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Although the costs of reproduction are predicted to vary with the quality of the breeding habitat thereby affecting population dynamics and life-history trade-offs, empirical evidence for this pattern remains sparse and equivocal. Costs of reproduction can operate through immediate ecological mechanisms or through delayed intrinsic mechanisms. Ignoring these separate pathways might hinder the identification of costs and the understanding of their consequences. We experimentally investigated the survival costs of reproduction for adult little owls (Athene noctua) within a gradient of habitat quality. We supplemented food to nestlings, thereby relieving the parents’ effort for brood provisioning. We used radio-tracking and Bayesian multistate modeling based on marked recapture and dead recovery to estimate survival rates of adult little owls across the year as a function of food supplementation and habitat characteristics. Food supplementation to nestlings during the breeding season increased parental survival not only during the breeding season but also during the rest of the year. Thus, the low survival of parents of unfed broods likely represents both, strong ecological and strong intrinsic costs of reproduction. However, while immediate ecological costs occurred also in high-quality habitats, intrinsic costs carrying over to the post-breeding period occurred only in low-quality habitats. Our results suggest that immediate costs resulting from ecological mechanisms such as predation, are high also in territories of high habitat quality. Long-term costs resulting from intrinsic trade-offs, however, are only paid in low-quality habitats. Consequently, differential effects of habitat quality on immediate ecological and delayed intrinsic mechanisms can mask the increase of costs of reproduction in low-quality breeding habitats. Intrinsic costs may represent an underrated mechanism of habitat quality affecting adult survival rate thereby considerably accelerating population decline in degrading habitats. This study therefore highlights the need for a long-term perspective to fully assess the costs of reproduction and the role of habitat quality in modifying these costs.

Habitat models are fundamental tools for designing evidence-based conservation measures, particularly for locating sites with high potential for promoting a species' recolonisation and occupancy. However, it remains challenging to respond to both the need for large-scale general rules, and for fine-scale information concurrently. Multi-level habitat models provide all-in-one surfaces that explicitly account for conditional dependencies among single-level selection probabilities. We integrated occurrence data obtained from citizen-science species observation data with radio-tracking data to develop multi-level resource selection functions for the little owl (Athene noctua), a species of conservation concern in Central Europe. The results of our habitat selection analyses confirmed that suitable little owl habitat is located in widely open agricultural landscapes that often exist in the vicinity of human settlements. We mapped habitats at fine resolution (40 × 40 m) over an area covering 77,313 km2 in Switzerland and Baden-Wurttemberg, Germany. We validated the models with external out-of-sample data, and we demonstrated good predictive ability and transferability over the broad landscape. Overall, a fifth of the modelled landscape was estimated to be suitable for little owls. Habitat suitability scores in Switzerland were generally lower than in Baden-Wurttemberg due to higher elevation, fewer orchards, and more forest patches. Extant populations currently occupy c. 15% of the potential suitable habitats in Baden-Wurttemberg, and 2% in Switzerland, suggesting that considerable space for recolonisation is available. However, while Baden-Wurttemberg offers vast open landscapes, lowlands in Switzerland show narrow swaths of habitat along valleys and lakes. We showed that the simultaneous integration of different levels of habitat selection behaviour into a multi-level habitat suitability map creates a promising tool for conservation planning of endangered species over large geographical areas. Our multi-level model allowed for identification of both large-scale habitat suitability patterns to develop conservation strategies, and fine-scale clusters of high quality habitats where conservation measures can be applied at once, thereby increasing relevance of such all-in-one habitat maps for policy makers, wildlife managers and conservations practitioners alike.

Food availability is a major characteristic of habitat quality, linking habitats with demographic parameters such as reproductive performance. Parent birds adjust their food provisioning to both habitat-specific food characteristics and the demands of their young. However, because habitat quality and the brood's food intake are often correlated, the underlying mechanisms of adjustments in parental provisioning remain entangled. How the relationship between habitat quality and parental provisioning behaviour affects the quantity of food available to nestlings and the resulting nestling growth and survival is therefore still incompletely established. We experimentally increased the food intake of little owl, Athene noctua, nestlings in two habitat types differing in food availability and used unsupplemented broods as controls. The food supplementation experiment allowed us to disentangle the effect of habitat type from the effect of the nestlings' food intake on parental provisioning behaviour. Camera traps recording a series of 10 consecutive images for each parental visit allowed us to quantify visiting rates and diet composition by applying a hierarchical multinomial model explicitly accounting for the observation process. Food supplementation caused parents to switch to smaller food items and to increase visiting rates, resulting in similar biomass brought to nestlings in supplemented and unsupplemented broods. Irrespective of the food supplementation, parents in low-quality habitats delivered 63% of the biomass delivered by those in high-quality habitats. Accordingly, we found an increase in nestling survival rates in response to food supplementation in low-quality habitats, but not in high-quality habitats. Our results show that habitat quality affects the biomass of prey delivered to the brood, whereas the nutritional state of the brood affects prey selection or foraging modes of parents. Reproductive output directly reflected habitat quality in terms of food availability, identifying food as the main factor underlying differential reproduction within and between populations.

Understanding the rules of habitat selection and the individual behavioural routines in the home-range is crucial for developing evidence-based conservation action. We investigated habitat selection and range use of adult little owls Athene noctua in relation to landscape configuration, habitat structure and resource distribution. We determined the preference of habitat structures by VHF-telemetry. Large- and fine-scale distribution patterns of voles – the main prey during the breeding season – were assessed by transect counts of signs of vole presence. An experiment using artificial perches was carried out to determine the fine-scale adjustment of the owls' range use in relation to prey abundance and vegetation height. Habitat selection and resource exploitation by little owls were structured at all spatial levels: (1) at the landscape scale, orchards were highly preferred over other areas. This accords with the patchy large-scale occurrence of voles, which were absent in cropland, but abundant in orchards and grassland; (2) within home-ranges, the spatial distribution of voles was highly inhomogeneous and structures with high prey abundance were used over-proportionally; (3) at the scale of foraging sites, little owls preferred patches with low vegetation over those with high prey abundance, establishing that prey availability is the crux. The results suggest that all levels of habitat selection and range use were related to farming practices and affected by current cultivation. Conservation measures should focus on the conservation and restoration of orchards on the landscape level and habitat management measures should focus on grasslands – the main food providers – by creating a mosaic of patches with short grass and tall grass. Together with other habitat structures providing food resources such as field edges, wildflower areas and structures facilitating access to prey, the quality of habitat patches in terms of food availability may be highly improved.

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.

