Nederlands
Herbivore foraging theory (7 Topics)
1. Relating food distribution, mapped by satellite imagery, to the distribution of bewick's swans
Key words
Carrying capacity, optimal foraging theory, Bewick's swans, fennel pondweed, fieldwork.
General context
Food densities differ dramatically throughout space. Foragers that are able to respond to this heterogeneity can potentially profit from it by putting more foraging effort in spots with a higher food density. In order to achieve the highest possible long term intake rate foragers have to decide on the amount of time to be spent at a location with a certain food density. This location might mean a small feeding patch or (e.g. as in the case of migratory birds) even a whole staging site.
The Bewick`s swan, our study species, forages on belowground tubers of fennel pondweed and therefore has no direct indication on the distribution of the food items. Consequently, the birds have to sample the sediment and on a certain moment decide to move on to the next patch. But what is the driving force behind this decision? Purely intake rate derived from food density? Or other factors, such as interference (with other swans and bird species), abiotic factors (for example sediment type, water depth) and social status are also playing a prominent role?
Project outline
This student project consists of (a lot of) fieldwork at the Lauwersmeer in the northern part of the Netherlands and consequent data analysis at the NIOO center in Nieuwersluis. The fieldwork takes place in the autumn with plant biomass sampling in September and November and observations of Bewick's swans in October.
Duration (indicative)
5-6 Months starting the latest in September (every year).
Contact
Abel Gyimesi, tel.: +31 (0)294 239 346, e-mail:a.gyimesi@nioo.knaw.nl
2. Change of foraging habits under interference
Key words
Foraging, optimal foraging theory, mallard, experimental setup, interference.
General context
Many foragers do not know the quality of a spot upon arrival but have to sample the environment to assess it. Basically, they can only make assumptions about the food density and about when it is the most advantageous to leave a certain foraging patch and move to another one.
However, we believe that there are several factors, among others interference, which have a large impact on this search pattern. The animals might follow other rules of thumb while foraging alone than foraging in company. The effect might also depend on the social status of the individuals and on the way food items are distributed throughout the environment. In an experimental study we want to explore how interference influences the foraging pattern and consequently food intake rate. Furthermore, we would like to derive general predictions about the foraging pattern of wild waterfowl in natural environments of different qualities.
Project outline
This student project is based on an experiment carried out with captive mallards at the NIOO center in Nieuwersluis. This project has a strong link to our study system of Bewick`s Swans foraging on submerged food items at the Lauwersmeer.
Duration (indicative)
Approximately 4 months, starting as soon as possible.
Contact
Abel Gyimesi, tel.: +31 (0)294 239 346, e-mail: a.gyimesi@nioo.knaw.nl
3. Diving ducks as competitors with bewick’s swans for food
Key words
Foraging ecology, interspecific competition, experimental setup, interference.
General context
We are planning to conduct a (very animal friendly) behavioural experiment with Bewick`s swans, trying to answer the question whether diving ducks can steal a considerable amount of food from foraging swans. The larger body-sized swans are capable of digging up tubers buried in the sediment. They trample with their feet to dig a pit and feed on the up-whirling tubers with their necks submerged. Diving ducks, which are normally not capable of utilizing this food source on their own, can greatly benefit from the floating tubers, basically by stealing from the swans. By gaining information on the extent of this kleptoparasitic behaviour, we might be able to understand better the habitat choices the swans make during their winter visit in the Netherlands.
Project outline
The project is available between February and July and will be carried out at the Centre of Terrestrial Ecology of the NIOO, situated in Heteren (Gelderland), nearby Arnhem. Student accommodations are available.
Duration (indicative)
Approximately 5-6 months, starting in early spring 2009.
Contact
Abel Gyimesi (e-mail:a.gyimesi@nioo.knaw.nl; tel: +31 (0)294 239 346)
4. How to forage in a flock?
Key words
Exploitative competition; flock-feeding; marginal value theorem; patch use; social foraging theory.
General context
A solitary forager is predicted to move successively between food patches and only to visit each patch once. According to the marginal value theorem, such a forager should leave a patch when the intake rate in the patch drops to the average intake rate for the habitat. When competitors are around, however, the optimal patch-leaving behaviour may be different, in particular when there is interference. When competitors do not interfere with each other, the gain rate of animals that stay in the patch does not increase after one of the other animals in the patch has left, so all individuals should still leave at the time predicted by the marginal value theorem. Still, under such so-called exploitative competition, the best tactic is presumably to "catch as catch can": to deplete patches down to the current average intake rate of the environment rather than down to the final quitting intake rate. Partly exploited patches are later revisited, so in effect flock-feeding birds are predicted to deplete patches in a few visits in succession. This hypothesis remains to be tested.
Project outline
In order to test the hypothesis, an experiment will be carried out in which captive mallards will be feeding in a basin on patches of grain, either alone or in pairs. The time devoted to the patches and the sequence of patch visits will be recorded and analysed with video analysis software. Food will be retrieved from the patches in order to determine the giving-up densities in the two cases. Experiments have to be approved by the Animal Welfare Committee.
