls. Broad coverage of a species range across Europe is reached only for restricted-range species. The key driver for monitoring, which is mostly coordinated by NGOs, is conservation, and the main end users are governmental institutions. International collaboration in the field of monitoring of raptors is mainly regional and not yet pan-European in scale. The involvement of volunteers in raptor monitoring was perceived as the main strength of many schemes, but insufficient manpower and a focus on rare species were recognised as the main weaknesses across Europe as a whole. Among priorities identified for the future development of monitoring schemes are: improvements to national coordination; support to increase the number of volunteers; and assurances of stable funding. Further analysis of EURAPMON questionnaires will identify knowledge gaps, which will steer good practice guidance on survey methodologies; the need for the latter was identified as the main benefit that National Coordinators expect to gain from international networking
The alternative prey hypothesis (APH) states that synchronous population fluctuations of small game are caused by varying predation impact. Voles that show 3-4 yr population cycles in C and N Fennoscandia, are the staple food of several predators. APH predicts that these predators partly switch their diet from voles to small game as the staple prey decrease. The authors collected food samples of breeding eagle and Ural owls (Bubo bubo, Strix uralensis) during 1973-87 in S Ostrobothnia (SO) and during 1965-80 in C Ostrobothnia (CO), Finland, and tested the following predictions of APH: 1) The yearly abundances of voles in the field should correlate positively with the proportions of voles in the diet. Data from eagle and Ural owls in SO were consistent with this. 2) The proportion of small game in the diet should be negatively related to the abundance of voles in the field. This was true for the Ural owl in SO. 3) The owls should take more small game in poor vole years than in good ones, independently of the proportion of voles in the diet. This was the case for the 2 owls in CO. 4) The proportion of small game in the diet is nearly independent of its abundance in the field. This held true for the eagle owl in CO and the Ural owl in the 2 areas, while most data supported the 4 predictions of APH, additional data on their predation impact on small game are needed to better assess how much these two owls are responsible for crashes of hare and grouse populations. -from Authors
In the temperate zone, food availability and winter weather place serious constraints on European Barn Owl Tyto alba populations. Using data collected over 22 years in a Swiss population, we analysed the influence of early pre-breeding food conditions and winter severity on between-year variations in population size and reproductive performance. To estimate pre-breeding food conditions, we attempted a novel approach based on an index that combines Tawny Owl Strix aluco reproductive parameters and the occurrence of wood mice Apodemus sp. in their diet. Tawny Owls breed earlier in the season than Barn Owls and are strongly dependent on the abundance of wood mice for breeding. This index was strongly positively associated with the number of breeding pairs and early breeding in the Barn Owl. Winter severity, measured by snow cover and low temperatures, had a pronounced negative influence on the size of the breeding population and clutch size. Food conditions early in the breeding season and winter severity differentially affect the Barn Owl life cycle. We were able to use aspects of the ecology and demography of the Tawny Owl as an indicator of the quality of the environment for a related species of similar ecology, in this case the Barn Owl.
Abstract The maintenance of genetic variation is a long-standing issue because the adaptive value of life-history strategies associated with each genetic variant is usually unknown. However, evidence for the coexistence of alternative evolutionary fixed strategies at the population level remains scarce. Because in the tawny owl (Strix aluco) heritable melanin-based coloration shows different physiological and behavioral norms of reaction, we investigated whether coloration is associated with investment in maintenance and reproduction. Light melanic owls had lower adult survival compared to dark melanic conspecifics, and color variation was related to the trade-off between offspring number and quality. When we experimentally enlarged brood size, light melanic males produced more fledglings but in poorer condition, and they were less often recruited in the local breeding population than those of darker melanic conspecifics. Our results also suggest that dark melanic males allocate a constant effort to raise their brood independently of environmental conditions, whereas lighter melanic males finely adjust reproductive effort in relation to changes in environmental conditions. Color traits can therefore be associated with life-history strategies, and stochastic environmental perturbation can temporarily favor one phenotype over others. The existence of fixed strategies implies that some phenotypes can sometimes display a "maladapted" strategy. Long-term population monitoring is therefore vital for a full understanding of how different genotypes deal with trade-offs.
Large-scale patterns and variations in the occurrence and breeding of the Tawny Owl Strix aluco in Finland was studied in relation to the availability of small voles and winter-ing birds as well as to general weather conditions of the preceding winter. In 19862000, pronounced three-year cycles in the numbers of occupied territories as well as nesting at-tempts recorded were less regular near the southern coast than elsewhere in Finland. On average, 64.7 ± 6.8 (SD)% of the Tawny Owl territories recorded were annually found to be occupied by breeding birds. The annual mean of the clutch size averaged 3.67 ± 0.46 (SD), and that of the fledglings produced per nesting attempt 2.73 ± 0.37 (SD). The annual mean fledgling production per occupied territory was significantly lower in the southern coastal region than elsewhere in Finland. The average long-term fledgling production of Finnish Tawny Owls was high, but not consistently higher compared to that of more southern populations. The clutch size and fledgling production followed mainly the cycli-cally fluctuating levels of small voles. High numbers of wintering birds seemed to have favourable effects on breeding, while benign weather conditions of the preceding winter tended to increase the number and proportion of breeding birds.
