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Ettiswil, Switzerland

Amrhein V.,Research Station Petite Camargue Alsacienne | Amrhein V.,University of Basel | Scaar B.,Research Station Petite Camargue Alsacienne | Baumann M.,Research Station Petite Camargue Alsacienne | And 4 more authors.
Methods in Ecology and Evolution

1. It is increasingly acknowledged that skewed adult sex ratios (ASRs) may play an important role in ecology, evolution and conservation of animals. 2. In birds, published estimates on ASRsmostly rely onmist netting data. However, previous studies suggested that mist nets or other trap types provide biased estimates on sex ratios, with males being more susceptible to capture than females. 3. We used data from a Constant Effort Site ringing scheme to show how sex ratios that are corrected for sex- and year-specific capture probabilities can be directly estimated by applying capture-recapture analysis, for example, in a Bayesian framework. 4. When capture data were pooled from the 19 years of study, we found that in the blackbird (Turdus merula) and the blackcap (Sylvia atricapilla), the observed proportions of males were 57% and 55%, respectively. However, when the observed annual proportions of males were corrected for the sex-specific capture probabilities, the proportions of males did not clearly differ from50%in most study years, and thus, the apparent male-bias in the ASRs almost completely disappeared. 5. We propose that published estimates on ASRs in birds should be re-evaluated if based solely on observed sex ratios frommist netting studies. 6. We further propose that data from national bird ringing schemes and in particular from Constant Effort Site ringing programs can provide valuable information on ASRs, if analysed using capture-recapture models. We discuss important assumptions of those models; for example, movements that may differ between sexes should be taken into account, as well as the occurrence of transient individuals that do not hold breeding territories within a study site. © 2012 The Authors. Methods in Ecology and Evolution, © 2012 British Ecological Society. Source

Knape J.,Swedish University of Agricultural Sciences | Korner-Nievergelt F.,Swiss Ornithological Institute | Korner-Nievergelt F.,Oikostat GmbH
Methods in Ecology and Evolution

N-mixture and occupancy models are often used to account for non-detections in population surveys. The consensus has been that the methods require data that are replicated in space, as well as within a short period of time while the population at each site remains closed, in order for parameters such as detection probabilities and expected abundances to be identifiable. The requirement of replication prohibits the use of N-mixture and occupancy models for many surveys in practice. Recently, some studies have argued that N-mixture and occupancy models for surveys with only one visit at each site are identifiable when covariates for both detection probabilities and expected abundances, with at least one distinct covariate for each, are available (Journal of Plant Ecology, 5, 2012, 22; Environmetrics, 23, 2012, 197). We investigate the reasons for why detection probabilities have traditionally been considered unestimable from non-replicated counts and how the new methods sidestep these issues. We further use simulations to investigate properties of the new estimators. We show that detection probabilities of the single-visit models with covariates are non-identifiable and that absolute abundances cannot be estimated when particular link functions are employed (log links for both expected abundance and detection probability). Further, assumptions about the range within which detection probabilities vary are necessary to render estimability. The possibility of estimating abundance from single-visit surveys therefore implicitly hinges on knowledge about the link functions. Simulations show that estimates of abundance can be highly variable and sensitive to the choice of link function. We further show how a reduced parameterization of an N-mixture model for surveys repeated over time, without replication under closure but where detection probabilities are constant over time, corresponds to a Poisson model. Non-robust estimation can result in misleading conclusions about population abundance. When estimating abundance from count data that are not replicated, it is therefore important to be aware of how imprecise estimators may be and how sensitive they are to model assumptions. © 2015 British Ecological Society. Source

Korner-Nievergelt F.,Oikostat GmbH | Korner-Nievergelt F.,Swiss Ornithological Institute | Brinkmann R.,Leibniz University of Hanover | Brinkmann R.,Freiburg Institute of Applied Animal Ecology | And 2 more authors.