Equivocal knowledge of the phase-specific drivers of natal dispersal remains a major deficit in understanding causes and consequences of dispersal and thus, spatial dynamics within and between populations. We performed a field experiment combining partial cross-fostering of nestlings and nestling food supplementation in little owls ( Athene noctua ). This approach disentangled the effect of nestling origin from the effect of the rearing environment on dispersal behaviour, while simultaneously investigating the effect of food availability in the rearing environment. We radio-tracked fledglings to quantify the timing of pre-emigration forays and emigration, foray and transfer duration, and the dispersal distances. Dispersal characteristics of the pre-emigration phase were affected by the rearing environment rather than by the origin of nestlings. In food-poor habitats, supplemented individuals emigrated later than unsupplemented individuals. By contrast, transfer duration and distance were influenced by the birds' origin rather than by their rearing environment. We found no correlation between timing of emigration and transfer duration or distance. We conclude that food supply to the nestlings and other characteristics of the rearing environment modulate the timing of emigration, while innate traits associated with the nestling origin affect the transfer phases after emigration. The dispersal behaviours of juveniles prior and after emigration, therefore, were related to different determinants, and are suggested to form different life-history traits.

Dispersal is crucial for population viability and thus a popular target for conservation measures. However, the ability of individuals to move between habitat patches is notoriously difficult to estimate. One solution is to quantify functional connectivity via realistic individual-based movement models. Such simulation models, however, are difficult to build and even more difficult to parameterize. Here, we use the example of natal little owl (Athene noctua) dispersal to develop a new analysis chain for the calibration of individual-based dispersal models using a hybrid of statistical parameter estimation and Approximate Bayesian Computation (ABC). Specifically, we use locations of 126 radio-tracked juveniles to first estimate habitat utilization by generalized additive models (GAMs) and the biased random bridges (BRB) method. We then include the estimated parameters in a spatially explicit individual-based model (IBM) of little owl dispersal and calibrate further movement parameters using ABC. To derive efficient summary statistics, we use a new dimension reduction method based on random forest (RF) regression. Finally, we use the calibrated IBM to predict the dispersal potential of little owls from local populations in southwestern Germany to suitable habitat patches in northern Switzerland. We show that pre-calibrating habitat preference parameters while inferring movement behavioral parameters via ABC is a computationally efficient solution to obtain a plausible IBM parameterization. We also find that dimension reduction via RF regression outperforms the widely used least squares regression, which we applied as a benchmark approach. Estimated movement parameters for the individuals reveal plausible inter-individual and inter-sexual differences in movement behavior during natal dispersal. In agreement with a sex-biased dispersal distance in little owls, females show longer individual flights and higher directional persistence. Simulations from the fitted model indicate that a (re)colonization of northern Switzerland is generally possible, albeit restricted. We conclude that the presented analysis chain is a sensible work-flow to assess dispersal connectivity across species and ecosystems. It embraces species- and individual-specific behavioral responses to the landscape and allows likelihood-based calibration, despite an irregular sampling design. Our study highlights existing, yet narrow dispersal corridors, which may require enhancements to facilitate a recolonization of little owl habitat patches in northern Switzerland.

Although biased offspring sex ratios are common in species with sexual size dimorphism, the proximate causes are often unresolved. This is because two general mechanisms operating in different ways and in various periods of reproduction can lead to the bias: sex-biased survival or parental sex-allocation. We examined nestling sex ratio patterns between hatching and fledging, sexual size dimorphism, and factors affecting nestling survival using growth and survival data of 846 individual little owl Athene noctua nestlings with known sex from 307 broods from Germany, the Netherlands and Denmark. Nestling sex ratio was female-biased, mainly due to a significant female bias in the first-hatched chicks. Females showed a higher body weight than male nestlings at ringing and body weight of nestlings decreased with hatching sequence. Nestling survival was higher in females (Φ = 0.91) than in males (Φ = 0.85), and survival rates were positively related to body mass and negatively to brood size. Although the observed lower survival of males can cause an overall female-biased sex ratio, the sex dimorphism and survival patterns found here are unlikely to explain the conspicuous sex ratio pattern with a female bias in the first-hatched nestlings and the increase in female bias across the season. Thus, these results point towards interacting mechanisms of parental sex allocation strategies and sex-specific survival. As the female bias was allocated to the first rank that is most likely to survive, the female bias will increase under suboptimal breeding conditions. We therefore suggest that under suboptimal ecological conditions, higher investment into females is adaptive in little owls. Biased sex ratios can have severe effects on the social behaviour and population dynamics of endangered species. However, the existence of subtle sex ratio bias is often unknown and its proximate mechanisms and ultimate consequences often remain unclear. Small sample sizes make the detection of subtle effects unlikely and often fail to disentangle diverging mechanisms such as sex-biased survival and parental sex allocation. We used a large dataset of 846 little owl nestlings from 307 broods from three countries to investigate offspring sex ratio patterns, sexual size dimorphism and nestling survival simultaneously. Our findings suggest interacting mechanisms of parental sex allocation strategies and sex-specific survival to drive biased offspring sex ratios in little owls. The context dependence of the sex ratio bias indicates that offspring sex ratio bias in little owls is both, a consequence of—and an adaptation to—suboptimal breeding conditions.

For altricial young, fledging is an abrupt step into an unknown environment. Despite increasing numbers of studies addressing the post-fledging period, our current knowledge of the causes and consequences of post-fledging survival remains fragmentary. Here, we review the literature on post-fledging survival of juvenile altricial birds, addressing the following main questions: Is low post-fledging survival a bottleneck in the altricial reproductive cycle? What is known of proximate and ultimate causal factors such as trophic relations (food and predation), habitat conditions, or abiotic factors acting in the post-fledging period? We analyzed weekly survival estimates from 123 data series based on studies of 65 species, covering weeks 1–13 post-fledging. As a general pattern, survival of fledglings was low during the first week post-fledging (median rate = 0.83), and improved rapidly with time post-fledging (week 4 median rate = 0.96). For ground-nesting species, survival immediately after leaving nests was similar to egg-to-fledging survival. For species breeding above-ground, survival during the first week post-fledging was substantially lower than during both the nestling period and later post-fledging stages. Thus, the early post-fledging period is a bottleneck of markedly elevated mortality for most altricial species. Predation was the main proximate cause of mortality. Various factors such as habitat, annual and seasonal variation in the environment, and the physical condition of fledglings have been found to affect post-fledging survival. Individual survival depended strongly on physical traits such as mass and wing length, which likely influence the ability of fledglings to escape predation. Trophic relationships at various levels are the main ultimate driver of adaptation of traits relevant to survival during the pre- and post-fledging periods. Spatiotemporal dynamics of food resources determine the physical development of juveniles and, in turn, their performance after fledging. However, predators can cause quick and efficient selection for fledgling traits and adult breeding decisions. Parental strategies related to clutch size and timing of breeding, and the age and developmental stage at which young fledge have substantial effects on post-fledging survival. The intensity and duration of post-fledging parental investment also influences fledgling survival. Post-fledging mortality is therefore not a random and inevitable loss. Traits and strategies related to fledging and the post-fledging stage create large fitness differentials and, therefore, are integral, yet poorly understood, parts of the altricial reproductive strategy.