Relationships between the wintering conditions, vole abundance, and food and breeding success of a population of Tawny Owls in Uusimaa, southern Finland, were studied during a 15-year period (1986-2000). There were no significant trends in the weather variables studied, though general experience suggested that winters were getting milder. The abundance of small microtine voles, the staple food for owls before the breeding season, fluctuated in three-year cycles, but the general level of vole abundance seems to have lowered after the 1980s . Correspondingly, during the first years of the study the proportion of small voles in the prey of owls was significantly higher than later. When voles were scarce, the proportion of alternative prey in the diet of owls was high. The number of nestings (denoting the breeding frequency of owls) could be predicted from the abundance of voles in spring . On average, the Tawny Owls of the district laid eggs and produced young slightly but not significantly more in "good" vole years than in "poor" ones . The difference was, however, significant in the breeding success of the local population studied. The number of young fledged correlated positively with the number of voles brought to the nest by parents. Significant relationships between the clutch size and fledgling production of owls and the vole abundance and weather variables studied emerged only when the data of the "poor" vole years were excluded . In such cases there was a significant negative relationship between the average clutch size and the "frost see-saw effect", the number of days in winter during which the temperature at least once fell from plus to minus °C . The latter variable was suggested to be negatively correlated with the abundance of voles. The relationship was similar also with the average fledgling production that was, however, largely determined by the clutch size.
Research on Tawny Owls (Strix aluco) in Kielder Forest, northern England, since 1981 demonstrated that field voles (Microtus agrestis) were their most important food. Here, field voles exhibited a 3-4 year cycle of abundance, and mean clutch size in Tawny Owls was significantly related to vole abundance in March. In this analysis we use variations in clutch size as a surrogate to explore whether vole abundance was synchronized over a larger spatial scale, in this case between Kielder Forest and another forest (Kershope) in an adjacent valley system. We show that mean clutch sizes were synchronized between study areas during 1987-1992, but not subsequently (1993-1996). Synchrony was broken in 1993 when voles in Kielder experienced an extended low phase to the cycle resulting in 4 years between peaks, whereas vole cycles in Kershope continued with 3-year periodicity. Thus, since 1993 vole cycles in the two valley systems have been out-of-phase by 1 year. We discuss possible mechanisms whereby vole abundance in nearby areas can oscillate in- and out-of-phase with one another.
Prey remains and regurgitated pellets collected from nests are the most common material for investigating the diet of birds of prey. Generally such data is thought to be biased with large prey overrepresented. However, there is no analysis investigating how systematic the error is in relation to prey size, abundance, species or method used. In this study we compared the diet composition of the Goshawk (Accipiter gentilis) and the Buzzards (the Common Buzzard Buteo buteo and the Rough-legged Buzzard B. lagopus) in northern Finland obtained indirectly (by collection of prey remains and pellets) and by direct methods (using a movie camera and a video recording system). In order to investigate the relationship between these two types of diet data more generally, we combined our own material and some published original data. Video and film images allowed us to identify according to class or family level most of the prey items delivered to the nests during the surveillance sessions, but identification according to genus or species level often was difficult. We found that small prey items were underestimated in remains as compared to large prey items. However, when none of the prey delivered to the nest is in large numbers, prey remains give fairly reliable idea of the real diet.
The paper presents the material on the Tawny Owl (Strix aluco) diet collected during the breeding period (AprilMay) in Ðakiai district. Prey remains from 11 nestboxes occupied by owls in 19861987 and 19972004 were analysed. The remains of 15 small mammal species, 18 bird species, two amphibian species and two genera of insects were recovered. The dominating small mammal species (n = 361) were Clethrionomys glareolus (24.6%), Microtus arvalis (20.5%), Apodemus flavicollis (11.6%), Microtus agrestis (8.8%) and Sorex araneus (8.6%). Among birds, the most often preyed were Turdus philomelos, T. merula, Fringilla coelebs and Coccothraustes coccothraustes. Frogs Rana arvalis and R. temporaria as well as insects Dytiscus sp. and Melolontha sp. were also present in the food remains. The species composition and the number of small mammals preyed in Ðakiai district were compared to the respective data from Këdainiai district, Kurtuvënai Regional Park and Kamanos Strict Nature Reserve. The diet of the Tawny Owl in Ðakiai district differed in comparison to the Kamanos Reserve (more C. glareolus, less Microtus and shrews), Këdainiai district (less shrews) and Kurtuvënai Regional Park (a less share of secondary prey species).
Diet composition and food niches of two common and widespread in Lithuania owl species, Tawny Owl (Strix aluco) and Long-eared Owl (Asio otus), were analysed from irregularly collected pellets. S. aluco was characterised by a more diverse diet and a wider food niche: 14 small mammal (93.1% of the recovered items) and two amphibian (5.2%) species, a few passerine birds (1.1%) and representatives of three Coleoptera groups (0.6%) were recovered, whereas for A. otus – nine small mammal and two Carabidea species. By numbers, primary food resources of S. aluco were C. glareolus (31.4%), M. arvalis (27.9%) and A. flavicollis (14.3%), whereas of A. otus - M. arvalis (70.8%), all Microtus voles constituting 95.2%. Diet diversity of S. aluco was more than twofold higher (Shannon’s H = 2.62 vs. 1.16). According to biomass, main foods consumed by S. aluco were C. glareolus, M. arvalis and A. flavicollis (27.6, 25.0 and 22.4%, respectively). For A. otus, Microtus voles represented 95.8% of biomass consumed (M. arvalis – 64.3%). Levins’ measure of food niche breadth for S. aluco was B = 5.51 and for A. otus – B = 2.22. The average number of prey items per pellet was 2.84 ± 0.11 (max. 10) for S. aluco and 1.75 ± 0.03 (max. 7) for A. otus.
