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

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

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

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

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

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

Time and travelling costs during chick‐rearing in relation to habitat quality in Little Owls Athene noctua

In many bird species, parents adjust their home‐ranges during chick‐rearing to the availability and distribution of food resources, balancing the benefits of energy intake against the costs of travelling. Over recent decades, European agricultural landscapes have changed radically, resulting in the degradation of habitats and reductions in food resources for farmland birds. Lower foraging success and longer foraging trip distances that result from these changes are often assumed to reduce the reproductive performance of parents, although the mechanisms are not well understood. We tested the behavioural response of chick‐rearing Little Owls Athene noctua to variation in habitat diversity in an agricultural landscape. We equipped females with GPS loggers and received adequate range‐use data for 19 individuals (6063–14 439 locations per bird). In habitats dominated by homogeneous cropland habitats, home‐ranges were over 12 ha in size, whereas in highly diverse habitats they were below 2 ha. Large home‐ranges were associated with increased flight activity (117% of that of birds in small home‐ranges) and distances travelled per night (152%), increased duration of foraging trips (169%) covering larger distances (246%), and reduced nest visiting rates (81%). The study therefore provides strong correlative evidence that Little Owls breeding in monotonous farmland habitats expend more time and energy for a lower benefit in terms of feeding rates than do birds in more heterogeneous landscapes. As nestling food supply is the main determinant of chick survival, these results suggest a strong impact of farmland characteristics on local demographic rates. We suggest that preserving and creating islands of high habitat diversity within uniform open agricultural landscapes should be a key target in the conservation of Little Owl populations.     

Habitat displacement effect between two competing owl species in fragmented forests

Many owl species use the same nesting and food resources, which causes strong interspecific competition and spatio-temporal niche separation. We made use of a recent colonisation of Ural Owls (Strix uralensis) in southern Poland to compare habitat preferences of Tawny Owls (Strix aluco) allopatry and sympatry with Ural Owls. We investigated spatial niche segregation of Ural Owl and the Tawny Owl in sympatry and compared habitat preferences of Tawny Owls breeding in allopatry and sympatry. Tawny Owls breeding in sympatry with Ural Owls occupied forests with higher canopy compactness, sites located closer to forest border and to built-up areas, as well as stands with a higher share of fir and spruce and a lower share of beech as compared to sites occupied by Ural Owls. Allopatric Tawny Owls occupied sites with lower canopy compactness and bred at sites located further from forest borders and in stands with lower share of fir and spruce and a higher share of deciduous as compared to sympatric Tawny Owls. As Ural owls are dominant in relation to Tawny Owls, this indicates that the presence of Ural Owls prevents Tawny Owls from occupying deciduous-dominated and old stands located in forest interior areas, far from buildings and forest edges. The results support habitat displacement between the two species when breeding in sympatry. We also show that protection of large forest patches is crucial for the Ural Owl, a species still rare in central Europe, while small patches are occupied by the abundant Tawny Owl.     

Predictive modeling for allopatric Strix (Strigiformes: Strigidae) owls in South America: determinants of their distributions and ecological niche‐based processes

Strix (Strigidae) is a worldwide genus of 17 owl species typical of forested habitats, including Rusty‐barred Owls (S. hylophila), Chaco Owls (S. chacoensis), and Rufous‐legged Owls (S. rufipes) in South America. These species are distributed allopatrically, but the ecological traits that determine their distributions remain largely unknown and their phylogenetic relationships are unclear. We used species distribution models (SDMs) to identify variables explaining their distribution patterns and test hypotheses about ecological divergence and conservatism based on niche overlap analysis. For Rusty‐barred Owls and Chaco Owls, climatic factors related to temperature played a major role, whereas a rainfall variable was more important for Rufous‐legged Owls. When niche overlaps were compared, accounting for regional similarities in the habitat available to each species, an ecological niche divergence process was supported for Chaco Owl‐Rusty‐barred Owl and Chaco Owl‐Rufous‐legged Owl, whereas a niche conservatism process was supported for Rusty‐barred Owl‐Rufous‐legged Owl. Different ecological requirements support current species delimitation, but they are in disagreement with the two main hypotheses currently envisaged about their phylogenetic relationships (Chaco Owls as the sister taxa of either Rufous‐legged Owls or Rusty‐barred Owls) and support a new phylogenetic hypothesis (Rufous‐legged Owls as sister taxa of Rusty‐barred Owls). Our findings suggest that speciation of Rusty‐barred Owls and Rufous‐legged Owls was a vicariant event resulting from Atlantic marine transgressions in southern South America in the Miocene, but their niche was conserved because habitat changed little in their respective ranges. In contrast, Chaco Owls diverged ecologically from the other two species as a result of their adaptations to the habitat they currently occupy. Ecological and historical approaches in biogeography can be embedded to explain distribution patterns, and results provided by SDMs can be used to infer historical and ecological processes in an integrative way.     

