Julia Stahl

Subordinates explore but dominants profit: resource competition in high Arctic barnacle goose flocks

Social dominance plays an important role in assessing and obtaining access to patchy or scarce food sources in group-foraging herbivores. We investigated the foraging strategies of individuals with respect to their social position in the group in a flock of nonbreeding, moulting barnacle geese, Branta leucopsis, on high Arctic Spitsbergen. We first determined the dominance rank of individually marked birds. The dominance of an individual was best described by its age and its sex-specific body mass. Mating status explained the large variation in dominance among younger birds, as unpaired yearlings ranked lowest. In an artificially created, competitive situation, subordinate individuals occupied explorative front positions in the flock and were the first to find sites with experimentally enriched vegetation. Nevertheless, they were displaced quickly from these favourable sites by more dominant geese which were able to monopolize them. The enhanced sites were subsequently visited preferentially by individuals that succeeded in feeding there when the exclosures were first opened. Data on walking speed of foraging individuals and nearest-neighbour distances in the group suggest that subordinates try to compensate for a lower energy intake by exploring and by lengthening the foraging bout. Observations of our focal birds during the following breeding season revealed that females that returned to the study area were significantly more dominant in the previous year than those not seen in the area again. Copyright 2001 The Association for the Study of Animal Behaviour.

Travelling to breed

Traditionally, investigation of the dynamics of avian migration has been heavily biased towards the autumn return trip to the wintering quarters. Since the migratory prelude to breeding has direct fitness consequences, the European Science Foundation recently redressed the balance and sponsored a workshop on spring travels. We here survey the findings elucidating the complications arising during migration directed towards the breeding quarters. The evidence that early nesting confers advantage is overwhelming, hence demands of reproduction pose a constraint on both time and energy resources during spring. Both during migration and upon arrival there must therefore be strong selection in favour of saving time. Experimental results (e.g. using supplementary feeding) show that the date of laying is generally proximately constrained by the inability of the female to find sufficient nutrients. A key question thus concerns the implementation of the ‘capital’ strategy for breeding, the female accumulating nutrient stores along the way to bridge periods of shortage upon arrival on the breeding grounds. Eight studies on waterfowl (geese and eiders) and shorebirds (turnstones and knots) nesting in the arctic combine tracking of individuals (satellite telemetry) with direct observation (marked birds) and reconstruction of the origin of nutrients laid down in the eggs or in the form of body stores of the female parent (stable isotope signatures of tissues compared to potential food sources). The consensus emerges that in most cases a mixed strategy prevails, with nutrients garnered locally supplementing ‘imported’ body stores. The species concerned face a shortage of feeding time during incubation and suffer an energy deficit. Successful breeding thus depends on adequate fat depots that form part of the ‘capital’ the parents bring with them. Some headway has been made in predicting the extent of dependence on body stores for breeding in relation to body mass and length of migration from rates of fat deposition during stopover and fat consumption during flight for waterfowl. This work poses a challenge to refine field studies accordingly. The pressure to save time en route highlights the need to effectively exploit rich food resources. Several case studies underline the crucial significance of a very limited set of stopover sites, each with a narrow time window of optimal harvest. The influence of man at such sites often verges on the disastrous, and ongoing climate change may unhinge the finely tuned timing long-distance migrants depend on. There is a real need to extend this work to provide the scientific basis to underpin adequate conservation actions.

Skipping the Baltic : the emergence of a dichotomy of alternative spring migration strategies in Russian barnacle geese

1. Since the early 1990s, an increasing proportion of barnacle geese, Branta leucopsis, bound for breeding sites in the Russian Arctic delay their departure from the wintering quarters in the Wadden Sea by 4 weeks. These late-migrating geese skip spring stopover sites in the Baltic traditionally used by the entire population. 2. Individual geese from an arctic colony tracked by satellite or light-level geolocators during spring migration in 2004 and 2005 predominantly followed the new strategy, but a minority still maintained the traditional pattern. Despite a spread of more than 50 days in departure date from the Wadden Sea, both early and late departing females laid their eggs within the short time-window conferring breeding success. 3. The spread of these new migration routines coincided with a strong increase of overall numbers and the exploitation of new spring staging resources in the Wadden Sea. Counts from Estonia demonstrate that numbers have levelled off recently at the Baltic staging sites, suggesting that the capacity of these staging sites in spring has been reached. Although onset of spring affects migratory timing in barnacle geese, it cannot explain the observed delay in departure from the wintering grounds. 4. We hypothesize that the new migratory strategy has evolved in response to increased competition for food at spring staging sites in the Baltic. According to an analytical model of optimal migration, the geese should skip the Baltic whenever the energy deposition rate falls below 88% of the Wadden Sea value.