We analysed the variation of small mammal species composition in the Tawny Owl Strix aluco L. diet in forest habitats of Cen-tral European Lowland. We used published and unpublished materials from forest-dominated landscapes in Lithuania (n = 7 locations), Po-land (n = 8) and East Germany (n = 1); marginal localities were ca. 870 km from each other. We recorded that in Central European Lowland the proportion of Arvicolidae in the Tawny Owl diet significantly increased, while that of Muridae de-creased toward north-east. The proportion of less common rodent species (including Gliridae and Sicita betulina Pallas) in the diet also increased significantly toward NE. We did not record any trend of small mammals diversity along the ana-lysed transect. We suggest that the change of Ar-vicolidae to Muridae ratio toward north-east can be caused by the replacement of mice with boreal vole species in small mammal community. Small mammal diversity in Central Europe is subject of discussion.
ABSTR½T.--Numerous studies of predatory birds worldwide report dietary proportions based on analyses of large numbers of pellets or prey remains. Such analyses are often severely biased, hence strictly unquantifiable, because some prey remains are more conspicuous or persistent than others. We investigated this bias for the bird- and micromammal-eating African Marsh Harrier (Circus ranivorus), using an essentially independent measure of diet, observed prey deliveries to the nest. Comparisons of the frequency of occurrence showed that bird prey, particularly large wetland species, were over-represented almost threefold among remains. Micromammals were under-represented about 1.5-fold, while fish, frogs and eggs were marginally over-represented. Analyses using pellets were also biased but in the opposite direction to that of remains. We show that by combining pellets and prey remains (collected with equal effort), accurate estimates of overall diet can be achieved. This was verified using month by month comparisons of micromammals, in which proportions derived from pellets and remains never differed by more than 10% from those established from direct observations.
From October 1986 through September 1989, the diet of tawny owls Strix aluco in the city of Pavia (northern Italy) was studied by examining 769 pellets coming from 4 pellet‐dropping stations. The results show that the owls fed mainly on birds (61.1% in biomass), whereas mammals accounted for 37%. Arthropods, especially ants, were numerous in the diet, mainly during summer although their biomass was slight. Seasonal changes in diet occurred throughout the year. During winter, the owls preyed on birds and mammals in the same proportions. Birds largely predominated in spring and sum mer, while mammals slightly prevailed in autumn. The diet of dif ferent individual tawny owls was quite similar qualitatively, but dif fered quantitatively, according to the habitats included in their respective territories.
Feeding of tawny owl, Strix aluco, was studied from 14 localities of northern and central Italy, where it preyed mainly upon mammals and birds, but amphibians and invertebrates were also frequently eaten. Predation upon mammals was more important from autumn to spring, when alternative prey (amphibians, birds and invertebrates) were less available. The food niche overlaps, the linear distances (in km) between the various localities were negatively correlated, and the mean size of prey taken increased with the proportion of forested territory. However, where tawny owl preyed upon large forest mammals, it also preyed upon larger mammals not typical of forests. Considering the relationships between forest structure and tawny owl diet, it was found that forest density was positively correlated with the proportion of arvicolids and negatively with prey size diversity and the proportion of myoxids. The amount of forest territory within 1 km radius from each pellet collection site largely explained diet variability. Results of discriminant analysis, in addition, suggested that tawny owl is likely to avoid thick Mediterranean coppice forests as hunting habitats.
In Finland, monitoring of ‘common’ birds of prey is based on two projects run by the Finnish Ringing Centre: the Raptor Grid (since 1982) and Raptor Questionnaire (since 1986). The Raptor Grid has produced sufficient data for analysing population trends in six of ten owl species. The overall trend during 1982–2007 was significantly negative in the Eagle Owl Bubo bubo (-2.2% per year), Long-eared Owl Asio otus (-4.7%) and Tengmalm's Owl Aegolius funereus (-3.1%), and significantly positive in the Ural Owl Strix uralensis (+1%); no significant trend was detected in the Pygmy Owl Glaucidium passerinum or Tawny Owl Strix aluco. The Eagle Owl population increased to the middle of 1990s, but has since decreased by 5% per year. The decrease coincides with the closing of 90% of local open rubbish dumps, which offered a stable and rich food supply to the Eagle Owls. The decrease in the Tengmalm's Owls can partly be attributed to the decrease in the amount of old forest. The Pygmy Owl population increased steeply (>5% per year) during 1994–2003, then crashed following a mass invasion and has since started to recover. The geographical distribution of the Raptor Grid study plots is not suitable for monitoring the northern nomadic species like the Snowy Owl Bubo scandiacus, Northern Hawk-Owl Surnia ulula, Great Grey Owl Strix nebulosa and Short-eared Owl Asio flammeus. Annual totals of active nests and occupied territories reported with the Raptor Questionnaire do not indicate any long-term population changes of these species. However, intensive cooperation over larger areas across national boundaries in northern Europe is urgently needed for reliable monitoring of these nomadic owls.
We compared techniques to assess diet at 17 Hen Harrier (Circus cyaneus) nests. Diet was measured directly from observations and compared to estimates from pellets, prey remains and a combination of pellets and remains. For data pooled across nests, pellets over-represented mammalian prey and under-represented avian prey. Prey remains over-represented large prey and under-represented small prey. Combining pellet and remains data did not eliminate these biases. Pellets gave higher diversity values than direct observations and detected more small prey species. For data analyzed on a nest by nest basis, estimates from pellets were significantly related to estimates from direct observations for three prey types by frequency and all types by biomass. These linear relationships were used to predict frequencies and biomass of prey types in the observed diet at five new nests. Our findings suggest that pellets are useful for estimates of prey diversity and as an index of the frequency of certain prey types in the diet, but direct observations are necessary to help quantify the biases inherent in diet estimates.
