| 11. |
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| 12. |
- Jonzén, Niclas, et al.
(författare)
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The timing of spring migration in trans-Saharan migrants: a comparison between Ottenby, Sweden and Capri, Italy
- 2006
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Ingår i: Ornis Svecica. - 1102-6812. ; 16:1-2, s. 27-33
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Tidskriftsartikel (refereegranskat)abstract
- Some migratory birds have advanced their spring arrival to Northern Europe, possibly by increasing the speed of migration through Europe in response to increased temperature en route. In this paper we compare the phenology of spring arrival of seven trans-Saharan migrants along their migration route and test for patterns indicating that migration speed varied over the season using long-term data collected on the Italian island of Capri and at Ottenby Bird Observatory, Sweden. There was a linear relationship between median arrival dates on Capri and at Ottenby. The slope was not significantly different from one. On average, the seven species arrived 15 days later at Ottenby compared to Capri. There was a (non-significant) negative relationship between the species-specific arrival dates at Capri and the differences in median arrival dates between Capri and Ottenby, possibly indicating a tendency towards faster migration through Europe later in the season. To what extent different species are able to speed up their migration to benefit from the advancement of spring events is unknown.
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| 13. |
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| 14. |
- Knape, Jonas, et al.
(författare)
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An analysis of hatching success in the great reed warbler Acrocephalus arundinaceus
- 2008
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Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; :117, s. 430-438
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Tidskriftsartikel (refereegranskat)abstract
- Hatching success is a potentially important fitness component for avian species. Previous studies of hatching success in natural populations have primarily focused on effects of inbreeding but a general understanding of variation in hatching success is lacking. We analyse data on hatching success in a population of great reed warblers Acrocephalus arundinaceus in Lake Kvismaren in south central Sweden. The effects of a range of covariates, including three measures of inbreeding as well as effects of classifications in the data (such as identities of individuals), on hatching success are analysed simultaneously. This is done by means of fitting Bayesian binomial mixed models using Markov chain Monte Carlo methods. Using random effects for each individual parent we check for unexplained variation in hatching success among male and female individuals and compare it to effects of covariates such as degree of inbreeding. Model selection showed that there was a significant amount of unexplained variation in hatching probability between females. This was manifested by a few females laying eggs with a substantially lower hatching success than the majority of the females. The deviations were of the same order of magnitude as the significant effect of parent relatedness on hatching success. Whereas the negative effect of parent relatedness on hatchability is an expression of inbreeding, the female individual effect is not due to inbreeding and could reflect maternal effects, that females differ in fertilisation and/or incubation ability, or an over representation of genetic components from the female acting on the early developing embryo.
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| 15. |
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| 16. |
- Knape, Jonas, et al.
(författare)
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Individual heterogeneity and senescence in Silvereyes on Heron Island
- 2011
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Ingår i: Ecology. - : John Wiley & Sons. - 0012-9658 .- 1939-9170. ; 92:4, s. 813-820
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Tidskriftsartikel (refereegranskat)abstract
- Individual heterogeneity and correlations between life history traits play a fundamental role in life history evolution and population dynamics. Unobserved individual heterogeneity in survival can be a nuisance for estimation of age effects at the individual level by causing bias due to mortality selection. We jointly analyze survival and breeding output from successful breeding attempts in an island population of Silvereyes (Zosterops lateralis chlorocephalus) by fitting models that incorporate age effects and individual heterogeneity via random effects. The number of offspring produced increased with age of parents in their first years of life but then eventually declined with age. A similar pattern was found for the probability of successful breeding. Annual survival declined with age even when individual heterogeneity was not accounted for. The rate of senescence in survival, however, depends on the variance of individual heterogeneity and vice versa; hence, both cannot be simultaneously estimated with precision. Model selection supported individual heterogeneity in breeding performance, but we found no correlation between individual heterogeneity in survival and breeding performance. We argue that individual random effects, unless unambiguously identified, should be treated as statistical nuisance or taken as a starting point in a search for mechanisms rather than given direct biological interpretation.
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| 17. |
- Knape, Jonas, et al.