Sward height and bite size affect the functional response of barnacle geese Branta leucopsis

Intake rate, the rate in which herbivores can process their food, is presumed to be an important factor in habitat selection down to the scale of the foraging patch. Much attention has been given to the selection of swards of high nutritional quality, but much less has been given to the influences of sward structure on patch selection in small herbivores. In this study we tested the effects of sward density and height on the functional foraging response of barnacle geese, Branta leucopsis. The functional response curve for herbivores describes how intake rate is affected by food availability. We conducted feeding trials to determine intake rate and bite size of barnacle geese on experimentally manipulated swards. Results indicate that intake rate is mainly dependent on sward height and that there is a strong correlation between bite size and intake rate. Sward density does not influence the rate of food consumption; it is, however, a crucial parameter affecting potential total yield. We conclude that bite size is the crucial parameter influencing intake rate. Bite size is explained both by sward height and individual differences in bill morphology. Furthermore, intake rate seems to be dependent on the physical structure of the grass species consumed.

Widespread Usutu virus outbreak in birds in the Netherlands, 2016

We report a widespread Usutu virus outbreak in birds in the Netherlands. Viral presence had been detected through targeted surveillance as early as April 2016 and increased mortality in common blackbirds and captive great grey owls was noticed from August 2016 onwards. Usutu virus infection was confirmed by post-mortem examination and RT-PCR. Extensive Usutu virus activity in the Netherlands in 2016 underlines the need to monitor mosquito activity and mosquito-borne infections in 2017 and beyond.

Habitat use of barnacle geese at a subarctic salt marsh in the Kolokolkova Bay, Russia

Along the east Atlantic migratory flyway, goose and swan species rely on the availability of suitable coastal habitats as staging sites during migration and for breeding. Especially for the Russian part of the flyway, detailed descriptions of these habitats in relation to use by herbivores are lacking. We report on salt-marsh characteristics and habitat use by barnacle geese in a recently established breeding site, near the village Tobseda at Kolokolkova Bay in the Pechora Delta, Russia. A classification of plant communities was made and both vegetation and soil properties were measured. Goose visitation was assessed by counting droppings. From the bay towards the dunes there was a gradient of decreasing salinity, along with a gradual shift in plant communities from salt marshes to freshwater bogs, tundra and dunes. Barnacle geese visited salt-marsh communities of the low and middle marsh more than the high marsh and non-saline communities. Two low marsh communities were preferred. Both had high tiller densities and low canopy heights and were dominated by Carex subspathacea, a species with high forage quality. Those characteristics are selected by geese, but might be maintained through grazing. The salt marsh at our site bears similarities to other marshes at similar latitudes along the Russian coast and in Canada. We discuss whether processes like grazing facilitation and habitat degeneration through overgrazing occur on our site.

Habitat preference of geese is affected by livestock grazing : Seasonal variation in an experimental field evaluation

The number of staging geese in northwestern Europe has increased dramatically. Growing goose numbers put strong grazing pressure on agricultural pastures. Damage to agricultural land may be mitigated by managing nature reserves in order to optimally accommodate large numbers of grazing geese. Livestock grazing has been shown to facilitate foraging geese; we take the novel approach of determining the effects of four different livestock grazing treatments in a replicated experiment on the distribution of geese. We present experimental field evidence that livestock grazing of a salt marsh in summer affects the habitat preference of foraging geese during autumn and spring staging. In an experimental field set-up with four different livestock grazing treatments we assessed goose visitation through dropping counts, in both autumn and spring. Grazing treatments included 0.5 or 1 horse ha-1 and 0.5 or 1 cattle ha-1 during the summer season. The livestock grazing regime affected goose distribution in autumn, just after livestock had been removed from the salt marsh. In autumn, goose visitation was highest in the 1 head ha-1 grazing treatments, where grazing intensity by livestock was also highest. In line with this result, goose visitation was lowest in the 0.5 head ha-1 livestock grazing treatments, where the grazing intensity by livestock was lowest. The differences in goose visitation among the experimental treatments in autumn could not be explained by the canopy height. In spring we did not find any effect of livestock grazing treatment on goose visitation. Differences in the distribution of geese over the experiment between autumn and spring may be explained by changes in the availability of nutrient-rich vegetation. Livestock summer grazing with a high stocking density, especially with horses, can be used to attract geese to salt marshes in autumn and potentially reduces damage caused by geese to inland farmland. From a nature conservation interest point of view, however, variation in structure of the vegetation is a prerequisite for other groups of organisms. Hence, we recommend grazing of salt marshes with densities of 0.5 head ha-1 of livestock when goose conservation is not the only management issue.