We tested a hypothesis on the influence of prey distribution on habitat selection by the Lesser Spotted Eagle Aquila pomarina in north-eastern Poland during the breeding season. We analysed the habitat composition in schematic territories around the nests of 116 breeding pairs of eagles (in a radius of 3 km) and related them to randomly selected sites. Next, we compared the habitat requirements of potential prey species with the proportion of different prey categories found in the eagle's diet. We demonstrated that, in contrast to random sites, eagle nests were located closer to the forest edges. The habitat composition of schematic territories of eagles was different from the random sites owing to the lower proportion of forest and higher proportion of meadows and agricultural land. The feeding habits of Lesser Spotted Eagles were opportunistic, and the diet was composed mainly of rodents (voles), insectivorous mammals (hedgehogs and moles), small birds, and amphibians. Small prey species (body mass below 50 g) and species indicating preferences for open habitats dominated in the diet of eagles (69% and 74% of prey captured respectively). Prey species inhabiting grasslands were hunted more frequently than species preferring agricultural areas. Moreover, eagle pairs nesting deep in the forest interior captured relatively more larger-sized species, whereas the proportion of small prey in the eagle's diet increased as the distance of nest from forest edge decreased. We hypothesize that eagles have to breed closer to the forest edge to minimize energy expenditure and time associated with prey capture and delivery to the nest.
The diet of tawny owls Strix
aluco was determined from pellets and prey items in owl nests in Kielder Forest, a planted spruce forest in northern England. Field voles Microtus
agrestis were their most important food, and formed the highest proportion of tawny owl diet in winter and early spring. Common shrews Sorex
araneus, common frogs Rana
temporaria and birds were taken more frequently in late spring and summer. Clear cuts, areas from which timber had been felled at the end of the rotation, provided the main field vole habitat in the forest and remained suitable for voles for 10–15 years after re-planting. Field vole abundance was measured three times a year on numerous clear cuts throughout the study area using a vole sign index based on fresh grass clippings in runways. Tawny owls responded functionally to the 3 to 4-year cycles of field vole abundance. In years when voles were scarce, adult owls took more common shrews and common frogs, as determined from pellet analysis. In contrast, more bird prey was fed to nestlings when field voles were scarce, as determined from prey items in nests. The proportions of the main prey in nests changed over a 19-year period. More bank voles Clethrionomys
glareolus and wood mice Apodemus
sylvaticus occurred in every year after 1992 than in any year before this. Numbers of wood mice in owl nests increased significantly throughout the study period, whereas bank vole numbers exhibited non-cyclic, multi-annual fluctuations that were unrelated to field vole cycles. It is argued that fluctuations in rodent prey reflected changes in rodent guilds in the study area; reasons for this are discussed. This is the first study of tawny owl diet in spruce forests in Britain and highlights the value of such large-scale dynamic habitats for rodent populations and their predators.
We studied a population of 23–25 Eagle Owl Bubo bubo pairs between 1994 and 2000 in a 1330-km2 study plot in the central-eastern Italian Alps. Compared to random sites, territories were located at lower elevation and closer to intensively cultivated-urbanized valley floors. Early laying was associated with low elevation and negatively affected productivity. Diet was dominated by rats, hedgehogs and dormice (n = 978 prey items), all of them typical of low-elevation habitats. Higher productivity was associated with a higher proportion of rats in the diet of individual pairs. Low availability of rats resulted in a more diverse diet, in turn associated with low productivity. Territories were occupied every year in a non-random fashion, and those most occupied were characterized by higher productivity and higher occurrence of the favoured prey types in the diet, suggesting they were of superior quality. Eagle Owls also paid a cost associated with nesting near human-altered habitats: the main cause of mortality reported to local authorities was electrocution. This is an increasing cause of death for many European populations and may be a cause for conservation concern. Human persecution is also an important cause of mortality in some parts of the European range. Apart from such costs, the study population appeared to have adapted well to the proximity of humans: estimates of density and productivity were comparable to those recorded elsewhere in Europe. The pattern found in our population also held at higher spatial scales: data from 17 European populations showed density to be highest in low-elevation, human-altered landscapes.
Few studies have quantified the dynamics of recovering populations of large raptors using long-term, spatially explicit studies. Using data collected over 37 years in the western Italian Alps, we assessed the trends in distribution, abundance, fecundity and breeding population structure of Golden Eagles Aquila chrysaetos. Using the spatial distribution of territory centroids in 2007, we found that the spatial distribution of eagle territories was over-dispersed up to 3 km. Although population size and total productivity increased from 1972 to 2008, the proportion of pairs that laid eggs showed a strong decline, falling to no more than 50% after 2003. On average, 15% of successful nests produced two fledglings, and productivity also declined over time. No significant relationship between population growth rate and total population size was detected, but the percentage of pairs that bred and breeding success showed evidence of density dependence, as they declined significantly with increasing density. Our results suggest that density dependence, operating across heterogeneous habitats, is currently regulating this population, while the carrying capacity may still be increasing. This may explain the apparent paradox of reduced breeding effort despite increasing total productivity.
