Breeding density,habitat selection and reproductive rates of the Peregrine Falcon Falco peregrinus in Álava (northern Spain)

A population of 33–35 pairs of Peregrine Falcon in álava (north Spain) was studied with the aim of assessing the habitat attributes that influence breeding density, habitat selection and breeding success. A strong relationship was found between density of the species in each UTM (Universal Transverse Mercator) square of 10 × 10 km and cliff availability. Habitat selection was analysed by comparing 15 variables in 33 occupied and 25 unoccupied cliffs located at least 2 km from the nearest Peregrine pair. Significant differences were found in five variables: cliff dominance, distance to the nearest Golden Eagle Aquila chrysaetos nesting cliff, steepness and altitude (all showing larger values in occupied cliffs), and cliff orientation, with occupied cliffs facing preferably south and east. Orientation, dominance and distance to the nearest Golden Eagle pair, and the distance to the nearest Eagle Owl Bubo bubo nesting cliff, were included in a discriminant analysis which classified 82.76% of the cliffs correctly. The productivity of the studied population was 1.44 young/territorial pair (n = 45), and no consistent relationship was found between breeding success and habitat variables.     

Distribution and behaviour of passerines around Peregrine Falco peregrinus eyries in western Greenland

This paper describes the distribution and behaviour of passerine birds within 1 km of Peregrine Falcon Falco peregrinus eyries in western Greenland. Passerine populations were censused in 1983 around six cliffs occupied by nesting Peregrines and at six sites on the open tundra, and in 1984 at two unoccupied sites suitable for Peregrine nesting.
Four passerine species accounted for over 90% of birds seen. Within 400 m of Peregrine occupied cliffs, three species, Lapland Longspur Calcarius lapponicus . Northern Wheatear Oenanthe oenanthe , and Common Redpoll Carduelisflammea , were found in lower than average abundance while the fourth, Snow Bunting Plectrophenax nivalis , was found in increased abundance. Our indirect tests suggest that Peregrine presence, rather than direct predation, was responsible for the distributions of the first three species. The Snow Bunting, however, behaviourally adapts to Peregrine presence in order to exploit nesting habitat available at the cliff base.
These results contrast strongly with those reported by other Arctic workers, where increased abundances of potential prey were observed in close proximity to nesting Peregrines.     

Characteristics of Peregrine and Lanner Falcon nesting habitats in South Africa

Jenkins, A.R. 2000. Choracteristics of Peregrine and Lanner Falcon nesting habitots in South Africa. Ostrich 71 (3&4): 416-424. Peregrine Falcon, Falco peregrinus, and Lanner Falcon, F. biarmicus, nesting habitats in South Africa are described in terms of nest ledge, nest cliff and scree slope parameters, ond the environmental context of each site. Data were collected in a tropical study area where Peregrines and Lannen occurred in sympatry, from an allopatric south temperate Peregrine population, and opportunistically for bath species from other sites throughout the country. Elevation, ledge size, cliff size ond vegetation structure accounted for about 80%of the variation in falcon nesting habitots between species and between populations within species. Peregrines used larger nest ledges on higher, more elevated cliffs than Lanners, overlooking mare open, structurally complex Vegetation. Temperate Peregrines used smaller ledges on lower cliffs, overlooking vegetation that provided less refuge for overflying prey than subtropical pairs. Temperote Peregrines selected nest sites to minimize the negative effects of spring roinfall. Tropical Peregrines may have selected sites to maximize hunting and provisioning efficiency.     

Hunting mode and success of African Peregrines Falco peregrinus minor,does nesting habitat quality affect foraging efficiency?