Evaluating cropland in the Canadian prairies as an ecological trap for the endangered Burrowing Owl Athene cunicularia

Anthropogenic development may influence the choices animals make and their resulting reproductive success and survival. If such choices are maladaptive, the impact of anthropogenic change can be catastrophic to small or declining populations. Over the past century, Canada's prairie landscapes have been altered dramatically, with over two-thirds of its native grasslands now having been converted to cropland. The decline of the endangered Burrowing Owl Athene cunicularia population is assumed to have resulted from this landscape change, yet no causal link has been demonstrated. One hypothesis to explain this population decline is that owls get caught in an ecological trap, whereby they prefer to establish nests at the start of each breeding season in landscapes that later confer lower reproductive success. Agricultural landscapes represent a plausible potential ecological trap because the short and sparse vegetation in annual crops (seeded each spring) is predicted to be attractive nesting/foraging habitat for Burrowing Owls when they arrive from northward migration, yet crops become substantially taller and denser over the growing season so prey are predicted to become less accessible by the time broods have hatched. We tested this ecological trap hypothesis in a 3-year study, involving 379 Burrowing Owl pairs, across the agricultural landscapes of southern Alberta and Saskatchewan. In support of the hypothesis, Burrowing Owls did prefer to settle in breeding home-ranges that contained a higher proportion of cropland, and their prey-delivery rates during brood-rearing were lower at nests with a higher proportion of cropland growing in the surrounding landscape. However, in contradiction to a key prediction, the number of fledglings produced (range = 0–9) was higher, not lower, for pairs with more actively growing cropland in their landscapes. Therefore, the decline of the Burrowing Owl in Canada does not appear to result from cropland forming an ecological trap during the breeding season. We also found a significant positive relationship between the amount of summer fallow within Burrowing Owl home-ranges and the quantity of vertebrate prey delivered to the nest and the number of juveniles fledged, highlighting the importance of this declining land use in raptor conservation.     

Using population viability analysis,genomics, and habitat suitability to forecast future population patterns of Little Owl Athene noctua across Europe

The agricultural scene has changed over the past decades, resulting in a declining population trend in many species. It is therefore important to determine the factors that the individual species depend on in order to understand their decline. The landscape changes have also resulted in habitat fragmentation, turning once continuous populations into metapopulations. It is thus increasingly important to estimate both the number of individuals it takes to create a genetically viable population and the population trend. Here, population viability analysis and habitat suitability modeling were used to estimate population viability and future prospects across Europe of the Little Owl Athene noctua, a widespread species associated with agricultural landscapes. The results show a high risk of population declines over the coming 100 years, especially toward the north of Europe, whereas populations toward the southeastern part of Europe have a greater probability of persistence. In order to be considered genetically viable, individual populations must count 1,000–30,000 individuals. As Little Owl populations of several countries count <30,000, and many isolated populations in northern Europe count <1,000 individuals, management actions resulting in exchange of individuals between populations or even countries are probably necessary to prevent losing <1% genetic diversity over a 100‐year period. At a continental scale, a habitat suitability analysis suggested Little Owl to be affected positively by increasing temperatures and urban areas, whereas an increased tree cover, an increasing annual rainfall, grassland, and sparsely vegetated areas affect the presence of the owl negatively. However, the low predictive power of the habitat suitability model suggests that habitat suitability might be better explained at a smaller scale.     