Harvesting often results in the destruction of both, the nest and the surrounding habitat of ground-nesting farmland birds. While the direct effect of harvesting on nest loss and productivity is well investigated, the behavioural responses of parent birds to harvest events remain ambiguous, though they might have profound consequences for farmland bird populations. The progressive harvest of fields within a pair’s home-range represents a gradual decrease in habitat quality. The response of breeding individuals to anthropogenic nest loss therefore might depend on the extent of remaining suitable habitat. Here, we radio-tracked a cohort of breeding whinchats Saxicola rubetra affected by anthropogenic nest loss due to harvesting. We used the meadow management as a natural experiment, destroying nests at various stages of the mowing process and leaving different relict areas of suitable habitat after nest loss. Divorce and within-season dispersal rates after nest loss were strongly increased in home-ranges with a high proportion of mown area. Re-nesting rates were six times higher in non-dispersing than in dispersing individuals. Thus, dispersing individuals contributed only little to the overall re-nesting rate and to the productivity of the population. This suggests that the habitat-dependent behavioural responses to harvesting can have major effects on farmland birds’ reproductive output. Since harvesting in agricultural landscapes affects many breeding pairs, the resulting changes in the social patterns, the spatial dynamics and the re-nesting rate should be considered in the conservation of farmland bird populations. 2015 Elsevier Ltd. All rights reserved.

In altricial birds, the great effort involved in supplying food to nestlings can create trade-offs in the allocation of resources between the current brood and parental self-maintenance. In poor foraging conditions, parents have to adjust their energy expenditure in relation to the increased foraging costs. However, intra-specific variation in parental energy expenditure has rarely been evaluated in the context of these trade-offs. Here, we quantified the daily energy expenditure (DEE) of parent Barn Swallows Hirundo rustica during the nestling period in relation to foraging conditions while controlling for differences in brood size and nestling age. DEE varied substantially with environmental conditions, increasing by 10kJ/day per 5 degrees C in ambient temperature, and by 11kJ/day per hour in day length. Parent birds did not compensate for a poor aerial insect supply on cool days, but reduced their DEE. Parents only slightly buffered a negative energy balance during chick provisioning with stored body reserves. They did not sacrifice their own energy demands to keep up a high energy flow to the brood when foraging conditions were poor. Instead they worked harder when foraging conditions allowed a surplus intake, fully compensating for their additional efforts, and made maximum use of the rich food supply, allowing the brood to accrue body reserves to compensate for low food intake on cold days. This strategy of energy management may have evolved in the context of the adaptation to the aerial foraging mode and to the ephemeral nature of aerial food resources.

In birds, energy supply during growth is a major predictor of the fledglings' physical condition and survival prospects. Differential quantity and quality of fledglings produced under varying nestling food supplies are likely to affect the number of offspring that recruit into the breeding population. However, the underlying mechanisms and associated consequences are still poorly known. Using a partial cross-fostering and food supplementation experiment, we estimated the effect of variation in food supply during growth on nestling survival and fledgling phenotypic traits of Little Owls Athene noctua. Survival to fledging was much higher in food-supplemented nestlings (98.6%) than in control nestlings (82.4%). Furthermore, supplemented nestlings were on average 8.9 g heavier and were more likely to develop subcutaneous fat deposits (99.4 vs. 73.7% of treatment and control nestlings, respectively). Supplemented nestlings also had on average longer wings than control nestlings, but tarsi and culmen did not differ significantly. Furthermore, experimentally supplemented fledglings struggled more when handled and emerged sooner from tonic immobility than control fledglings. The irises of supplemented fledglings were less intensely coloured. The experimentally induced changes in nestling development probably affect individual performance beyond fledging. Nestlings from orchard-dominated habitats were larger than those from habitats dominated by arable land. As nestling food supply is largely determined by natural food availability, we conclude that habitat quality affects Little Owl productivity and offspring quality, and ultimately, population dynamics.

In migrant birds, survival estimates for the different life-history stages between fledging and first breeding are scarce. First-year survival is shown to be strongly reduced compared with annual survival of adult birds. However, it remains unclear whether the main bottleneck in juvenile long-distant migrants occurs in the postfledging period within the breeding ranges or en route. Quantifying survival rates during different life-history stages and during different periods of the migration cycle is crucial to understand forces driving the evolution of optimal life histories in migrant birds. Here, we estimate survival rates of adult and juvenile barn swallows (Hirundo rusticaL.) in the breeding and nonbreeding areas using a population model integrating survival estimates in the breeding ranges based on a large radio-telemetry data set and published estimates of demographic parameters from large-scale population-monitoring projects across Switzerland. Input parameters included the country-wide population trend, annual productivity estimates of the double-brooded species, and year-to-year survival corrected for breeding dispersal. Juvenile survival in the 3-week postfledging period was low (S = 0.32; SE = 0.05), whereas in the rest of the annual cycle survival estimates of adults and juveniles were similarly high (S > 0.957). Thus, the postfledging period was the main survival bottleneck, revealing the striking result that nonbreeding period mortality (including migration) is not higher for juveniles than for adult birds. Therefore, focusing future research on sources of variation in postfledging mortality can provide new insights into determinants of population dynamics and life-history evolution of migrant birds.

Dispersal is crucial for population viability and thus a popular target for conservation measures. However, the ability of individuals to move between habitat patches is notoriously difficult to estimate. One solution is to quantify functional connectivity via realistic individual-based movement models. Such simulation models, however, are difficult to build and even more difficult to parameterize. Here, we use the example of natal little owl (Athene noctua) dispersal to develop a new analysis chain for the calibration of individual-based dispersal models using a hybrid of statistical parameter estimation and Approximate Bayesian Computation (ABC). Specifically, we use locations of 126 radio-tracked juveniles to first estimate habitat utilization by generalized additive models (GAMs) and the biased random bridges (BRB) method. We then include the estimated parameters in a spatially explicit individual-based model (IBM) of little owl dispersal and calibrate further movement parameters using ABC. To derive efficient summary statistics, we use a new dimension reduction method based on random forest (RF) regression. Finally, we use the calibrated IBM to predict the dispersal potential of little owls from local populations in southwestern Germany to suitable habitat patches in northern Switzerland. We show that pre-calibrating habitat preference parameters while inferring movement behavioral parameters via ABC is a computationally efficient solution to obtain a plausible IBM parameterization. We also find that dimension reduction via RF regression outperforms the widely used least squares regression, which we applied as a benchmark approach. Estimated movement parameters for the individuals reveal plausible inter-individual and inter-sexual differences in movement behavior during natal dispersal. In agreement with a sex-biased dispersal distance in little owls, females show longer individual flights and higher directional persistence. Simulations from the fitted model indicate that a (re)colonization of northern Switzerland is generally possible, albeit restricted. We conclude that the presented analysis chain is a sensible work-flow to assess dispersal connectivity across species and ecosystems. It embraces species- and individual-specific behavioral responses to the landscape and allows likelihood-based calibration, despite an irregular sampling design. Our study highlights existing, yet narrow dispersal corridors, which may require enhancements to facilitate a recolonization of little owl habitat patches in northern Switzerland.