(författare)
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Multivariate state space modelling of bird migration count data
- 2009
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Ingår i: Environmental and Ecological Statistics. - Berlin : Springer. - 1352-8505 .- 1573-3009. ; 3:1, s. 59-79
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Tidskriftsartikel (refereegranskat)abstract
- We analyse 54 year long time series data on the numbers of common redstart (Phoenicurus phoenicurus), common whitethroat (Sylvia communis), garden warbler (Sylvia borin) and lesser whitethroat (Sylvia curruca) trapped in spring and autumn at Ottenby Bird Observatory, Sweden. The Ottenby time series could potentially serve as a reference on how much information on population change is available in count data on migrating birds. To investigate this, we combine spring and autumn data in a Bayesian state-space model trying to separate demographic signals and observation noise. The spring data are assumed to be a measure of the breeding population size, whereas the autumn data measure the population size after reproduction. At the demographic level we include seasonal density dependence and model winter dynamics as a function of precipitation in the Sahel region, south of the Sahara desert, where these species are known to spend the winter. Results show that the large fluctuations in the data restrict what conclusions can be drawn about the dynamics of the species. Annual catches are highly correlated between species and we show that a likely explanation for this is that trapping numbers are strongly dependent on local weather conditions. A comparative analysis of a related data set from the Courish Spit, Russia, gives rather different dynamics which may be caused by low information in the two data sets, but also by distinct populations passing Ottenby and the Courish Spit. This highlights the difficulty of validating results of the analyses when abundance indices derived by other methods or from other populations do not agree.
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| 18. |
- Knape, Jonas, et al.
(författare)
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On observation distributions for state space models of population survey data
- 2011
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Ingår i: Journal of Animal Ecology. - : Wiley. - 0021-8790 .- 1365-2656. ; 80:6, s. 1269-1277
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Tidskriftsartikel (refereegranskat)abstract
- 1. State space models are starting to replace more simple time series models in analyses of temporal dynamics of populations that are not perfectly censused. By simultaneously modelling both the dynamics and the observations, consistent estimates of population dynamical parameters may be obtained. For many data sets, the distribution of observation errors is unknown and error models typically chosen in an ad-hoc manner. 2. To investigate the influence of the choice of observation error on inferences, we analyse the dynamics of a replicated time series of red kangaroo surveys using a state space model with linear state dynamics. Surveys were performed through aerial counts and Poisson, overdispersed Poisson, normal and log-normal distributions may all be adequate for modelling observation errors for the data. We fit each of these to the data and compare them using AIC. 3. The state space models were fitted with maximum likelihood methods using a recent importance sampling technique that relies on the Kalman filter. The method relaxes the assumption of Gaussian observation errors required by the basic Kalman filter. Matlab code for fitting linear state space models with Poisson observations is provided. 4. The ability of AIC to identify the correct observation model was investigated in a small simulation study. For the parameter values used in the study, without replicated observations, the correct observation distribution could sometimes be identified but model selection was prone to misclassification. On the other hand, when observations were replicated, the correct distribution could typically be identified. 5. Our results illustrate that inferences may differ markedly depending on the observation distributions used, suggesting that choosing an adequate observation model can be critical. Model selection and simulations show that for the models and parameter values in this study, a suitable observation model can typically be identified if observations are replicated. Model selection and replication of observations, therefore, provide a potential solution when the observation distribution is unknown.
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| 19. |
- Knudsen, Endre, et al.
(författare)
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Characterizing bird migration phenology using data from standardized monitoring at bird observatories
- 2007
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Ingår i: Climate Research. - : Inter-Research Science Center. - 1616-1572 .- 0936-577X. ; 35:1-2, s. 59-77
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Forskningsöversikt (refereegranskat)abstract
- Long-term data from standardized monitoring programmes at bird observatories are becoming increasingly available. These data are frequently used for detecting changes in the timing of bird migration that may relate to recent climate change. We present an overview of problematic issues in the analysis of these data, and review approaches to and methods for characterizing bird migration phenology and its change over time. Methods are illustrated and briefly compared using autumn data on garden warbler Sylvia borin from a standardized mist-netting programme at Lista bird observatory, southern Norway. Bird migration phenology is usually characterized rather coarsely using a small number of sample statistics such as mean, median and selected quantiles. We present 2 alternative approaches. Smoothing methods describe the within-season pattern in the data at an arbitrary level of detail, while fitting a parametric seasonal distribution curve offers a coarse description of migration phenology relatively robust to sampling effects. Various methods for analyzing linear trends in the timing of bird migration are reviewed and discussed. Exploratory studies using long-term data gathered at bird observatories can yield more detailed insight into the phenomenon of bird migration and how phenologies relate to climate. Methodological advances are needed, particularly in order to better characterize the shape of phenological distributions and separate between sampling effects and 'true' phenology.
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| 20. |
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