Environmental factors affecting patterns of distribution and co‐occurrence of two competing raptor species

Habitat selection is a complex process, that is affected by several factors, including habitat characteristics, environmental conditions, and both intra‐ and interspecific interactions. We analysed habitat preferences of two top avian predators, Peregrine Falcon Falco peregrinus, a medium‐sized diurnal raptor, and Eagle Owl Bubo bubo, a large nocturnal raptor. These two species are known to compete for preferred nest‐sites, and proximity to cliffs with Eagle Owls may reduce Peregrine breeding output through predation of young Falcons. We investigated the environmental factors affecting occurrence and coexistence of the two species and the potential role of habitat suitability in favouring co‐occurrence in 3519 km² of the central pre‐Alps of Italy, where the two species breed on cliffs and sometimes co‐occur on the same cliff. Peregrines settled on long, steep and favourably orientated cliffs in woodland landscapes close to urban areas. Eagle Owls settled on topographically similar cliffs, but in lower rainfall areas compared with cliffs occupied by Peregrines and cliffs unoccupied by either species. Sites where the two species co‐occurred were characterized by more horizontally extended cliffs compared with sites of exclusive occurrence of each species. An analysis of relative habitat suitability revealed that sites where the two species co‐occurred had the highest predicted probability of occupancy for both species, suggesting that those sites should be regarded as high‐quality sites. Breeding productivity of Eagle Owls was negatively affected by the co‐occurrence of Peregrines, whereas the effect of Eagle Owl proximity on Peregrine productivity varied according to cliff suitability for the Peregrines. Habitat selection had fitness consequences for Eagle Owls because breeding productivity increased with cliff length. Environmental conditions, particularly climatic factors, could allow the widespread coexistence of these competing raptors at the landscape scale, whereas at the local scale co‐occurrence could take place only on larger cliffs. These were preferred sites for both species, presumably because breeding at such sites offsets the costs of settling close to the competitor species.     

The decline of Common Kestrels Falco tinnunculus in a forested area of northern England: the role of predation by Northern Goshawks Accipiter gentilis

We have previously documented the decline of the Common Kestrel Falco tinnunculus over a 23‐year period in a large coniferous forest in northern England. Kestrels fed predominantly on Field Voles Microtus agrestis, which were most abundant in young plantations (1–11 years old). Over the 23 years, voles remained abundant in the study area, but their numbers fluctuated cyclically. Here we consider whether the decline of Kestrels was linked to predation by Northern Goshawk Accipiter gentilis. Goshawks first bred in the study area in 1973 and increased until 1989, after which numbers stabilized. We use a number of approaches to explore the role of Goshawk predation, all of which are correlative, but independent. First, there was a significant negative relationship between Kestrel and Goshawk numbers after controlling for a decline in vole habitat. Short‐eared Owls Asio flammeus, which also hunt by day, declined over the same period as Kestrels. Second, numbers of Tawny Owl Strix aluco and Long‐eared Owl Asio otus did not decline as Goshawk numbers increased. These two species are also vole‐dependent, but active by night, and less vulnerable to Goshawk attack. Third, six species of raptor comprised 4.5% of 5445 Goshawk prey items during the breeding season, but more Kestrels were killed than the combined total of all other raptors. Goshawks not only killed many adult Kestrels in early spring, prior to breeding, when it would have most impact on population levels, but there was also a temporal trend for predation on Kestrels to be inversely density‐dependent. Finally, we estimated that Goshawks removed more Kestrels than were recorded each spring in the study area. We interpreted this as indicating that immigrant Kestrels were being removed continually, mostly before they could breed. We conclude that the decline of Kestrels (and possibly Short‐eared Owls) was mainly due to predation by Goshawks. This study provides some of the strongest evidence yet of the role of predation in the hierarchical structuring of raptor communities.     