The formation of tree-cavities is an important ecological factor since many animals in woody habitats are cavity users. Recent research focuses on tree-cavity formation and the associated cavity networks in forest ecosystems. However, although traditional European orchards are important habitats for secondary cavity users, ecological research on the factors associated with the occurrence of cavities in fruit trees is widely missing. In particular, fruit tree pruning management may affect decay-cavity formation due to the pruning wounds allowing heart rot and decay to enter the tree. Here, we present a cross-sectional study investigating 608 fruit trees in 30 study plots of three European fruit-growing regions to identify factors associated with decay-cavity occurrence in fruit trees. Presence of decay-cavities was positively related to trunk diameter. Moreover, fruit trees of low vitality and with woodpecker-cavities featured more often decay-cavities than trees of high vitality or without woodpecker-cavities. Apple trees featured higher numbers of cavities at younger age than other fruit trees. Occurrence of decay-cavities was also related to the past removal of large main branches. Accordingly, traditional orchards are cavity-rich habitats if they comprise high proportion of old fruit trees, in particular apple trees, and if pruning management produces large pruning wounds. Thus, differential tree pruning and fruit-growing traditions across Europe result in different cavity densities in traditional orchards. Preservation of existing and potential cavity trees and selective removal of large branches from apple trees are recommended as conservation measures establishing high cavity densities and increasing the associated biodiversity in the agricultural landscape.

Roost site selection is a state-dependent process, affected by the individual's costs and benefits of roosting at a specific site in the available environment. Costs and benefits of different roost sites vary in relation to intrinsic factors and environmental conditions. Thus, the cost–benefit functions of roost sites are expected to differ between seasons and life-history stages, resulting in adjustments in roost site selection. Studying roost site selection throughout the year therefore provides information about year-round habitat requirements at different life-history stages. However, little is known about the roosting behaviour of birds. Here, the roost site selection of Little Owls Athene noctua was studied by repeated daytime location of 24 adult and 75 juvenile radiotagged individuals from July to November. Little Owls preferred sheltered roost sites such as tree cavities with multiple entrances. They increasingly used sheltered sites from summer to winter and preferentially used sheltered roost sites with low ambient temperatures. Juveniles used significantly less sheltered sites during dispersal than before and afterwards, and used less sheltered sites than adults within their home-range. The survival probability of birds roosting frequently at exposed sites was reduced. Roost site selection is probably driven by the two mechanisms of predator avoidance and thermoregulation, and the costs of natal dispersal may include increased predation threat and higher energy expenditure for thermoregulation. We suggest that adequate roost sites, such as multi-entrance tree cavities, are an important habitat requirement for Little Owls and that habitat quality can be affected by manipulating their availability.

The microclimate of potential roost-sites is likely to be a crucial determinant in the optimal roost-site selection of endotherms, in particular during the winter season of temperate zones. Available roost-sites for birds and mammals in European high trunk orchards are mainly tree cavities, wood stacks and artificial nest boxes. However, little is known about the microclimatic patterns inside cavities and thermal advantages of using these winter roost-sites. Here, we simultaneously investigate the thermal patterns of winter roost-sites in relation to winter ambient temperature and their insulation capacity. While tree cavities and wood stacks strongly buffered the daily cycle of temperature changes, nest boxes showed low buffering capacity. The buffering effect of tree cavities was stronger at extreme ambient temperatures compared to temperatures around zero. Heat sources inside roosts amplified Δ T (i.e., the difference between inside and outside temperatures), particularly in the closed roosts of nest boxes and tree cavities, and less in the open wood stacks with stronger circulation of air. Positive Δ T due to the installation of a heat source increased in cold ambient temperatures. These results suggest that orchard habitats in winter show a spatiotemporal mosaic of sites providing different thermal benefits varying over time and in relation to ambient temperatures. At cold temperatures tree cavities provide significantly higher thermal benefits than nest boxes or wood stacks. Thus, in winter ecology of hole-using endotherms, the availability of tree cavities may be an important characteristic of winter habitat quality.

Recoveries of marked animals hold long‐term, large‐scale information on survival and causes of mortality, but are prone to bias towards dead recoveries and casualties in the range of presence of potential finders. Thus, accounting for circumstance‐related recovery probabilities is crucial in statistical approaches. For the little owl, a species of conservation concern in central Europe, raw ring recoveries suggested a strong human‐related impact on survival. We analysed the proportions of the main causes of death using a large sample of radio‐tracked birds as a reference. We compared ring recoveries in southern Germany collected 1950–2012 (n = 465 dead recoveries of 2007 recoveries of 30 623 ringed birds) with data from a radio‐tracking study in the same region 2009–2012 (n = 177 dead recoveries of 377 tagged individuals). Two assumptions of multi‐state ring recovery modelling were unrealistic. First, not all dispatched rings remained available to potential finders. Instead, 34% of tracked birds were displaced to sites where rings were irretrievable, resulting in biased estimates of recovery probability. Second, the proportions of irretrievable rings were disproportional, with 48% in predated birds and 5% in human‐induced mortality. Consequently, the sample of rings from which recoveries were drawn differed from the sample of dispatched rings. Accounting for these biases in a multi‐state model, we estimated the frequencies of main causes of mortality to 45% for predation, 20% for casualties due to traffic and at buildings and 34% for all other causes. In radio‐tracked birds, predation was even more dominant (76%). Integrating mark–recapture data and telemetry observations allowed detecting and quantifying so far unknown recovery bias and resulted in improved estimates of key population parameters. The demography of little owls likely depends mainly on predator–prey relationships rather than on human‐induced deaths. published

The amount of high-quality habitat patches, their distribution, and the resource accessibility therein play a key role in regulating habitat effects on reproductive success. Heterogeneous habitats offer non-substitutable resources (e.g. nest sites and food) and substitutable resources (e.g. different types of food) in close proximity, thereby facilitating landscape complementation and supplementation. However, it remains poorly understood how spatial resource separation in homogeneous agricultural landscapes affects reproductive success. To fill this gap, we investigated the relationships between farmland heterogeneity and little owl (Athene noctua) reproductive success, including potential indirect effects of the heterogeneity-dependent home-range size on reproduction. Little owl home-ranges were related to field heterogeneity in summer and to structural heterogeneity in winter. Clutch size was correlated with the amount of food-rich habitat close to the nest irrespective of female home-range size, suggesting importance of landscape complementation. Nestling survival was positively correlated with male home-range size, suggesting importance of landscape supplementation. At the same time, fledgling condition was negatively correlated with male home-range size. We conclude that decreasing farmland heterogeneity constrains population productivity by two processes: increasing separation of food resources from nest or roost sites results in low landscape complementation, and reduction of alternative food resources limits landscape supplementation. Our results suggest that structural heterogeneity affects landscape complementation, whereas the heterogeneity and management of farmland fields affect landscape supplementation. Thus, to what extent a reduction of the heterogeneity within agricultural landscapes results in species-specific habitat degradation depends on the ecological processes (i.e. landscape complementation or supplementation) which are affected.