Duration (indicative)
4-6 Months, preferably planned in advance.
Contact
Bart Nolet, e-mail: b.nolet@nioo.knaw.nl
5. Functional response and patch choice of grass-feeding Bewick's swans
Key words
Functional response; grass; grazing; herbivory; patch choice; swan.
General context
Movement patterns of free-ranging animals are often dictated by energy and nutrient requirements. In order to understand constraints on energy gain better, we need to establish the relation between food abundance and energy intake, i.e. the functional response. In our working group we try to link up individual movement patterns of Bewick's swans (Cygnus columbianus bewickii) to their population dynamics and we therefore need to quantify how energy intake of swans varies with food abundance and quality. For their main diet in winter, grass, this relation is yet unknown.
Project outline
We will experimentally offer a range of grass types (height and quality) to captive Bewick's swans and record their intake rate. At the same time, the swans will be offered choices between different fields of grass and their preference will be recorded. At NIOO-CTE in Heteren we have 6 individual swans available for this purpose. Behaviour (step and bite rates) will be recorded on videotapes, which will be analysed in detail using video-analysis package The Observer (Noldus). Quality and height of the grass will be sampled and measured later on in the lab at NIOO-CL in Nieuwersluis.
Practical information
The approximate starting date is as soon as possible, but this can be changed to some extend. The project lasts about 6 months and constitutes in chronological order of experimentation (2 months), video and data analyses (2 months), and writing and presenting a report (2 months). If required, student residents are available both in Heteren and in Nieuwersluis. In case you are interested in the subject and like to work with these pretty birds, please contact Jan van Gils at NIOO-CL (e-mail: j.vangils@nioo.knaw.nl; tel.: +31 (0)294 239 344).
6. Fuelling on grass by Bewick's swans: what are the bottlenecks?
Keywords
Fieldwork; Bewick's swan; foraging; grass; migration.
General context
When fuelling for spring migration, Bewick's swans (Cygnus columbianus bewickii) mostly feed on grass. Compared to the food they consume during the rest of the year (tubers; sugarbeets), grass is rather indigestible. One way out for the birds would be to postpone their fuelling until the grass starts growing in late winter/early spring. Young shoots of grass contain relatively low amounts of indigestible fibers and swans may be able to take advantage of this so called 'green wave'. However, being big birds, Bewick's swans have relatively low speeds of migration and therefore they presumably cannot afford to wait for the earliest grass growth period. However, swans seem to take an alternative pathway in order to enhance the digestibility of their food. Carcass dissections have shown that Bewick's nearly double the length of their guts in the course of winter. In this way, they can increase the retention time of the food in their gastrointestinal tract, which enhances digestive efficiency.
Project outline
In the field (Arkenheem polder), we will measure digestive efficiency, food retention times and grass quality. We will do so by making detailed protocols of foraging Bewick's swans, during which we will score bite and dropping rates. Since quite some free-living swans carry a uniquely coded neck- or leg-band, changes in digestive efficiency can be traced within individual birds.
Remarks
Driving license is required; a car is available at our centre.
Duration (indicative)
More or less 5-6 months during wintertime.
Contact
Jan van Gils (e-mail: j.vangils@nioo.knaw.nl; tel.: +31 (0)294 239 344).
7. Tracking individual Bewick's swans throughout their wintering range using GPS-loggers
Keywords
Fieldwork; Bewick's swan; GPS-loggers; avian spatial ecology.
General context
Within the Avian Spatial Ecology project we aim to understand spatial distributions of animals. We approach the problem from an individually-based angle and ask questions such as: where should animals feed in relation to the abundance of food, conspecifics and roosting sites. We hope to be able to disentangle 'rules of thumb' that animals follow in their daily whereabouts and we assume a key role for the individual's energy budget. Bewick's swans (Cygnus columbianus bewickii) are an ideal species to study these problems as many are individually coded by leg or neck band and attract a dense network of amateur observers. The recent miniaturization of global positioning systems (GPS) will allow us to study individual movements of Bewick's swans in even greater detail. Eventually, we hope to be able to quantify the outcome of movement-decisions in units of fitness, which will enable us to scale-up to dynamics in their population.
Project outline
Detailed field-observations on fuelling rates of GPS-equipped Bewick's swans. Estimates on fuelling rates can be obtained on a short-term scale by quantification of bite and dropping rates. Changes in the abdominal profile index (API) yield estimates on long-term average fuelling rates. After the field work, individual energy budgets will be reconstructed in combination with geographical analyses of movement patterns.
Remarks
Driving license is required.
Duration (indicative)
More or less 5-6 months during wintertime.
Contact
Jan van Gils (e-mail: j.vangils@nioo.knaw.nl; tel.: +31 (0)294 239 344).