Variation in coloration with a strong underlying genetic basis is frequently found in birds, insects, anurans, molluscs and plants. Although such a variation can be large, little is known about its functional value. Correlative data, however, can help suggest testable hypotheses about potential covariation between reproductive parameters and a colour polymorphism displayed by individuals belonging to a single population. In this context, we studied two Swiss populations of tawny owls Strix aluco, a polymorphic species that varies in coloration from reddish-brown to grey. Observations in the first population showed that although greyer females had shorter tarsi, they produced heavier offspring in two of three years. Pairing with respect to plumage coloration was not significantly disassortative, indicating that these correlations were probably not inflated by plumage coloration of the mate. In the second population, where breeding females had been monitored for 14 years, the proportion of all breeding females that were reddish-brown was greater in years when the breeding density was lower. Capture-recapture analyses show that the latter result is explained by the fact that greyish females bred less often than reddish-brown females, although their survival probability was similar. The number of greyer breeding females was greater when spring/summer temperatures were lower. When combined, the results from the two populations lend support to the hypothesis that grey females do not breed every year, but produce offspring of higher quality. Whatever the mechanism underlying the correlations reported in this study, colour polymorphism in female tawny owls appears to reflect some components of individual quality.
We studied the nest site selection and distribution pattern at landscape level of the German Osprey population, and demonstrated
how to test the predictions of the ideal free distribution theory and its derivatives on such an expanding population. Information
about the location and breeding success of each Osprey nest site between 1995 and 2005 was collected through a long-term monitoring
programme. Data of land cover types were acquired from the administrations of each federal state and the CORINE Land Cover
database. The results showed that Ospreys preferred landscapes with more water bodies and forests. Such sites were also occupied
earlier and had higher local population density. However, in the study period of 11years, there was a gradual shift from
forest-dominated landscapes to agricultural land-dominated landscapes. The breeding success increased over time, with no difference
in the breeding success between pairs nesting on trees and poles, whereas there was higher breeding success at nest sites
surrounded by more agricultural land and less forest. The more efficient foraging in eutrophic lakes in agricultural landscapes
was the most likely cause for the higher breeding success. The distribution pattern of the Ospreys did not match the resource
allocation, which deviated from the models tested. We suggested that the proximate cues used for nest site selection mismatched
site quality due to anthropogenic environmental changes.
Food samples of breeding Kestrels (Falco tinnunculus) and Long-eared Owls (Asio otus) were collected in the peak and low phase of their preferred prey (Microtus voles) in western Finland. Diets of pairs that bred as neighbours (1 km) with interspecifics were compared with those of non-neighbours. In both species, neighbouring pairs fed less on Microtus voles and more on alternative prey than did non-neighbours. Competition theory predicts that diet overlap should be lower during prey shortage and that diet similarity should be especially reduced in neighbouring pairs. Observations were consistent with expectations: diet similarity was lower in the low vole years and neighbouring pairs showed less diet overlap that non-neighbours. Differences in habitat composition and prey availability at the sample sites should not confuse the results. In addition to the high diet similarity, hunting habitats and nest sites of the species overlapped almost completely; they only showed clear temporal segregation in hunting. Probably because of food competition, the neighbouring pairs of both species produced significantly fewer young than the non-neighbours. These results contrast with the view that the diet composition and dietary shift of rodent-feeding predatory birds can be interpreted in terms of simple opportunistic foraging. In the breeding season, interspecific competition for food seems to be an important factor that affects the niches of these species, especially in northern areas, where the seasonal low phase of voles in spring and the number of alternative prey are lower than in more southern areas.
Tawny owl reproduction and offspring sex ratios have been considered to depend on the abundance of small voles. We studied
reproductive performance (laying date, clutch and brood size) during 1995–2003 and offspring sex ratios from 1999 to 2003
in relation to the abundance of small voles and food delivered to the nest in a tawny owl population in southern Finland.
Abundance of small voles (field and bank voles) was based on trappings in the field, and estimates of food delivery was based
on diet analysis of food remains in the nest boxes. In this population, reproductive output was not related to the abundance
of small voles. Analysis of food delivered to the nest showed that the prey weight per offspring varied more than twofold
between years and revealed that this difference was mainly related to the proportion of water voles in the diet. Only the
number of water voles correlated with laying dates. Offspring sex ratios were weakly male biased (55%) but did not differ
from parity. Sex ratios were not related to the abundance of small voles, and we found no evidence that parents delivered
more food to nests with proportionally more offspring of the larger (female) sex. Our results underline the notion that populations
may differ in their sex allocation pattern, and suggest such differences may be due to diet.