Interactions between top predators and mesopredators of the same guild often result in habitat segregation restricting interactions to shared habitat edges. Although negative edge effects are recognized as important spatial patterns in the ecology of fragmented landscapes, the underlying mechanisms of predator–prey interactions resulting in negative edge effects remain unknown. To disentangle top‐down effects of intraguild predators and bottom‐up effects of shared resources on mesopredator spatial distribution, we recorded the occurrence of tawny owls Strix aluco in forests and their prey, the little owl Athene noctua in adjacent open areas over 2 yr across 687 km2 in Southern Germany. We developed a new, asymmetrical dynamic two‐species occupancy model investigating spatial interactions while accounting for imperfect detection. Little owl occupancy was strongly reduced within 150 m of forests, but only in the presence of tawny owls. Analysis of over 30 000 telemetry locations of 275 little owls showed that little owls strongly avoided areas closer than 150 m from the forest during range use. These results suggest that the negative edge effect is due to forest edge avoidance rather than direct predation. Potential confounding mechanisms such as food depletion or habitat avoidance at forest edges can be ruled out. Thus, top‐down effects caused by avoidance of intraguild top predators shape the spatial distribution of mesopredators such as the little owl. While habitat complexity mitigates multitrophic interactions within habitats, it is expected to reinforce multitrophic interactions between habitats, potentially leading to the suppression of mesopredators from suitable habitats.

Many farmland-breeding wader species have declined across Europe, probably due to reductions in reproductive output caused by high nest losses as a result of agriculture or predation, or low chick survival between hatching and fledging. Most studies have focused on nest failures, and the factors affecting post-hatching survival of chicks are poorly known. In an experimental approach, we fenced parts of the arable foraging areas of Northern Lapwing Vanellus vanellus families to quantify chick survival simultaneously in the presence and absence of ground predators. Lapwing chicks were radiotagged to estimate survival probabilities by daily locations, applying multistate capture–recapture models. During the night, chick survival was considerably lower outside fenced plots than within. During the day, chick survival was higher than at night and did not differ between protected and unprotected plots. This suggests that nocturnal ground predators such as Red Foxes Vulpes vulpes were responsible for a significant proportion of chick mortality. Cumulative survival probability from hatching to fledging was 0.24 in chicks within fenced plots, but virtually zero in chicks outside fenced plots. In farmland, temporary electric fences can be effective in minimizing the impact of ground predators and offer a promising short-term method to increase fledging success of precocial birds.

Summary 1. An innate migration strategy guides birds through space and time. Environmental variation further modulates individual behaviour within a genetically determined frame. In particular, ecological barriers could influence departure direction and its timing. A shift in the migratory direction in response to an ecological barrier could reveal how birds adjust their individual trajectories to environmental cues and body condition. 2. Northern wheatears of the Greenland/Iceland subspecies Oenanthe oenanthe leucorhoa arrive in Western Europe en route from their West African winter range. They then undergo an endogenously controlled shift in migratory direction from north to north-west to cross a large ecological barrier, the North Atlantic. We radiotracked these songbirds departing from Helgoland, a small island in the North Sea, over an unprecedented range of their journey. 3. Here, we show that both birds’ body condition and the wind conditions that they encountered influenced the departure direction significantly. Jointly high fuel loads and favourable wind conditions enabled migrants to cross large stretches of sea. Birds in good condition departed early in the night heading to the sea towards their breeding areas, while birds with low fuel loads and/or flying in poor weather conditions departed in directions leading towards nearby mainland areas during the entire night. These areas could be reached even after setting off late at night. 4. Behavioural adjustment of migratory patterns is a critical adaptation for crossing ecological barriers. The observed variation in departure direction and time in relation to fuel load and wind revealed that these birds have an innate ability to respond by jointly incorporating internal information (body condition) and external information (wind support).

In seasonal environments with limited time and energy resources, double-brooded birds face trade-offs in the timing of their two reproductive attempts and in the effort allocated to the first and the second broods. In the Barn Swallow Hirundo rustica a long care period for the first brood enhances the survival of first-brood chicks, but also delays the start of the second brood, which in turn reduces the survival prospects of second-brood chicks. Probably as a response to this trade-off, double-brooded Barn Swallows reduce the period of post-fledging care for first-brood fledglings. By radiotracking whole families, we investigated the determinants of this behaviour and its consequences for the survival of the first-brood fledglings. The end of the females’ investment in post-fledging care of the first brood was related to the beginning of egg synthesis for the second clutch. With the start of egg synthesis, females significantly reduced provisioning rates to the firstbrood fledglings to less than one-fifth of the previous rates, while the proportion of time they spent foraging remained high. Assuming that the females’ foraging success was constant, we conclude that their energy income was allocated to egg production rather than fledgling provision. Males did not compensate for the females’ reduced feeding rates. Thus the start of egg production for the second clutch had a marked effect on the quantity of food received by first-brood fledglings. In parallel with the changes in parental behaviour and provisioning rates, we observed a marked drop in the daily survival rate of first-brood chicks. These results support the hypothesis that females face a strong tradeoff in the allocation of energy to subsequent broods. Energy allocation to a second clutch involves a cost in terms of reduced provisioning, and as a result the survival of first-brood chicks is compromised. This is probably outweighed by the improved success of an early second brood.

In seasonal environments, avian reproductive performance almost generally declines in the course of the season. Quantifying the associated fitness consequences of timing of breeding, i.e. of date-related factors, is important for understanding the evolution of temporal patterns in avian life-histories and for predicting consequences of climate change. The seasonal decline can also be caused by an effect of parental quality: individuals with high phenotypic quality may breed early. The results of existing experimental studies investigating whether date or quality effects cause the seasonal decline are inconsistent, indicating that both mechanisms might be involved. However, it remains unclear to what extent the confounding effect of quality occurs and what the fitness consequences of timing per se over a whole breeding episode are. In a cross-fostering experiment using the barn swallows' second broods we evaluated the causes for the seasonal decline in reproductive performance for three distinct periods of a reproductive attempt, the early nestling period, the late nestling period and the post-fledging period, and we assessed the overall fitness consequences of timing per se. A seasonal decline in juvenile feather growth rate was mainly due to date effects in the late nestling period, although we determined quality effects during early nestling development. Date effects on survival were present in the post-fledging period, but not in the nestling period. The decline in feather length due to date effects in the nestling period accounted for 9% of the seasonal decline in post-fledging survival, whereas date effects arising only in the post-fledging period caused 91% of the decline. These results suggest that date effects increase in the course of a reproductive episode. Thus, the benefits of an early timing of breeding can be quantified only when considering also the post-fledging period. We suggest that the timing of breeding evolved through a trade-off between date-related benefits and quality-related costs of early breeding.