Tawny Owl Strix aluco as an indicator of Barn Owl Tyto alba breeding biology and the effect of winter severity on Barn Owl reproduction

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.     

Is the large‐scale decline of the starling related to local changes in demography?

The decline of one farmland bird, the migratory European starling, has been attributed to both agricultural intensification and farmland abandonment and to factors operating both during the winter and during the breeding season. We analysed population data from thirty‐three Swedish nestbox colonies over more than two decades to determine if the national decline was caused by a common factor affecting all colonies or by local changes in the breeding grounds affecting starling colonies. We found that numbers of breeding starling had declined significantly, but at different rates in different colonies. The local population sizes were affected by previous years’ productivity at both national and local scales, suggesting that changes in habitat quality at both scales could affect local population trends. There were no long‐term trends in reproductive output, but fledgling production was lowest at intermediate years. The local population changes were positively related to local changes in reproductive output, but only when including complete nest‐failures. A relationship between population declines and low mean local productivity was the result of the association between population sizes and reproductive success over time, since decline rates of starlings were not related to the average success during the first part of the study, but to the average success during the later part of the study. The relationship between population change and changes in reproductive output was evident, but fledgling production showed negative density‐dependence. In conclusion this study suggests that the decline of the starling population in Sweden has been affected by processes at small spatial scales during the breeding season affecting reproductive success, but does not exclude an additional role for processes at large spatial scales or outside the breeding season.     

Habitat does not influence breeding performance in a long‐term Barn Owl Tyto alba study

Capsule This study examines the relationship between habitat variables and various aspects of breeding and foraging performance for 257 Barn Owl breeding attempts involving both released and wild birds, at 86 different nest‐sites over a 14‐year period.

Aims The study aimed to: (1) provide evidence for or against the importance of foraging habitat in Barn Owl breeding performance; (2) enable identification of areas which can, and those which cannot, be expected to sustain Barn Owl populations; (3) inform the compilation of any future guidelines such that Barn Owl release schemes are more likely to succeed; and (4) allow the revision of untested concerns in terms of the likely survival or otherwise of Barn Owls in a given area.

Methods Three data sources are used to assess the proportions of various habitat types and lengths of linear features in the vicinity of each nest: (1) The Centre for Ecology and Hydrology Land Cover Map; (2) Ordnance Survey Strategi dataset; and (3) Agricultural Census data. These are linked to various aspects of breeding performance.

Results Despite the size of the dataset, the number of significant correlations between habitat type and aspects of Barn Owl breeding success was similar to that expected by chance. Sites with more unimproved grassland within 1 km of the nest did not differ from those with less, except by a significant advancement of first‐egg date.

Conclusion The paucity of significant results may be evidence that Barn Owls are in fact rather catholic and adaptable in their habitat requirements, and not as dependent upon large areas of Field Vole (or other) habitat as has often been stated.     

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

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

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

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

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

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

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

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

Simple signals indicate which period of the annual cycle drives declines in seasonal populations

For declining wild populations, a critical aspect of effective conservation is understanding when and where the causes of decline occur. The primary drivers of decline in migratory and seasonal populations can often be attributed to a specific period of the year. However, generic, broadly applicable indicators of these season‐specific drivers of population decline remain elusive. We used a multi‐generation experiment to investigate whether habitat loss in either the breeding or non‐breeding period generated distinct signatures of population decline. When breeding habitat was reduced, population size remained relatively stable for several generations, before declining precipitously. When non‐breeding habitat was reduced, between‐season variation in population counts increased relative to control populations, and non‐breeding population size declined steadily. Changes in seasonal vital rates and other indicators were predicted by the season in which habitat loss treatment occurred. Per capita reproductive output increased when non‐breeding habitat was reduced and decreased with breeding habitat reduction, whereas per capita non‐breeding survival showed the opposite trends. Our results reveal how simple signals inherent in counts and demographics of declining populations can indicate which period of the annual cycle is driving declines.     