The understanding of the dynamics of animal populations and of related ecological and evolutionary issues frequently depends on a direct analysis of life history parameters. For instance, examination of trade-offs between reproduction and survival usually rely on individually marked animals, for which the exact time of death is most often unknown, because marked individuals cannot be followed closely through time. Thus, the quantitative analysis of survival studies and experiments must be based on capture-recapture (or resign ting) models which consider, besides the parameters of primary interest, recapture or resighting rates that are nuisance parameters. Capture-recapture models oriented to estimation of survival rates are the result of a recent change in emphasis from earlier approaches in which population size was the most important parameter, survival rates having been first introduced as nuisance parameters. This emphasis on survival rates in capture-recapture models developed rapidly in the 1980s and used as a basic structure the Cormack-Jolly-Seber survival model applied to an homogeneous group of animals, with various kinds of constraints on the model parameters. These approaches are conditional on first captures; hence they do not attempt to model the initial capture of unmarked animals as functions of population abundance in addition to survival and capture probabilities. This paper synthesizes, using a common framework, these recent developments together with new ones, with an emphasis on flexibility in modeling, model selection, and the analysis of multiple data sets. The effects on survival and capture rates of time, age, and categorical variables characterizing the individuals (e.g., sex) can be considered, as well as interactions between such effects. This "analysis of variance" philosophy emphasizes the structure of the survival and capture process rather than the technical characteristics of any particular model. The flexible array of models encompassed in this synthesis uses a common notation. As a result of the great level of flexibility and relevance achieved, the focus is changed from fitting a particular model to model building and model selection. The following procedure is recommended: (1) start from a global model compatible with the biology of the species studied and with the design of the study, and assess its fit; (2) select a more parsimonious model using Akaike's Information Criterion to limit the number of formal tests; (3) test for the most important biological questions by comparing this model with neighboring ones using likelihood ratio tests; and (4) obtain maximum likelihood estimates of model parameters with estimates of precision. Computer software is critical, as few of the models now available have parameter estimators that are in closed form. A comprehensive table of existing computer software is provided. We used RELEASE for data summary and goodness-of-fit tests and SURGE for iterative model fitting and the computation of likelihood ratio tests. Five increasingly complex examples are given to illustrate the theory. The first, using two data sets on the European Dipper (Cinclus cinclus), tests for sex-specific parameters, explores a model with time-dependent survival rates, and finally uses a priori information to model survival allowing for an environmental variable. The second uses data on two colonies of the Swift (Apus apus), and shows how interaction terms can be modeled and assessed and how survival and recapture rates sometimes partly counterbalance each other. The third shows complex variation in survival rates across sexes and age classes in the roe deer (Capreolus capreolus), with a test of density dependence in annual survival rates. The fourth is an example of experimental density manipulation using the common lizard (Lacerta vivipara). The last example attempts to examine a large and complex data set on the Greater Flamingo (Phoenicopterus ruber), where parameters are age specific, survival is a function of an environmental variable, and an age × year interaction term is important. Heterogeneity seems present in this example and cannot be adequately modeled with existing theory. The discussion presents a summary of the paradigm we recommend and details issues in model selection and design, and foreseeable future developments.
1. The habitat heterogeneity (HHH) and individual adjustment (IAH) hypotheses are commonly proposed to explain a decrease in reproduction rate with increasing population density. Higher numbers of low-quality territories with low reproductive success as density increases lead to a decrease in reproduction under the HHH, while more competition at high density decreases reproduction across all territories under the IAH.
2. We analyse the influence of density and habitat heterogeneity on reproductive success in eight populations of long-lived territorial birds of prey belonging to four species. Sufficient reliability in distinguishing between population-wide, site-specific and individual quality effects on reproduction was granted through the minimal duration of 20 years of all data sets and the ability to control for individual quality in five of them.
3. Density increased in five populations but reproduction did not decrease in these. Territory occupancy as a surrogate of territory quality correlated positively with reproductive success but only significantly so in large data sets with more than 100 territories.
4. Reproductive success was always best explained by measures of territory quality in multivariate models. Direct or delayed (t−1) population density entered very few of the best models. Mixed models controlling for individual quality showed an increasing reproductive performance in older individuals and in those laying earlier, but measures of territory quality were also always retained in the best models.
5. We find strong support for the habitat heterogeneity hypothesis but weak support for the individual adjustment hypothesis. Both individual and site characteristics are crucial for reproductive performance in long-lived birds. Proportional occupancy of territories enables recognition of high-quality territories as preferential conservation targets.
The ongoing climate change has improved our understanding of how climate affects the reproduction of animals. However, the interaction between food availability and climate on breeding has rarely been examined. While it has been shown that breeding of boreal birds of prey is first and foremost determined by prey abundance, little information exists on how climatic conditions influence this relationship. We studied the joint effects of main prey abundance and ambient weather on timing of breeding and reproductive success of two smaller (pygmy owl Glaucidium passerinum and Tengmalm's owl Aegolius funereus) and two larger (tawny owl Strix aluco and Ural owl Strix uralensis) avian predator species using long-term nation-wide datasets during 1973-2004. We found no temporal trend either in vole abundance or in hatching date and brood size of any studied owl species. In the larger species, increasing late winter or early spring temperature advanced breeding at least as much as did high autumn abundance of prey (voles). Furthermore, increasing snow depth delayed breeding of the largest species (Ural owl), presumably by reducing the availability of voles. Brood size was strongly determined by spring vole abundance in all four owl species. These results show that climate directly affects the breeding performance of vole-eating boreal avian predators much more than previously thought. According to earlier studies, small-sized species should advance their breeding more than larger species in response to increasing temperature. However, we found an opposite pattern, with larger species being more sensitive to temperature. We argue that this pattern is caused by a difference in the breeding tactics of larger mostly capital breeding and smaller mostly income breeding owl species.
Journal is Ökologie der Vögel
The regional synchrony of short-term population fluctuations of small rodents and small game has usually been explained by varying impacts of generalist predators subsisting on both voles and small game (the “alternative prey hypothesis” APH). APH says that densities of predators increase as a response to increasing vole densities and then these predators shift their diet from the main prey to the alternative prey when the main prey decline and vice versa. We studied the diet composition of breeding common buzzards Buteo buteo during 1985–92 in western Finland. Microtus voles were the main prey and water voles, shrews, forest grouse, hares and small birds the most important alternative prey. Our data from the between-year variation in the diet composition of buzzards fulfilled the main predictions of APH. The yearly proportion of main prey (Microtus voles) in the diet was higher in years of high than low vole abundance. The proportion of grouse in the diet of buzzards was negatively related to the abundance of Microtus voles in the field and was nearly independent of grouse abundance in the field. In addition, buzzards mainly took grouse chicks and young hares which is consistent with the prediction of APH. Therefore, we conclude that buzzards are able to shift their diet in the way predicted by the APH and that buzzards, together with other generalist predators, may reduce the breeding success of small game in the decline phase of the vole cycle, and thus substantially contribute to the existence of short-term population cycles of small game.