Brood overlap can be an optimal solution to the intraseasonal trade-off in the allocation of time between successive broods. In line with life-history theory, the extent of brood overlap should be adjusted according to the individual costs and benefits of the simultaneous investment into 2 broods. Phenotypic quality can affect these costs and benefits. In barn swallows (Hirundo rustica), male tail streamer length is a phenotypic quality trait, which is related to reproductive decisions, resulting in pairs with long-tailed males showing higher annual output than those with short-tailed males. We predict that the extent of brood overlap in barn swallows is positively associated with the male's tail ornamentation. Using radiotelemetry, we assessed the duration of postfledging care and juvenile survival and quantified brood overlap with unprecedented accuracy. Broods of pairs with highly ornamented males overlapped to a larger extent than those of pairs with short-tailed males. Brood overlap depended on the reproductive value of the previous brood and on female phenotypic quality. Clutch initiation after a short interbrood interval was associated with a great brood overlap. Short duration of postfledging care but not brood overlap reduced fledgling survival in first broods, and earlier second broods were associated with enlarged clutches. Thus, pairs with long-tailed males managed to increase clutch size and survival of the second brood by an early onset without incurring survival costs in the first brood. This is in line with the hypothesis that brood overlap enforces differential annual reproductive output among individuals varying in phenotypic quality. Copyright 2010, Oxford University Press.

Understanding individual space use remains a major issue in ecology, and it is complicated by definitions of spatial scale and the interplay of multiple factors. We quantified the effect of habitat and biotic and individual factors on space use by amphibians (Bufo bufo spinosus [BB] and Bufo viridis [BV]) that were radio-tracked in their terrestrial summer habitat. We analyzed two spatial scales, 50% core areas and 95% home ranges (excluding 50% core areas), thought to represent resting and foraging areas, respectively. The 50% core area of BB was best explained by habitat structure and prey density, whereas the 50% core area of BV was determined solely by habitat structure. This suggests that the resting and foraging areas of BB are not spatially separated. The 95% home range of BB was determined by prey density, while for BV both habitat structure and prey density determined home range size. We conclude that the terrestrial area requirements of amphibians depend on the productivity and spatiotemporal complexity of landscapes and that differential space use may facilitate their co-occurrence. Behavior-based a priori hypotheses, in combination with an information-theoretic approach and path analyses, provide a promising framework to disentangle factors that govern individual space use, thereby advancing home range studies.

Meadow breeding birds such as the whinchat Saxicola rubetra have been declining due to increased farming intensity. In modern grassland management, the first mowing and the bird’s breeding cycle coincide, causing high nest destruction rates and low productivity of grassland bird populations. However, it is virtually unknown whether the mowing process directly affects adult survival by accidentally killing incubating females. We studied adult survival of an Alpine whinchat population during two breeding seasons using either colour-ringing or radio-tracking of 71 adults. Assessing territories, mowing phenology and nest destruction from 1988 to 2007 allowed changes in the factors associated with female mowing mortality to be estimated. Adult survival over 5-day-periods was Φ = 0.986, but during the period of mowing female survival was strongly reduced (Φ = 0.946). As a result, 80.6% of the males, but only 68.4% of the females survived the breeding season. Mowing undoubtedly killed two of 20 radio-tagged females when they were laying or incubating. In the 20-year period, an increasing proportion of nests were destroyed before the chicks hatched and this change was associated with an increased distortion of the adult sex ratio. Modelling the population growth rate showed that including the additional effect of mowing on female mortality resulted in a 1.7 times faster local population decline. These results are consistent with the hypothesis that the extinction of whinchat populations in the lowlands of central Europe was caused not only by habitat degradation and low productivity, but also by increased man-made female mortality.

The timing of breeding and the chicks' fledging condition are major fitness-relevant traits in the Great Tit Parus major. Yet, the proximate mechanisms relating these traits to differential survival and recruitment are largely unknown. We analysed the range use of Great Tit family groups during the first 20 days after fledging, i.e. the period of post-fledging dependence, in relation to the juveniles' body condition. Radio-telemetry and colour marks were used to track 25 families with 107 chicks. The post-fledging home-ranges of families with fledglings of good condition (average body mass 19 g) were c. 3 times larger than of those with poorly nourished (average 15 g) young. The rate of movements exceeding 50 m per hour was also positively correlated to the chicks' mean fledging mass. We hypothesise that the fledglings' body condition resulted in large differences in flight performance, and consequently, spatial behaviour of the family groups. The slow movements and the close grouping of chicks of poor bo...

In altricial birds, parental care after fledging should be terminated as soon as the benefits from deserting the young outweigh those from continuing parental care. In seasonal environments, multiple breeders may trade off fitness benefits of the time invested in the care of fledglings of one brood against the benefits of an advanced start of the subsequent clutch. Using radiotelemetry of 494 fledglings in 114 families (of 88 pairs) of the double-brooded barn swallow, Hirundo rustica, we found that, irrespective of the offspring quality, the duration of postfledging parental care was shorter for first broods than for single and second broods. A shorter duration of postfledging care was associated with a steeper decline in feeding rates after fledging. Postfledging parental care also depended on the pair's breeding season length and on the phenotypic quality of the female parent. We suggest that pairs that start other reproductive bouts in the same season and pairs with high chances of survival to subsequent years reduce parental effort in the current brood to maximize lifetime reproductive output. The results support the hypothesis that multibrooded birds adjust the duration of postfledging care in relation to future reproductive attempts because of an intraseasonal trade-off in the allocation of time between successive broods.

This paper presents a model to predict the daily spatio-temporal variation in the abundance of airborne insects in agricultural landscapes on the basis of publicly available environmental data. Data on the abundance of insects flying near ground level were collected by a robust and effective point-count method. The validation of the model using extra data revealed a high correspondence between predicted and observed insect density. The final model showed that the abundance of airborne insects depended largely on the daily weather conditions. Agricultural habitats differed in the abundance of airborne insects, with peak insect numbers found along hedgerows and trees. This accumulation pattern was most pronounced at low temperatures and in windy conditions. Efforts to enhance insect abundance in agricultural landscapes therefore need to increase the number of structural elements such as hedgerows, single trees and orchards. This study highlights how weather conditions and agricultural land-use affect aerial food webs in agri-environments, including the dynamics of food patches for all kinds of aerial foragers.

B. Naef-Daenzer, Swiss Ornithological Institute, Luzernerstrasse 6, CH-6204 Sempach, Switzerland. E-mail: beat.naef@vogelwarte.chHarnesses are indispensable for a long-term attachment of instruments to animals, particularly birds. The exactfit is crucial for both the reliability of the tags and the animals’ health and natural behaviour. Using data from 19bird species ranging from 8 400 g, I present an allometric function to calculate the dimension of leg-loopharnesses for birds on the basis of body mass data. The model greatly facilitates determining the correct loopspan for new species without time-consuming experiments. More importantly, the equation permits calculationof the excess span that is required for tagging juvenile, still growing birds.

SUMMARY We have developed a miniaturized very high frequency (VHF) transmitter design for radio-tagging small animals. The average mass of the circuitry is 0.084 g (range 0.081-0.087), hence, with the smallest power cell, complete tags weigh 0.2 g and have a life of 18-22 days. We further demonstrate that with such small tags the technique of attaching the device to the animal's body strongly affects the effective radiated power, and thus transmission ranges. Ideally the attachment couples the electronics to the animal's body as a ground plane for improved radiation. The transmitter allows the application of radio-tracking to be expanded to new taxa whose spatial behaviour and population dynamics are largely unknown, for example arthropods, reptiles or amphibians. The new design is also suitable for miniature implants and signal modulation with sensors.