Habitat composition as a determinant of reproductive success of Tengmalm's owls under fluctuating food conditions

The effect of landscape composition on the breeding success of vole-eating Tengmalm's owl ( Aegolius funereus ) was studied in western Finland at five different spatial scales (250–4000 m) around the nests during two consecutive three-year population cycles of voles. Landscape composition had strongest effects on owl breeding in the decrease phase of vole cycles. Significant variation in owl breeding occurred along the productivity gradient from farmland predominated areas to barren hinterland. Owls tended to produce earlier clutches on territories predominated by agricultural areas in increasing vole years. A similar trend was observed in the decreasing phase of the vole cycle; owls breeding on barren hinterland seemed to delay breeding compared to owls breeding near agricultural areas. Surprisingly, nestling survival and fledgling production in the decreasing phase declined steeply with increasing proportion of farmland. Clutch size was not significantly related to landscape composition. The number of fledglings decreased with increases in clear-cut and sapling areas in the decrease phase. During the declining years of vole abundance nestling survival increased from western farmland areas towards the eastern outlying district. These results indicate sudden summer decline of vole populations on farmland predominated habitats. This is probably due to that the number of vole-eating predators, and hence their impact on vole populations is apparently higher in farmland areas than on forested hinterland. This finding gives support for the 'spill-over' hypothesis, which states that predators and their exploitation tends to 'spill over' from luxuriant habitats to the barren habitats.     

Blood parasites in owls with conservation implications for the Spotted Owl (Strix occidentalis)

The three subspecies of Spotted Owl (Northern, Strix occidentalis caurina; California, S. o. occidentalis; and Mexican, S. o. lucida) are all threatened by habitat loss and range expansion of the Barred Owl (S. varia). An unaddressed threat is whether Barred Owls could be a source of novel strains of disease such as avian malaria (Plasmodium spp.) or other blood parasites potentially harmful for Spotted Owls. Although Barred Owls commonly harbor Plasmodium infections, these parasites have not been documented in the Spotted Owl. We screened 111 Spotted Owls, 44 Barred Owls, and 387 owls of nine other species for haemosporidian parasites (Leucocytozoon, Plasmodium, and Haemoproteus spp.). California Spotted Owls had the greatest number of simultaneous multi-species infections (44%). Additionally, sequencing results revealed that the Northern and California Spotted Owl subspecies together had the highest number of Leucocytozoon parasite lineages (n = 17) and unique lineages (n = 12). This high level of sequence diversity is significant because only one Leucocytozoon species (L. danilewskyi) has been accepted as valid among all owls, suggesting that L. danilewskyi is a cryptic species. Furthermore, a Plasmodium parasite was documented in a Northern Spotted Owl for the first time. West Coast Barred Owls had a lower prevalence of infection (15%) when compared to sympatric Spotted Owls (S. o. caurina 52%, S. o. occidentalis 79%) and Barred Owls from the historic range (61%). Consequently, Barred Owls on the West Coast may have a competitive advantage over the potentially immune compromised Spotted Owls.     

Effect of variation in forest harvest intensity on winter occupancy of Barred Owls and Eastern Screech‐Owls in deciduous forests of the east‐central United States

Responses of resident owls to changes in forest structure associated with forest management in the Central Hardwood Forest Region of the United States have not been widely studied. We estimated the winter occupancy of Barred Owls (Strix varia) and Eastern Screech‐Owls (Megascops asio) in forests with varying levels of timber harvest in southern Indiana. Surveys were conducted from 2009 to 2013 in two state forests undergoing active harvesting, a national forest unit with little recent harvesting, and a forested state park with no harvesting since about 1940. We hypothesized that 1) Barred Owl occupancy would vary inversely with harvesting intensity because they prefer mature forest, 2) Eastern Screech‐Owl occupancy would be greatest in the actively managed forests given their affinity for edge habitat, and 3) Eastern Screech‐Owls would avoid Barred Owl sampling areas and, thus, Barred Owl predation. Barred Owl occupancy was lowest in the non‐harvested state park relative to other areas, and Eastern Screech‐Owls were less likely to co‐occur in Barred Owl sampling areas. We found no evidence that forest management was inversely related to Barred Owl occupancy or that active edge creation associated with forest management increased the magnitude of Eastern Screech‐Owl occupancy. Reduced rates of Barred Owl occupancy in the state park may have resulted from high levels of recreational activity, whereas increased rates of Eastern Screech‐Owl occupancy in the park may have resulted from ample hunting opportunities in an environment with fewer Barred Owl predators. Recreational activity, prey availability, and interspecific antagonism likely have greater influences than timber harvesting on winter site occupancy of these owls in our study areas.     

