Project title:	Energy expenditure of Bewick swans
Collaborators:	Dr Marcel Klaasen & Dr Bart Nolet, NIOO
Funding:	Royal Society

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Background

In most situations animals strive to maximise their net energy gain, i.e. the difference between their energy benefits (food gained) and costs (energy expended obtaining that food). In the case of a predator foraging in a patchy environment, the long-term rate of energy gain will be maximised when the predator leaves a patch when the marginal rate equals the average rate that the predator achieves in the habitat as a whole¹. As the predator's intake rate usually increases with prey density, this model predicts that animals foraging in a patchy environment should equalise resource density over all exploited food patches.

Tundra Swans Cygnus columbianus feed on the below-ground tubers of Sago Pondweed Potamogeton pectinatus² during both their spring and autumn migration. In accordance with the optimal foraging model, swans graze the pondweed in exploited fields down to the same density as that found in unexploited fields³. However, this grazing threshold varied between two different lakes, which was attributed to the difference in the sediment type (sand versus clay) at the two lakes. It was hypothesised that the swan uses more energy to dig for tubers in a clay sediment than in a sandy one⁴. Recent fieldwork has revealed that such a sediment-dependent difference in grazing threshold is also present within the confines of an individual lake. This finding therefore excludes the possibility that the difference seen in the threshold levels is caused by variations in the alternative food source i.e. where the swans go when they stop foraging in an area⁵. In addition, the water depth influenced the grazing threshold. This phenomenon of site-dependent grazing thresholds is very interesting since the regrowth of the plant (from sprouting tubers) is also affected by the same factors. For example, at deep sites the sprouting tubers receive less light than at shallow ones, while sites with a clay sediment are richer in nutrients than those with a sandy one⁶.

Hypotheses tested

In the proposed project, we wanted to test experimentally whether the differences in foraging costs could explain the site-dependent differences in grazing threshold. We hypothesised that the net energy gain by the bird at each site would be the same. However, we also hypothesised that the swans would stop foraging at the deep or clay sediment sites at higher prey densities. This we hypothesised was due to the higher costs associated with extracting the prey than at shallow or sandy sites. In order to test this, we measured both the energy intake rate of the swans in relation to water depth and sediment type and also their energy expenditure in relation to these factors. We used heart rate to estimate the energy expenditure^7,8,9 of the swans. This method has been shown to estimate accurately the rate of energy expenditure when the heart rate of individual birds has been calibrated under a range of exercise levels^7,8,9.

Experimental set-up

Experiments were were carried out at the NIOO bird facilities in Heteren and at field facilities set up in the Lauwersmeer a man-made lake in the North of Holland. For the experiments, we used four captive Bewick's Swans C. c. bewickii. During the energetic studies, their behavioural responses under the different conditions were monitored as well as their assimilation of pondweed tubers. This enabled us to calculate the costs and benefits of foraging at different sites for each individual separately.

The research involves three main aspects.

1. Calibrations

2. Basin experiments

3. Fieldwork

Initially, the relationship between heart rate and oxygen consumption (and hence energy expenditure) for each swan was determined by measuring these variables simultaneously during rest (night- and daytime) and at several different walking speeds on a treadmill. Subsequently, the heart rate of each swan was measured while foraging in a T-shaped basin under four conditions (sandy-shallow, sandy-deep, clay-shallow, clay-deep). The foraging behaviour was monitored by video and analysed using the Observer package. Finally, the heart rates of all the swans were monitored when foraging under natural conditions in the Lauwersmeer, a normal foraging area of the migrating birds.

Some initial results

Table 1. Comparison between heart rates (beats.min^-1) of swans foraging on the basin on different sediments