We analysed the spatial distribution of home-ranges and the habitat use of re-introduced grey partridges Perdix perdix in relation to newly established ecologically enhanced areas, i.e. wild-flower strips and hedges, within an intensively cultivated area in Switzerland from which the species had become extinct. All birds settled within the ca. 30% of the study area where the proportion of enhanced areas was highest (5.8%). At the level of the individual family group, we found a significant over-proportional use of enhanced areas throughout the year. Furthermore, when the birds used the agricultural fields, the use densities strongly declined with distance from the nearest enhanced area. Thus, the availability and spatial distribution of ecologically enhanced areas were the main determinants of the partridges’ range use. These results indicate that a framework of ecologically enhanced areas is a key habitat structure for grey partridges, and that these enhancements, even if small in proportion, were the crucial first measure to restore the research area to an appropriate grey partridge habitat and, thus, the basis for successful re-introduction in this intensively cultivated area.

Over the past 50 years European populations of the lesser horseshoe bat Rhinolophus hipposideros have severely declined, probably because of the loss of foraging habitat. To date, studies of the foraging behaviour of this species have been limited as its low mass (4–8 g) precluded the use of radio-telemetry because commercially available radio-transmitters exceeded 10% of its body mass. In this study, radio-transmitters weighing < 0.35 g were built. These increased the body mass of the animals from 4.5% to 8.1%, with no demonstrable adverse effect on their flight behaviour. The habitat selection of eight female lesser horseshoe bats was studied in Monmouthshire, U.K. The bats had foraging ranges between 12 and 53 ha (100% kernel). Although one bat foraged 4.2 km from the roost, for 50% of the time tracked bats were recorded within 600 m of the nursery roost. The estimated density within 200 m of the roost was 5.8 foraging bats/ha. This decreased to 0.01 bats/ha at 1200 m. Compositional analysis revealed that this species used woodlands, predominately broadleaf, more than any other habitat. In addition, the bats foraged in areas of high habitat diversity. Conservation management of this species should concentrate on such areas within 2.5 km of the nursery roost.

Summary 1 We present a multivariate model of the post-fledging survival of juvenile great and coal tits (Parus major L., P. ater L.) in relation to chick body condition and timing of breeding. Radio-telemetry and colour marks were used to track tit families during 20 days from fledging, that is, the period of post-fledging dependence. Data on 342 chicks of 68 broods were obtained. 2 Forty-seven per cent of juveniles died during the observation period, predation being the main cause of mortality. In the first 4 days after fledging the mortality rate was 5–10% per day. 3 Survival of juveniles was positively correlated with fledging mass. Furthermore, survival strongly decreased during the season. In the second half of June, mortality was five times the rate of mid-May. The differential survival resulted in selection for both early fledging and high fledging mass. Juvenile condition was less important for survival in birds that had fledged early in the season. Their survival rates exceeded 70% in all weight classes, whereas in late broods only the heaviest individuals survived equally well. The survival of birds fledging both late and in poor condition was below 20%. Thus, selection for high fledging mass was much stronger in the late season than in early broods. 4 We conclude that the impact of predation after leaving the nest results in selection for early breeding and, particularly in the late season, for high fledging mass. This may explain why the earliest broods have been found to produce most recruits into the breeding population even if they did not profit from maximum food availability during the nestling period. On the other hand, energetic limitations may constrain the begin of egg laying in adult birds. Thus, counteracting evolutionary responses to the seasonal development of food availability (the caterpillar peak) and to the risk of post-fledging mortality (the peak in post-fledging mortality) may have focused the period of optimal reproduction to a narrow time-window.

1 We propose a quantitative zonation of Switzerland based on the distribution of breeding birds. We use a combined dataset from two nationwide bird censuses 20 years apart to account for distribution changes between the censuses. This approach approximates the ‘potential’ distribution of species more closely than a dataset from one period only. 2 A numerical approach with correspondence analysis and non-hierarchical clustering resulted in five avifaunal regions, consisting of two lowland regions north of the alpine arch, two alpine regions and one region of southern alpine valleys. 3 We identified characteristic species for each region using the indicator value analysis of Dufrene & Legendre (1997) and determined environmental conditions separating the regions with discriminant analysis. 4 Our zonation partly corresponds with previous subdivisions of Switzerland, but has the advantage of being based on a numerical approach instead of expert knowledge. 5 We discuss avifaunal and ecological differences between the regions and relate our zonation to the classical biogeographic regions of Europe. Finally, we evaluate the importance of the zonation for bird conservation.

The European nightjar is one of the most endangered bird species in Switzerland. As its ecology in the country is poorly understood, we collected data on resource exploitation in the upper Rhone valley (Alps). The diet of two adult birds and one nestling consisted primarily of moths (81 and 93% of biomass), which were also the most abundant prey sampled at the study site. Three radiotracked nightjars selectively exploited oak scrubland compared to vineyards and pine forests; vineyard monocultures harbour presumably insufficient moth populations, whereas dense pine stands probably do not provide the flying and foraging requirements of nightjars. The survival of nightjars in Valais probably depends both on the existence of sufficient populations of moths and on the availability of semi-open natural habitats, such as oak scrubland, which seem to offer the best suitable foraging and nesting places.

The range use of both sexes of the Aquatic Warbler was studied during two breeding seasons by radio-tracking. Males used home ranges of up to 8 ha. These home ranges over-lapped by up to 74 %, (mean 51 %) between individuals. During the mating period and prior to nesting, females used isolated sites of 2.8 to 6.4 ha (mean: 4.2 ha) within activity ranges of 100 to 160 ha (mean: 120 ha). Home ranges of different females overlapped by up to 20 %. However, during nest-building and incubation, the average size of home ranges in females was only 1.6 ha. Since no constant ranges were observed, we conclude that Aquatic Warblers are not territorial in the breeding season.

Naef-Daenzer, L., Naef-Daenzer, B. and Nager, R. G. 2000. Prey selection and foraging performance of breeding Great Tits Parus major in relation to food availability. ‐ J. Avian Biol. 31: 206‐214. We studied the nestling diet and the foraging performance of Great Tits in relation to prey abundance in the field. Numerous experimental studies present data on foraging decisions in captive Great Tits. Little is, however, known about prey selection in the field in relation to the food available and the consequences this has for the food delivery rate to nestlings. Since the foraging performance of the parents is one of the main determinants of fledging weight and juvenile survival, foraging behaviour is an important part of Great Tit reproduction. During the early breeding season up to 75% of the prey biomass delivered to the nestlings were spiders, which is in contrast with other studies. Only when caterpillars reached a size of 10‐12 mg (approximately the average size of the spiders caught at that time) did the Great Tits change their preferences and 80‐90% of the delivered prey masses were caterpillars, as reported by other authors. This ‘switching’ between prey occurred within a few days. It was not related to the changes in abundance but to size of caterpillars. The rate at which caterpillars were delivered to the nestlings (in mg:nestling:h) was strongly correlated with the caterpillar biomass available (in mg:m of branches) and nestling growth rate was significantly influenced by the mass of available caterpillars. The results provide evidence why perfect timing of breeding is so important for the Great Tit, and contribute to the understanding of the causal link between food supply, growth and breeding success.