Peregrine Falcon Falco peregrmus hunting behaviour was studied at nest-sites in three areas of South Africa over eight years. In Africa, resident Peregrines are mostly restricted to high cliffs, possibly because these structures provide optimal conditions for hunting. This hypothesis is examined in terms of the influence of nest-site quality, particularly cliff height, on foraging efficiency. Foraging mode varied considerably between sites, and males foraged more actively than females but there was little variation in the design of hunts between sexes, seasons or study areas. Individually, Peregrines spent 30–50% of the day on or near the nest cliff. On average, about 0.5 hunts were recorded per hour of observation. Foraging mode was not correlated with cliff height or elevation above the surrounding terrain, but Peregrine pairs occupying higher cliffs achieved greater hunting success rates. Most hunts were initiated from elevated perches on the nest cliff, and perch hunts were more successful than strikes made from the air. Success was highest in strikes at doves and small passerines, and over habitats with moderate cover. The height difference between Peregrine and prey at the start of a hunt positively and significantly affected hunting success. Overall, Peregrines were relatively sedentary and made extensive use of the nesting habitat as a foraging area. High nest cliffs contributed to foraging success by providing perch-hunting falcons with an effective height advantage over their prey.     

Conservation status and community structure of cliff-nesting raptors and ravens on the Cape Peninsula,South Africa

We detail the sizes, spatial distributions and trends in nest site selection of cliff-nesting raptor and raven populations resident in the mountains of the Cape Peninsula, South Africa. We also assess the conservation value of these populations to inform the future management of the newly-established Table Mountain National Park (TMNP), and examine the structure and interrelations within the raptor community. The combined number, dispersion and density of nests (n = 96 nests, mean inter-nest distance = 0.59km, density = 30.0 pairs/100km²) are comparable with those of high-density raptor populations studied elsewhere in Africa and the world. Densities of Verreauxs' Eagle Aquila verreauxii (n = 2, 12.01km, 0.6 pairs/100km², respectively) and Jackal Buzzard Buteo rufofuscus (n = 9, 4.63, 2.8 pairs/100km²), are low, Rock Kestrel Falco rupicolus (n = 44, 1.75km, 13.8 pairs/100km²) high and Peregrine Falcon Falco peregrinus (n = 19, 3.13km, 5.9 pairs/100km²) exceptional, relative to populations of the same or similar species in other areas. There are no comparable data for White-necked Raven Corvus albicollis (n = 22, 3.22km, 6.9 pairs/100km²). All species combined, and Peregrines in particular, significantly prefer high cliffs from the available habitat. Peregrines generally dominate the other species, may affect cliff site selection and dispersion in the rest of the community, and tend to locate their nests close to those of White-necked Ravens. Numbers of Verreauxs' Eagle are lower than recent historical levels, perhaps because key prey populations are depleted. Any future recovery of this large predator could subtly affect the entire assemblage. This cliff-nesting raptor community is a significant asset of the TMNP, and should be considered in management decisions taken in the Park, particularly those concerning the regulation of leisure activities in the vicinity of nesting areas.     

Factors affecting breeding success of Peregrine and Lanner Falcons in South Africa

Jenkins, A.R. 2000. Factors affecting breeding success of Peregrine and Lonner Falcons in South Africa. Ostrich 71 (384): 385-392. Breeding success was recorded for three Peregrine Falcon, Falco peregrinus, populations in South Africa over nine years, and for Peregrine and Lanner Falcon, Ebiarmicus, populations in an area of sympatry over three years. The objectives of the study were to measure geographic and interspecific variation in reproductive performance, and determine environmental correlates of productivity. Territory occupancy, the frequency of breeding per occupied territory and clutch size did not vary significantly between the three Peregrine populations. However, Peregrine breeding success was generally lower on the Cape Peninsula (1. 11 young fledged per territorial poir), higher in the Soutpansberg (1.36) and highest on the Orange River (1.70). Overall, fledging rates of Soutpansberg Peregrines and Lanners were not significantly different, although annual productivity of the Lonner population was consistently higher. Neither species' breeding success was significantly depressed by the presence of close neighbouring pairs of the other, suggesting that they were not active competitors. Breeding performance of Peregrines on the Cape Peninsula correlated strongly with spring weather conditions: egg and hatchling survival was lower in wet years, and fledging rates were higher in warm years. Annual productivity of Orange River Peregrines correlated positively with the height of the river at the onset of breeding, and productivity of Soutpansberg Peregrines was higher in seasons following years of high rainfall. Elements of the physical structure of the nesting habitat (exposure of the nest ledge, height of the nest cliff) correlated positively with Peregrine breeding performance. Breeding success of Soutpansberg Lanners was largely unaffected by any of the environmental variables considered. Overall, it is proposed that Peregrine productivity reflected variation in the physical environment and its affect on prey availability.     