Top-down vs. bottom-up control on vegetation composition in a tidal marsh depends on scale

The relative impact of top-down control by herbivores and bottom-up control by environmental conditions on vegetation is a subject of debate in ecology. In this study, we hypothesize that top-down control by goose foraging and bottom-up control by sediment accretion on vegetation composition within an ecosystem can co-occur but operate at different spatial and temporal scales. We used a highly dynamic marsh system with a large population of the Greylag goose (Anser anser) to investigate the potential importance of spatial and temporal scales on these processes. At the local scale, Greylag geese grub for below-ground storage organs of the vegetation, thereby creating bare patches of a few square metres within the marsh vegetation. In our study, such activities by Greylag geese allowed them to exert top-down control by setting back vegetation succession. However, we found that the patches reverted back to the initial vegetation type within 12 years. At large spatial (i.e. several square kilometres) and temporal scales (i.e. decades), high rates of sediment accretion surpassing the rate of local sea-level rise were found to drive long-term vegetation succession and increased cover of several climax vegetation types. In summary, we conclude that the vegetation composition within this tidal marsh was primarily controlled by the bottom-up factor of sediment accretion, which operates at large spatial as well as temporal scales. Top-down control exerted by herbivores was found to be a secondary process and operated at much smaller spatial and temporal scales.

Long-term effects of scrub clearance and litter removal on the re-establishment of dry alvar grassland species

Many characteristic dry alvar grassland species disappear after cessation of livestock grazing as a result of encroachment by Juniperus communis. We studied the re-establishment of these species after scrub clearance with and without the removal of the layer of litter and mosses in long-term (14 years) permanent plots. Most of the species belonging to the community species pool of dry alvar grassland species before clearance were found in permanent plots between 2 and 14 years after the clearance. A large part originated from vegetative spread of already occurring species in the established vegetation in the surroundings. Only a small part of the long-term persistent soil seed bank resulted in the re-establishment of alvar species. There was no significant difference in the traits soil seed bank, seed weight and long-distance dispersal by wind, dung or fur of animals of established and non-established species. Removal of litter and mosses positively affected the re-establishment of alvar species.

Environmental parameters linked to the last migratory stage of barnacle geese en route to their breeding sites

The migration timing of birds can be controlled by endogenous parameters. However, little is known about how environmental parameters influence the timing of migration and which have the greatest influence at different stages of migration. In this study we identified the main environmental parameters that correlate with the timing of the last stage of spring migration for the barnacle goose, Branta leucopsis. GPS tracking data were registered for 12 barnacle geese (in 2008–2010) on the Russian flyway and 17 (2006–2010) on the Svalbard flyway. A linear mixed-effect model and principal component analysis were used to retrieve statistically significant parameters. Departure date from the last staging site on the Russian flyway was related to daylength, temperature, cloud cover and barometric pressure, and on the Svalbard flyway to a food availability index and daylength. Arrival date at the Russian breeding site was related to cloud cover and barometric pressure en route and the food availability index and temperature at the breeding site. For the Svalbard flyway, temperature and cloud cover en route and the food availability index, wind, temperature and cloud cover at the breeding site were significantly related to arrival date at the breeding site. Our study highlights the importance of environmental parameters including food, weather and daylength for the last stage of goose spring migration. We found different priorities in selecting the environmental parameters in migration timing decisions between Svalbard and Russian barnacle geese which fly over sea and over land, respectively. Identifying the key factors that act as cues during the final stages of spring migration is important when assessing the possible effects of climate change on the timing of migration for a highly selective herbivore such as the barnacle goose.