Sex-biased survival of nestlings under unfavourable conditions drives secondary sex ratio in little owls (Athene noctua)

Identifying variation in key life history traits is crucial to predict population dynamics of endangered species. An increased bias in sex ratio as a result of low habitat quality can reinforce population decline and drive small populations to extinction. This is particularly true for monogamous bird species such as the little owl (Athene noctua). However, the underlying mechanisms driving biased sex ratios often remain elusive. Using a robust sample of molecular sex determination data recorded at two nest visits within each of 247 broods we examined the development in brood sex ratio, sexual size dimorphism and factors affecting the survival of little owl nestlings. Brood sex ratio in little owl broods was female-biased at fledging but not before the middle of the nestling period and it was female-biased at the end but not at the beginning of the breeding season. Female nestlings showed higher body mass than male nestlings of the same hatching rank. A survival analysis corroborated that late-hatched nestlings of low body mass showed reduced survival under adverse habitat conditions, resulting in a male-biased nestling mortality. Our conclusions therefore support theoretical predictions according to which the more sensitive sex will be disproportionally affected by unfavourable conditions that were previously identified by feeding experiments in the study system. This pattern of condition-dependent secondary brood sex ratio can severely affect the demographic parameters of endangered species and provides important implications for conservation.peerReviewe

Response of a free-flying songbird to an experimental shift of the light polarization pattern around sunset

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 deg shift of the band of maximum polarization during sunset, whereas control birds experienced the polarization pattern as under natural conditions. Full view of the sunset cues near the horizon was provided during the cue conflict exposure. Here we show that both the experimental and the control birds being released after nautical twilight departed consistently towards south-southeast. Radiotelemetry allowed tracking of the first 15 km of the birds' outward journey, 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 suggest 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

Reproductive consequences of farmland heterogeneity in little owls (Athene noctua)

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 fll this gap, we investigated the relationships between farmland heterogeneity and little owl (Athene noctua) reproductive success, including potential indirect effects of the heterogeneitydependent home-range size on reproduction. Little owl home-ranges were related to feld 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 felds affect landscape supplementation. Thus, to what extent a reduction of the heterogeneity within agricultural landscapes results in species-specifc habitat degradation depends on the ecological processes (i.e. landscape complementation or supplementation) which are affected

Bias in ring-recovery studies : causes of mortality of little owls Athene noctua and implications for population assessment

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.publishe

Reproductive consequences of farmland heterogeneity in little owls (Athene noctua)

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 fll this gap, we investigated the relationships between farmland heterogeneity and little owl (Athene noctua) reproductive success, including potential indirect effects of the heterogeneitydependent home-range size on reproduction. Little owl home-ranges were related to feld 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 felds affect landscape supplementation. Thus, to what extent a reduction of the heterogeneity within agricultural landscapes results in species-specifc habitat degradation depends on the ecological processes (i.e. landscape complementation or supplementation) which are affected

Brood overlap and male ornamentation in the double-brooded barn swallow

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.

Migration confers winter survival benefits in a partially migratory songbird

To evolve and to be maintained, seasonal migration, despite its risks, has to yield fitness benefits compared with year-round residency. Empirical data supporting this prediction have remained elusive in the bird literature. To test fitness related benefits of migration, we studied a partial migratory population of European blackbirds (Turdus merula) over 7 years. Using a combination of capture-mark-recapture and radio telemetry, we compared survival probabilities between migrants and residents estimated by multi-event survival models, showing that migrant blackbirds had 16% higher probability to survive the winter compared to residents. A subsequent modelling exercise revealed that residents should have 61.25% higher breeding success than migrants, to outweigh the survival costs of residency. Our results support theoretical models that migration should confer survival benefits to evolve, and thus provide empirical evidence to understand the evolution and maintenance of migration

Little owls in big landscapes: Informing conservation using multi-level resource selection functions

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-Württemberg, 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-Württemberg due to higher elevation, fewer orchards, and more forest patches. Extant populations currently occupy c. 15% of the potential suitable habitats in Baden-Württemberg, and 2% in Switzerland, suggesting that considerable space for recolonisation is available. However, while Baden-Württemberg 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