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.     

THE INFLUENCE OF HABITAT ON THE DISTRIBUTION AND ABUNDANCE OF PEREGRINE AND LANNER FALCONS IN SOUTH AFRICA

Jenkins, A.R. 1994. The influence of habitat on the distribution and abundance of Peregrine and Lanner Falcons in South Africa. Ostrich 65: 281–290.

The distribution and abundance of Peregrine and Lanner Falcons in South Africa was compared using recorded sightings from various sources, including the Southern African Bird Atlas Project. Falcon distributions were compared with the distribution of cliffs and vegetation, to quantify differences in the habitat preferences of the two species in the breeding and the non-breeding seasons. Lanner Falcons outnumbered Peregrine Falcons in most areas by at least 10:1. Peregrine Falcons were more habitat specific than Lanner Falcons, in terms of topographic and biotic requirements. Peregrine Falcons were largely restricted to high cliff areas throughout the year and there probably were no large-scale seasonal movements within the population. The bulk of the resident Peregrine Falcon population was found in the fynbos biome, in the southwestern Cape. Outside of this area, Peregrine Falcons were concentrated in woodlands. Lanner Falcons were less dependent on high cliffs, although cliff availability was important in defining the ranges of both species. Lanner Falcons were most common in the sour grasslands in the east of the country in the breeding season, with apparent movements in the non-breeding season into the fynbos, the Nama Karoo and the southern Kalahari. Overall, Peregrine Falcons favoured relatively closed habitats and Lanner Falcons favoured relatively open habitats. The differences in the two species' habitat preferences are proximate factors influencing distribution and abundance.     

Multi‐season occupancy models identify biotic and abiotic factors influencing a recovering Arctic Peregrine Falcon Falco peregrinus tundrius population

Critical information for evaluating the effectiveness of management strategies for species of concern include distinguishing seldom occupied (or low‐quality) habitat from habitat that is frequently occupied and thus contributes substantially to population trends. Using multi‐season models that account for imperfect detection and a long‐term (1981–2002) dataset on migratory Arctic Peregrine Falcons Falco peregrinus tundrius nesting along the Colville River, Alaska, we quantified the effects of previous year's productivity (i.e. site quality), amount of prey habitat, topography, climate, competition and year on occupancy dynamics across two spatial scales (nest‐sites, cliffs) during recovery of the population. Initial occupancy probability was positively correlated with area of surrounding prey habitat and height of nest‐sites above the Colville River. Colonization probability was positively correlated with nest height and negatively correlated with date of snowmelt. Local extinction probability was negatively correlated with productivity, area of prey habitat and nest height. Colonization and local extinction probabilities were also positively and negatively correlated, respectively, with year. Our results suggest that nest‐sites (or cliffs) along the Colville River do not need equal protection measures. Nest‐sites and cliffs with historically higher productivity were occupied most frequently and had lower probability of local extinction. These sites were on cliffs high above the river drainage, surrounded by adequate prey habitat and with southerly aspects associated with early snowmelt and warmer microclimates in spring. Protecting these sites is likely to encourage continued occupancy by Arctic Peregrine Falcons along the Colville River and other similar areas. Our findings also illustrate the importance of evaluating fitness parameters along with climate and habitat features when analysing occupancy dynamics, particularly with a long‐term dataset spanning a range of annual climate variation.     

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.     

