The relationship between population means and variances of reproductive success differs between local populations of white stork (Ciconia ciconia)

Diet analysis and the assessment of plastic and other indigestible anthropogenic litter in the white stork pellets

Pollution by anthropogenic litter is a major threat to global ecosystems. Seabirds are frequently used as environmental monitors of litter ingestion, but similar research is rare for terrestrial birds. Here, we focused on pellet analysis from 117 nests of an iconic bird of the Western Palearctic, the white stork (Ciconia ciconia), breeding in southern and southwestern Poland in a farmland landscape, far away from large dumps and landfills. We found that most prey items in the diet of white storks were invertebrates (particularly from orders Coleoptera, Orthoptera, and Hymenoptera) but vertebrate prey comprised most of the biomass. Further analysis revealed that anthropogenic litter was found in 22.7% of pellets (34.2% of breeding pairs) with plastic (8.4%) and cigarette filters (6.9%) being most prevalent. This study represents the first assessment through pellet analysis of the ingestion of anthropogenic litter by live wild storks in Poland and also by a migratory population of white storks. Our study indicates a potentially significant transfer of plastic and other anthropogenic material through terrestrial food webs.

Do agricultural environments increase the reproductive success of White Stork Ciconia ciconia populations in South-Western Poland?

As populations of White Stork Ciconia ciconia have decreased at different rates in Europe, the specific environmental drivers that influence breeding success are a matter of controversy. Here we use body size, blood, and environmental data of a total of 1226 stork nestlings of different ages from a total of 363 nests in three different habitats (meadows, forest-edges, open agricultural landscapes) in S-W Poland to ask whether: 1) natural grassland environments increase the reproductive output in comparison with agricultural landscapes, 2) nestlings from agricultural landscapes differ in health status from nestlings in more natural habitats, 3) differences in environmental stress translate into respective differences in reproductive output and health status of nestlings. There was no significant difference in age corrected body mass and in the temporal increase in nestling mass between the study sites. Clutch sizes were highest and age corrected total clutch mass lowest at the meadow sites while agricultural and woodland sites did not significantly differ. Hemoglobin and red blood cell content were lowest and white blood cell and blood antioxidant concentrations were highest in the meadows indicating higher degrees of environmental stressors. These blood parameters varied strongly among study years. Our study does not confirm that agricultural landscapes are less suited for stork breeding success. We even find some indication of a better health status of nestlings in agricultural environments that might compensate smaller clutch sizes. Our data indicate that reproductive output is multifaceted. As we found some indication of a trade-off between clutch size and health status we argue that only multiple metrics of reproductive success are able to assess the long-term effect of habitat choice on fitness.

Linking land cover satellite data with dietary variation and reproductive output in an opportunistic forager: Arable land use can boost an ontogenetic trophic bottleneck in the White Stork Ciconia ciconia

Determining how the progressive loss of resources due to agricultural intensification and habitat degradation affect individual fitness and population persistence is a matter of urgency. Here we explored three major questions in order to extend knowledge of the relationship between reproduction rate, diet and energy intake in White Storks Ciconia ciconia based both on our own analysis of pellets and landscape properties sampled in 52 nests in south-western Poland, and published literature data. (1) How many individual prey items are needed to meet the daily energy requirements of nestlings over the brood rearing period? (2) How do the dietary patterns vary under different habitat conditions and what is the spatial scale responsible for these relationships? (3) Is reproductive output related to variations in landscape traits, and is diet variability related to intraspecific competition resulting from colonial breeding? In our estimation, the energy requirements of nestlings during the brood rearing period showed that the most profitable invertebrate prey items were Orthoptera and earthworms. Owing to the nestlings' gape-size constraint (precluding consumption of vertebrate prey items of the size of Common Voles), these most likely comprise the staple diet enabling survival during the first 20 days of life. The total energy content across all the pellets was a simple function (a negative correlation) of %arable land within a distance of 5 km around the nests. White Storks from nests of high-productivity pairs (with 3-4 fledglings and less %arable around) consumed equal %biomasses of invertebrate and vertebrate prey, while invertebrates prevailed in the diet of the low-productivity pairs. Our results suggest that a two-level ontogenetic trophic bottleneck may explain the low productivity of White Stork pairs in simplified landscapes with predominant arable land use. As a result of this, parent birds are unable to satisfy the growing energy demands of nestlings (1) by gathering a sufficient volume of abundant small-sized prey (early nestlings) and (2) by delivering energetically more profitable vertebrate prey at the time of the diet switch.

Climate and spatio-temporal variation in the population dynamics of a long distance migrant, the white stork

1. A central question in ecology is to separate the relative contribution of density dependence and stochastic influences to annual fluctuations in population size. Here we estimate the deterministic and stochastic components of the dynamics of different European populations of white stork Ciconia ciconia. We then examined whether annual changes in population size was related to the climate during the breeding period (the 'tap hypothesis' sensu Saether, Sutherland & Engen (2004, Advances in Ecological Research, 35, 185 209) or during the nonbreeding period, especially in the winter areas in Africa (the 'tube hypothesis'). 2. A general characteristic of the population dynamics of this long-distance migrant is small environmental stochasticity and strong density regulation around the carrying capacity with short return times to equilibrium. 3. Annual changes in the size of the eastern European populations were correlated by rainfall in the wintering areas in Africa as well as local weather in the breeding areas just before arrival and in the later part of the breeding season and regional climate variation (North Atlantic Oscillation). This indicates that weather influences the population fluctuations of white storks through losses of sexually mature individuals as well as through an effect on the number of individuals that manages to establish themselves in the breeding population. Thus, both the tap and tube hypothesis explains climate influences on white stork population dynamics. 4. The spatial scale of environmental noise after accounting for the local dynamics was 67 km, suggesting that the strong density dependence reduces the synchronizing effects of climate variation on the population dynamics of white stork. 5. Several climate variables reduced the synchrony of the residual variation in population size after accounting for density dependence and demographic stochasticity, indicating that these climate variables had a synchronizing effect on the population fluctuations. In contrast, other climatic variables acted as desynchronizing agents. 6. Our results illustrate that evaluating the effects of common environmental variables on the spatio-temporal variation in population dynamics require estimates and modelling of their influence on the local dynamics.