1. We analysed the effect of prey density and size on the foraging performance of great and blue tit (Parus major L., P. caeruleus L.) parents, and its consequences for the growth and fledging weight of nestlings. Because fledging weight is a determinant of subsequent survival and therefore fitness, foraging decisions of the parents play a key role in the reproductive system of tits. The analysis quantifies (i) the rate at which energy is delivered to the nestlings in relation to prey size and abundance, and (ii) the growth rates of nestlings and the resulting fledging weight in relation to the rate of food delivery by the parents. 2. The searching time per prey item increased exponentially with decreasing prey biomass. During the peak abundance of caterpillars, the average searching time per item was 2·5–3 min instead of 5–6 min before and after the peak. Searching time was significantly reduced when the birds returned to the foraging site where the preceding prey was found. This accords with the clumped distribution of caterpillars within the canopy. 3. The foraging performance (in mg caterpillars per min) was maximal when caterpillars were both abundant and large, i.e. shortly before they left the trees for pupation. The high feeding frequency and the large prey then caused a peak energy flow rate to the nestlings of 4–5 times the rate before or after the caterpillar peak. This suggests that the foraging success and rate of food delivery by tit parents was primarily determined by the abundance and size of prey. 4. The growth rate of nestlings, as well as their fledging weight was correlated with the rate of food delivery. Low feeding performance of the parents resulted therefore in poor relative growth rates of only 0·3–0·6 of the rate achieved under optimal conditions and, as a consequence, in a low fledging weight. This indicates that tit parents have restricted options to adjust prey delivery rates according to the requirements of the brood. 5. The results give insight into the chain of causal mechanisms through which an environmental factor (availability of food) has a strong and immediate effect on fitness (growth, fledging weight and, thus, survival of the nestlings). The importance of caterpillar size for foraging success and prey delivery rates of parent tits makes clear, why the phase of best foraging conditions is shorter than the period during which caterpillars are available. The relationships we quantified give a proximate explanation for the great effects that temperature and caterpillar growth have on the between-year variation in selection intensity for laying date observed in other studies.

The breeding populations of the Swallow Hirundo rustica show long-term declines in most European countries (BirdLife International 2004, Hagemeijer & Blair 1997, Tucker & Heath 1994). An international project, coordinated by EURING, is in progress to investigate the causes. Part of the work is studying annual reproduction and survival rates of adults and their young (Jenni 1998). The physical condition of Swallows just before autumn migration may be an important indicator of the probability that they will survive to the next breeding season. Within young Swallows, fitness could be age-dependent, with older individuals that fledge from early broods being more experienced in using environmental resources than juveniles from late broods. To compare fitness data not only between adults and juveniles but also between recently-fledged and older juvenile Swallows we looked for an externally-visible character to distinguish these two cohorts in the field. So far, no such criteria seem to have been described (Svensson 1992, Suter 1985, Cramp et al 1988). Here we present data to demonstrate that scoring the stage of development of the bill edges is a useful method for that purpose. During the nestling period, the yellow gape and the whitish, swollen bill edges stimulate parental feeding and are therefore important for nestling growth and survival. After fledging and especially after juveniles have reached their independence, bill edges regress as their function becomes irrelevant. Up to now, it is unclear to what extent and at which age this occurs. Within the Swiss Swallow project, we used, over a period of six years (1998-2003), data from retrapped young Swallows ringed as nestlings of known age to quantify the process of bill edge regression. Nestling age was estimated with an accuracy of ±1 day by using observations during weekly visits to the nests as well as the key for ageing nestlings in Jenni (1998). Birds were

Prologue The restoration of grey wolves Canis lupus to the Yellowstone ecosystem is one of the largest and best-monitored experiments addressing links between individual habitat use and processes at the population level. After some 70 wolf-free years, 31 wolves were released in 1995–1996, so that Yellowstone again hosts all native large carnivores. Although large efforts to predict the ecological effects of wolves had been made on the basis of extant evidence, models and expert knowledge (Mack and Singer, 1992; Boyce, 1993; Cook, 1993) no one expected that these few individuals would change so much of the ecosystem. After only a decade the re-introduction has given rise to manifold ecological changes. Many of these are related to individual behaviour of the new predator or individual responses of potential prey. Proximate impacts, such as predation on elk Cervus elaphus , the main prey, were as predicted (Boyce, 1993; Smith et al ., 2003). It was a surprise, however, that the re-introduction of a top predator would so quickly influence Yellowstone’s vegetation. The presence of wolves had profound effects on the trophic web, operating remarkably fast and reaching down to primary production. Interestingly, these effects were mainly related to behavioural processes (such as habitat use and spatial distributions) rather than to demographic processes (for example, population sizes and survival rates) due to predation. How did these relatively few wolf individuals cause so many changes at various levels of the ecosystem? Primarily, as an anti-predator response, elk altered their habitat selection and use of resources. Simultaneous GPS tracking of wolves and elk revealed that the elk responded to the presence of wolves at a distance of up to 1 km. They moved into wooded areas, avoiding the open grassland (Creel et al ., 2005). The vegetation responded quickly to the resulting relief from browsing, and various plant species re-colonised areas from which they were absent for decades, for example willows Salix spp. and aspen Populus spp. (Ripple et al ., 2001; Ripple and Beschta, 2006; Beyer et al ., 2007).

Summary The magnetic field, the sun, the stars and the polarization pattern of visible light during twilight are important cues for orientation in nocturnally migrating songbirds. As these cues change with time and location on Earth, the polarization pattern was put forward as a likely key reference system calibrating the other compass systems. Whether this applies generally to migratory birds is, however, controversially discussed. We used an experimental approach in free-flying birds to study the role of polarization for their departure direction in autumn. Experimental birds experienced a 90° shift of the band of maximum polarization during sunset, whereas control-birds experienced the polarization pattern as under natural condition. Full view of the sunset cues near the horizon was provided during the cue conflict exposure. Here we show both the experimental and the control-birds being released after nautical twilight departed consistently towards south-southeast. Radio telemetry allowed tracking first 15 km of birds' way out, thus the intrinsic migration direction as chosen by the birds was measured. We found no recalibration of the magnetic compass after pre-exposure to a cue conflict between the natural magnetic field and the artificially shifted polarization pattern at sunset. The lacking difference in the departure direction of both groups may suggests that birds did not recalibrate any of the compass systems during the experiment. As free-flying migrants can use all available orientation cues after release, it remains unknown whether our birds might have used the magnetic and/or star compass to determine their departure direction.