Habitat suitability and demography,a time‐dependent relationship

The habitat suitability index, which reflects spatial variability in species occurrence probability, has been shown to exhibit various contrasting relationships with local demographic performances (survival, productivity) in several species. One proposed explanation for these discrepancies is that the link between the habitat suitability index and demography is influenced by density‐dependent, temporally variable processes. Based on the survival rates of more than 3,000 nests monitored over 12 years in the North African Houbara Bustard, we investigated whether the habitat suitability index computed over the species breeding range is related to nest survival throughout the breeding season, accounting for variation in meteorological conditions. We found that the relationship between the habitat suitability index and nest survival progressively changes along the breeding season and that this intra‐annual variation is consistent between years. Our results support the hypothesis that variation in space use occurs intra‐annually and that biotic interactions throughout the breeding season strongly influence the habitat suitability index–demography relationship.     

Landscape and anti-predation determinants of nest-site selection, nest distribution and productivity in a Mediterranean population of Long-eared Owls Asio otus

Nest predation is an important determinant of owl breeding success. We studied Long-eared Owl Asio otus productivity and attributes of nest-sites at the microhabitat and landscape scales in a Mediterranean locality over an 8-year period. We examined the effect on nest location and productivity of protective cover in concealing the nest from aerial and terrestrial predators. A dense cover of ivy and tree-foliage at canopy level favoured nest location but not productivity. By contrast, high shrub cover beneath the nest was selected by Owls and was positively related to both the site reoccupancy rate and the overall number of young fledged. Pre-fledging Owls use the ground, where they are exposed to terrestrial predators, which are much more abundant in the study area than are aerial predators. Our results therefore support the hypothesis that Owls adapt nest-site choice to local sources of predation risk. As reported elsewhere, Long-eared Owls in our study area showed restricted territoriality and nested in clusters. As active nest-sites during the same breeding season were more than 1 km apart on average, and their productivity was never greater for clustered nests than for more isolated nests, nest aggregation could not be interpreted as a case of facultative colonial breeding, which has been reported for this species in other areas. Neither landscape variables indicative of the availability of foraging areas nor structural attributes that protect young from predators explained the remarkable scarcity of nests in half of the study area. Unmeasured factors such as human disturbance could explain the pattern of distribution of Long-eared Owl nests.     

Noise pollution and decreased size of wooded areas reduces the probability of occurrence of Tawny Owl Strix aluco

At present, urban areas cover almost 3% of the Earth's land surface, and this proportion is constantly increasing along with human population growth. Although urbanization leads to biodiversity decline, at the same time it creates a novel and extensive environment that is exploited by whole assemblages of organisms. These include predators, which use the matrix of different habitat types within the urban environment for breeding and/or foraging. This study investigated how attributes of the urban landscape influence the distribution pattern of a nocturnal acoustic predator, the Tawny Owl Strix aluco. The probability of occurrence of this species was correlated with the presence of natural forests, and the increasing size of wooded habitat patches within the urban landscape; however, Tawny Owls were less likely to occur at sites with high noise levels at night. Our study suggests that the distribution pattern of acoustic predators is shaped by the availability of primary habitat but reduced by noise intensity (which may decrease hunting efficiency). The Tawny Owl is a top predator in the urban environment and its presence/absence may therefore affect populations of other species; this provides clear evidence of the indirect effect of noise pollution on animal populations inhabiting urban environments.