FIRST ANNUAL REPORT ON THE RHODESIAN ORNITHOLOGICAL SOCIETY'S NEST RECORD SCHEME FOR THE YEAR ENDED 30 JUNE, 1956

MENDELSOHN, J. M. 1989. Habitat preferences, population size, food and breeding of six owl species in the Springbok Flats, South Africa. Ostrich 60:183-190.

Information on the habitat preferences, population size, food and breeding of Barn, Grass, Whitefaced, Marsh, Pearlspotted and Spotted Eagle Owls was obtained in a 6900-ha area in the Springbok Flats, South Africa. Seventy-two per cent of the area consisted of cultivated fields not usually used by owls. Hunting, roosting and nesting requirements were largely met in 1930 ha of verges, farmyards and patches of wood land ant grassland, here was an estimated total population of 303 owls in the area, giving an overall density of 22,7 ho/owl for the whole area or 6.4 ha/owl for those areas used by owls. These high densities were attributed to an abundance of Mastomys natalensis, the most important prey item for all except Pearlspotted Owls. Rates of predation on M. natalensis varied in relation to their population density, as indicated by rodent trapping results. Marsh Owls ate more insects in summer than at other times. Barn and Marsh Owls usuall laid in March-April and August-September, while other species started breeding in July-October. de timing of breeding of some owls may be related to changes in rates of recruitment of juvenile M. natalensis. Most Marsh Owl nests were placed on the southwestern sides of grass clumps or shrubs.     

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.     

Identifying key habitats for the conservation of Bonelli's Eagle Aquila fasciata using radiotracking

Identifying the priority habitats of endangered species is crucial to implementing effective conservation actions. We characterize the key habitats used by Bonelli's Eagle Aquila fasciata, an umbrella and flagship species that is endangered in Mediterranean countries. We radiotracked 17 breeding individuals (10 males and seven females) in Catalonia (NE Iberian Peninsula) and used compositional analysis to determine the key habitats in home‐ranges of both sexes. The main habitats identified within the home‐range area were scrubland, coniferous forests, cropland, sclerophyllous forests, rock outcrops and urban areas, with little difference in habitat use between the sexes. Bonelli's Eagles preferred rocky habitats as breeding areas and scrubland as foraging areas, as these hold the highest abundance of their main prey, Rabbits Oryctolagus cuniculus and Red‐legged Partridges Alectoris rufa. Habitat selection varied seasonally in foraging areas: scrubland was the most preferred habitat type during the breeding season, whereas rocky areas were preferred during the non‐breeding season (although scrubland was also highly selected). Urban areas were avoided both as breeding and as foraging areas. Home‐range size was inversely correlated with the proportion of scrubland, suggesting that this is a key habitat for Bonelli's Eagle. To conserve this species effectively, policies that ensure the preservation of the cliffs used as breeding sites, as well as suitable management of the scrubland used for foraging, should be implemented in the areas inhabited by this species. The promotion of traditional land uses and management techniques that will enhance open areas in Mediterranean landscapes should in the future play an essential role in the conservation of Bonelli's Eagle in Mediterranean habitats.     

Surprise as a winter hunting strategy in Sparrowhawks Accipiter nisus,Peregrines Falco peregrinus and Merlins F. columbarius