ABSTRCT.--The diet of the Eurasian Eagle-Owl (Bubo bubo) was assessed by collecting pellets and prey remains near the nesting cliffs of 21 pairs in the central-eastern Italian Alps between 1993-97. Taxo- nomic and prey mass composition of the diet was compared between two methods of analysis, pellets and prey remains, to assess biases associated with these techniques. When compared with pellets, remains overestimated avian occurrence, underestimated mammals, and completely failed to detect fish occur- rence (P < 0.0001). Large prey were also over-represented in remains (P < 0.002). Overall, pellets gave a more realistic and diverse picture of Eurasian Eagle-Owl diet, but failed to detect 26 avian species and 12 avian families identified in remains. Biases associated with the two methods may be lowered by pooling items collected by both methods, assuming the minimum possible number of individuals per species per collection event. However, care must be taken to show the relative contribution of each method in the pooled sample. Further research is needed to quantify biases in diet study methods, by using controlled feeding of captive owls. Similar biases may apply to the study methods commonly employed to assess the diet composition of other owls and predatory birds.
Ne st boxes for breeding tawny owls Strix aluco were sited in a mixed oak/hornbeam/beech forest located in the Pilis Biosphere Reserve, 30 km northwest of Budapest, Hungary, during the period 1992-2000. We hypothesized that both within-pair differences in age composition and weather conditions affect the breeding success of the parents. To test our hypothesis, we marked the owl parents in their first known breeding year and related their reproductive performance was to within-pair age composition through three subsequent breeding seasons. Sex-related age differences affected the breeding performance of the parents in their first and second known breeding year: number of eggs and hatching success were influenced by the age of females, while fledging success was influenced by the age of males. Within-pair age differences did not affect the breeding performance at their third known breeding year. Reproductive performance was lower in snowy years than in years without snow cover in all of their three subsequent breeding seasons. On basis of the lower mean body mass and shorter mean tarsi of the nestlings before fledging recorded in adverse weather conditions, we suggest that parents favour the sex that demands less investment in care. As a consequence, the sex-ratio becomes skewed towards the male in a poor food environment.
Capsule In the areas studied, Tawny Owls showed extreme flexibility and opportunism, occupying most available vegetation types and elevations below the tree-line.
Aims To assess habitat relationships for the species so as to forecast the potential impact of habitat changes.
Methods We censused a total of 321 territories in 12 study areas covering the regional variation in elevation and the associated heterogeneity in vegetation types and landscape configurations.
Results The species occupied virtually all vegetation types and landscape configurations below the tree-line. Nests were mostly located on cliffs, but also in tree cavities, buildings, and stick nests of other raptors. Density varied from 12 to 60 territories/100 km² and mean nearest-neighbour distance from 0.7 to 1.8 km, depending on the availability of broadleaved woodland. Compared to availability, nests were nearer to cliffs and had higher richness of habitat edges and vegetation types, which positively affected chick production. Diet composition varied widely according to habitat and was mostly dominated by Edible Dormouse Glis glis, voles and mice.
Conclusion The conservation status of the species was satisfactory. Future habitat changes may favour an increase in density. Silvicultural practices encouraging more old trees with large cavities would further favour this opportunistic species.
The diet of Tawny Owls during the breeding seasons 1949-52 in Wytham Woods, near Oxford, was determined (a) from analysis of pellets collected, (b) from observation at night, by the use of a red floodlight, of prey brought to the nest and (c) from records of prey left in the nest made during daily visits to weigh the young.
Analysis of pellets showed an increase in the proportion of moles and beetles (mainly cockchafen) in the diet after the first week of May (the time when, on average, the young owls are about half grown) and a decrease in the proportion of mice and voles.
These changes were confirmed in a more emphatic way from observations of food being brought to the nest and from records of prey left in the nest.
This greater emphasis suggests that tho food brought to the young may differ from that which the adults eat themselves.
The fact that no moles were observed being brought to nests at night, wherean many were recorded as surplus prey in the nest, showed that diurnal hunting is regular during the breeding reaaon.
A true assessment of prey taken by Tawny Owls during the breeding season should be based both on analyeis of pellets cast by the adults and on records of food brought to the nest throughout 24 hours. Such records could best be obtained with an automatic camera and flash and a design of nest-box which is described.
The spatial dispersion and social interactions were studied in 11 neighbouring pairs of radio-tagged tawny owls Strix aluco in a deciduous wood in Denmark from 1998–2001. The numbers and shapes of territories were stable throughout the survey and similar to a mapping made 40 years earlier. The home ranges of mates were of equal size and overlapped 82% in summer and 56% in winter. The inter-mate distances were on average 2.7% shorter than expected by chance. The activity distribution of neighbouring pairs overlapped 9% (95% CI: 2–15%) on average. Males and females did not differ in overlap with neighbours, and there was a similar overlap between neighbours of the same and opposite sex. Both sexes vocalized more often in the peripheries than in the centres of their territory. The vocal activity during May–September varied extensively among years and months in accordance with variation in the density of juvenile floaters. Males and females vocalized equally often and were involved in disputes with neighbours at similar rates. Usually, neighbouring disputes involved either one individual from each pair or all four. Disputes involving all four owls more often involved chasing and fighting than those involving one owl only from each pair. The dispute rate between neighbouring pairs correlated positively with home-range overlap. The total annual mortality was 21% (95% CI: 6–33%). Dead owners were usually replaced within 1–2 months. Two out of four cases of radio-tagged owls disappearing from their territory because of natural causes was due to take-overs by invading owls, suggesting that the risk of losing fitness resulting from eviction was important. The apparent co-operative territorial behaviour of tawny owl pairs is probably due to improved resource holding potential of pair coalitions compared to single individuals.