Environmental impacts of wind energy facilities increasingly cause concern, a central issue being bats and birds killed by rotor blades. Two approaches have been employed to assess collision rates: carcass searches and surveys of animals prone to collisions. Carcass searches can provide an estimate for the actual number of animals being killed but they offer little information on the relation between collision rates and, for example, weather parameters due to the time of death not being precisely known. In contrast, a density index of animals exposed to collision is sufficient to analyse the parameters influencing the collision rate. However, quantification of the collision rate from animal density indices (e.g. acoustic bat activity or bird migration traffic rates) remains difficult. We combine carcass search data with animal density indices in a mixture model to investigate collision rates. In a simulation study we show that the collision rates estimated by our model were at least as precise as conventional estimates based solely on carcass search data. Furthermore, if certain conditions are met, the model can be used to predict the collision rate from density indices alone, without data from carcass searches. This can reduce the time and effort required to estimate collision rates. We applied the model to bat carcass search data obtained at 30 wind turbines in 15 wind facilities in Germany. We used acoustic bat activity and wind speed as predictors for the collision rate. The model estimates correlated well with conventional estimators. Our model can be used to predict the average collision rate. It enables an analysis of the effect of parameters such as rotor diameter or turbine type on the collision rate. The model can also be used in turbine-specific curtailment algorithms that predict the collision rate and reduce this rate with a minimal loss of energy production. © 2013 Korner-Nievergelt et al. Source

Saggese K.,University of Basel | Saggese K.,University of Oslo | Korner-Nievergelt F.,Swiss Ornithological Institute | Korner-Nievergelt F.,Oikostat GmbH | And 3 more authors.
Animal Behaviour

Supplementary feeding of wild birds during winter is one of the most popular wildlife management activities, and is likely to have profound influence on the behavioural ecology of a species. At garden bird feeders, birds are now often fed well into the breeding season. Providing food within an established songbird territory, however, is likely to influence the territorial behaviour of the resident male. We used song performance during the dawn chorus in early spring to study behavioural changes in food-supplemented great tits, Parus major. After 2 weeks of continuous food supply within their territory, supplemented males started dawn singing later than control males, and thus postponed their regular dawn chorus before sunrise. This effect was maintained 2 weeks after food supplementation had ended. However, we did not find an effect of long-term feeding on song output. Our results were largely unexpected because formal models and field studies on short-term food supplementation suggested an earlier start of dawn singing or a higher dawn song output. Because we did not observe great tits visiting the feeders before sunrise or food supplementation increasing the numbers of conspecific intruders, the reasons for the delay in the start of dawn singing remain unclear; possible explanations include the presence of predators at feeding stations and the quality of the supplementary food itself. Delaying dawn singing could potentially affect the reproductive success of supplemented males, for example if females base extrapair mating decisions on dawn song performance of their mates. © 2010 The Association for the Study of Animal Behaviour. Source

Schmaljohann H.,Institute of Avian Research Vogelwarte Helgoland | Korner-Nievergelt F.,Swiss Ornithological Institute | Korner-Nievergelt F.,Oikostat GmbH | Naef-Daenzer B.,Swiss Ornithological Institute | And 5 more authors.
Frontiers in Zoology

Introduction: In long-distance migrants, a considerably higher proportion of time and energy is allocated to stopovers rather than to flights. Stopover duration and departure decisions affect consequently subsequent flight stages and overall speed of migration. In Arctic nocturnal songbird migrants the trade-off between a relatively long migration distance and short nights available for travelling may impose a significant time pressure on migrants. Therefore, we hypothesize that Alaskan northern wheatears (Oenanthe oenanthe) use a time-minimizing migration strategy to reach their African wintering area 15,000 km away. Results: We estimated the factors influencing the birds' daily departure probability from an Arctic stopover before crossing the Bering Strait by using a Cormack-Jolly-Seber model. To identify in which direction and when migration was resumed departing birds were radio-tracked. Here we show that Alaskan northern wheatears did not behave as strict time minimizers, because their departure fuel load was unrelated to fuel deposition rate. All birds departed with more fuel load than necessary for the sea crossing. Departure probability increased with stopover duration, evening fuel load and decreasing temperature. Birds took-off towards southwest and hence, followed in general the constant magnetic and geographic course but not the alternative great circle route. Nocturnal departure times were concentrated immediately after sunset. Conclusion: Although birds did not behave like time-minimizers in respect of the optimal migration strategies their surplus of fuel load clearly contradicted an energy saving strategy in terms of the minimization of overall energy cost of transport. The observed low variation in nocturnal take-off time in relation to local night length compared to similar studies in the temperate zone revealed that migrants have an innate ability to respond to changes in the external cue of night length. Likely, birds maximized their potential nightly flight range by taking off early in the night which in turn maximizes their overall migration speed. Hence, nocturnal departure time may be a crucial parameter shaping the speed of migration indicating the significance of its integration in future migration models. © 2013 Schmaljohann et al.; licensee BioMed Central Ltd. Source

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