Sparrowhawks Accipiter nisus, Peregrines Falco peregrinus and Merlins Falco columbarius were studied hunting Redshanks Tringa totanus, Dunlins Calidris alpina and Skylarks Alauda arvensis over three winters on a small Scottish estuary. Most Sparrowhawk and Merlin hunts consisted of a single attack (mean = 1.0 and 1.1, respectively), whereas Peregrine hunts often consisted of several attacks (mean = 1.8). Most hunts were short (<1 min), but Peregrine and Merlin hunts occasionally lasted over 5 min. In general, all three raptor species attacked by surprise, although Peregrines were more likely to make nonsurprise attacks. Prey attacked were usually initially very close to the raptor (<100 m); Peregrines attacked prey most often at long distances (>500 m). Chase lengths were mostly <5 second in length, although Peregrines, and particularly Merlins hunting Skylarks, often chased for several minutes. Peregrines attacked most prey in flight from flight, while Merlins and Sparrowhawks attacked birds on the ground with a flight from a perch. All three raptor species preferentially attacked larger Dunlin flocks, but Peregrines also favoured single birds. Capture rates of Redshanks and Dunlins were similar for the three raptor species (C. 10%), but for Skylarks, capture rate by Merlins was much higher (12%) than by Sparrowhawks (3%) or Peregrines (0%). Capture rates were highest when raptors attacked by surprise, particularly for a Peregrine hunting in the first minute of arrival on the study site if no Peregrines had been hunting there for the previous hour (16% success for the first minute compared with 2% in subsequent minutes). Sparrowhawks were more successful when attacking small rather than large Dunlin flocks. The use of short surprise attacks interspersed with long periods of inactivity was common to the three raptor species and was interpreted as a strategy to minimize the amount of energy and risk involved in hunting during the nonbreeding season.     

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

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

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.     

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.     

Components of breeding performance in two competing species: habitat heterogeneity, individual quality and density-dependence

Density‐dependent breeding performance due to habitat heterogeneity has been shown to regulate populations of territorial species, since the progressive occupation of low quality territories as breeding density increases may cause a decline in the mean per capita fecundity of a population while variation in fecundity increases. Although the preemptive use of sites may relegate low quality individuals to sites of progressively lower suitability, few studies on density dependence have tried to separate the effects of territory quality from individual quality, and none have simultaneously considered the effects of heterospecific competitors. Using two long‐term monitored populations, we assessed the relative contribution of habitat heterogeneity and bird quality (in terms of age) on the productivity of sympatric golden Aquila chrysaetos and Bonelli's eagles Hieraaetus fasciatus under different scenarios of intra‐ and inter‐specific competition. Productivity (number of offspring fledged) varied among territories and average annual productivity was negatively related to its variability in both species and populations, thus giving some support to the habitat heterogeneity hypothesis. However, the effect of habitat heterogeneity on productivity became non‐significant when parental age and local density estimators were included in multivariate analyses. Therefore, temporal changes in bird quality (age) combined with intra‐ and interspecific competition explained variability in territory productivity rather than habitat heterogeneity among territories per se. The recruitment of subadult breeders, a surrogate of mortality in eagles, strongly varied among territories. Habitat heterogeneity in productivity may thus arise not because sites differ in suitability for reproduction but because of differences in factors affecting survival. Territories associated with high mortality risks have a higher probability of being occupied by young birds, whose lower quality, interacting with the density competitors, leads to a reduction of productivity. Site‐dependent variability in adult survival and interspecific competition may be extensive, but so far largely overlooked, factors to be seriously considered for the site‐dependent population regulation framework.     

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.     

Nest-site selection of the Long-eared Owl Asio otus in northwestern Switzerland

Capsules Sites are selected as part of an antipredator strategy.

Aims To assess if site choice depends on habitat variables at nest sites and if habitat quality influences reproduction.

Methods Breeding density was explored in northwestern Switzerland from 1992 to 1996. Habitat variables were examined at 38 breeding sites and were compared with data collected from random sites. Habitat quality was estimated using the discriminant function scores of the nesting sites.

Results Breeding density was found to vary between years; more pairs bred and raised more young in 1993, a year of high vole abundance. Long-eared Owls tended to avoid the vicinity of buildings; they occupied sites with denser forest edges, greater canopy cover, and with more conifers than random sites. I found no statistical evidence that they used less optimal sites when the population was high. The number of fledglings increased with habitat quality, but did not vary with any of the habitat variables taken separately.

Conclusion Long-eared Owl selects nesting habitat as part of an anti-predator strategy, but the measures of territory quality did not seem to be a limiting factor for the population.