1. This paper follows over eight years the diet of some 20 pairs of Tawny Owls Strix aluco living in woodland.2. Pellets were collected regularly and the prey-remains in them were identified. Much information about the territorial and hunting habits of Tawny Owls was gained.3. There is a regular seasonal swing between small rodents (especially Wood Mice and Bank Voles) and larger prey (young Moles, Rabbits and rats). The former predominate in winter and spring, the latter in summer and autumn. This fluctuation is regular year after year with these animals, whereas shrews and small birds are eaten in small numbers steadily throughout the year and from year to year.4. Small rodents are eaten most when ground cover is sparsest, which may account for the early breeding season of the Tawny Owl. By the beginning of May, when vegetation is becoming dense and mice and voles are difficult to find, the young are already well-grown and can deal with the food brought to them. The hen is, therefore, free to go hunting herself and both parents must work hard for the next three months while the young are still dependent on them.In more open country Barn and Short-eared Owls seem to time their breeding later so that they are feeding their young on the big crops of Short-tailed Voles and–to a smaller extent–of shrews.5. This long period of dependence of young Tawny Owls falls when there are plenty of young Moles, often a large crop of cockchafers and generally plenty of earthworms to fall back on. The brownish, fibrous pellets of Tawny Owls are now known to contain mainly the chaetae of earthworms, which are sometimes brought to the young in very large quantities.6. Analysis of the results by locality shows that Tawny Owls take only the prey species which are available in their immediate vicinity. This and other evidence proves them to be strongly territorial and it is suggested that only by having a territory and knowing it intimately can a Tawny Owl survive.
A population of 17 to 20 pairs of Tawny Owls was studied from 1959–79 in Grunewald forest, West Berlin. The breeding success of the Owls (21–77% of pairs raising young per year) and the proportion of yellow-necked field mice Apodenzus flavicollis in their pellets (13–48% of prey items each year) exhibited the same three-year cycle. Long-term trends in the diet of the Owls were apparent and reasons for these are discussed.
A long-term study was made (1947-59) of the numbers and breeding success of the Tawny owl (Strix aluco L.) in a woodland habitat near Oxford. Parallel studies were made of the numbers and distribution of the owl's two main prey species, the Wood mouse (Apodemus sylvaticus (L.)) and the Bank vole (Clethrionomys glareolus (Schr.)).
The life-cycles of all these three interrelated species were worked out and special attention was paid to the habit of strict territoriality of the owl (on the evidence of vocalizations and of the recovery, from the regurgitated pellets of the owl, of marking rings placed on the rodents) and to the sequence of losses which occurred during the breeding of the owl. The owls' vocal defence of their territories enabled an accurate census to be made of the adult population each spring and the fact that the fledged young remained for a long time in their parents' territories made it possible to count the number of young produced each year. Investigation of the breeding habits of the owls showed that the number of fledged young produced fell far short of what was possible: some pairs of owls refrained from breeding at all, others laid eggs but failed to hatch them, yet others hatched young but failed to rear them to fledging. By and large this degree of reproductive “failure” was associated with the numbers and availability of rodent prey. At an exceptionally low density of prey no owls even attempted to breed; as densities increased, correspondingly greater numbers of fledged young were produced up to a ceiling where no more young were produced however much higher the level of abundance of the rodents rose.
In spite of these wide variations in the numbers of prey and in the numbers of fledged young produced, the population of adult owls remained notably stable, increasing from a low level of 17 pairs in 1947, owing to the emergency mortality of an exceptionally hard winter, to about 30 pairs in 1955, after which the population remained stationary until the end of the study in 1959.
It is clear that this stability is maintained by those young which fail to find a territory either starving or moving outside the study area, which in most cases amounts to the same thing. Thus regulation of the population studied was due to territorial intolerance acting to produce subsequent mortality, though it is not certain that this applies over a larger area.
The demographic consequences of within-population variability in predator foraging are not well understood. We assessed the relationship between the degree of diet specialization and two demographic parameters, population density and reproductive output, within a single population of Imperial Eagles Aquila heliaca at a nature reserve in north-central Kazakhstan. Nearest-neighbour distances between eagle nests throughout the reserve, and thus population density, were correlated with the degree to which diets were specialized. Diet diversity showed an extensive regional variability that was linked to prey distributions, but within-year analyses of reproductive output did not show similar linkages. However, multi-year analyses of breeding performance showed inter-regional differences in nesting success that were paralleled, and probably driven by, similar trends in diet diversity. In contrast, brood size at fledging was not linked to diet diversity and was more probably driven by reserve-wide influences such as weather. Finally, the decision to initiate breeding was associated neither with diet diversity nor with environmental variability. Our results indicate that the degree of dietary specialization is linked to the demographics of Imperial Eagle populations. For these and other raptor populations, it is possible that management could be used separately to increase or decrease nesting success, brood size at fledging, and the likelihood that a pair will initiate breeding.