Summary 1.In a system where depletion drives a habitat shift, the hypothesis was tested that animals switch habitat as soon as the average daily net energy intake (or gain) drops below that attainable in the alternative habitat. 2. The study was performed in the Lauwersmeer area. Upon arrival during the autumn migration, Bewick's swans first feed on below-ground tubers of fennel pondweed on the lake, but subsequently switched to feeding on harvest remains in sugar beet fields. 3. The daily energy intake was estimated by multiplying the average time spent foraging per day with the instantaneous energy intake rate while foraging. In the case of pondweed feeding, the latter was estimated from the functional response and the depletion of tuber biomass. In the case of beet feeding, it was estimated from dropping production rate. Gross energy intake was converted to metabolizable energy intake using the assimilation as determined in digestion trials. The daily energy expenditure was estimated by the time-energy budget method. Energetic costs were determined using heart rate. 4. The daily gain of pondweed feeding at the median date of the habitat switch (i.e. when 50% of the swans had switched) was compared with that of beet feeding. The daily gain of beet feeding was calculated for two strategies depending on the night activity on the lake: additional pondweed feeding (mixed feeding) or sleeping (pure beet feeding). 5. The majority of the swans switched when the daily gain they could achieve by staying on the pondweed bed fell just below the average daily gain of pure beet feeders. However, mixed feeders would attain an average daily gain considerably above that of pondweed feeders. A sensitivity analysis showed that this result was robust. 6. We therefore reject the hypothesis that the habitat switch by swans can be explained by simple long-term energy rate maximization. State-dependency, predation risk, and protein requirements are put forward as explanations for the delay in habitat switch.

We tested whether the spatial variation in resource depletion by Tundra Swans (Cygnus columbianus) foraging on belowground tubers of sago pondweed (Potamogeton pectinatus) was caused by differences in net energy intake rates. The variation in giving‐up densities within the confines of one lake was nearly eightfold, the giving‐up density being positively related to water depth and, to a lesser extent, the silt content of the sediment. The swans' preference (measured as cumulative foraging pressure) was negatively related to these variables. We adjusted a model developed for diving birds to predict changes in the time allocation of foraging swans with changes in power requirements and harvest rate. First, we compared the behavior of free‐living swans foraging in shallow and deep water, where they feed by head‐dipping and up‐ending, respectively. Up‐ending swans had 1.3–2.1 times longer feeding times than head‐dipping swans. This was contrary to our expectation, since the model predicted a decrease in feeding time with an increase in feeding power. However, up‐ending swans also had 1.9 times longer trampling times than head‐dipping swans. The model predicted a strong positive correlation between trampling time and feeding time, and the longer trampling times may thus have masked any effect of an increase in feeding power. Heart rate measurements showed that trampling was the most energetically costly part of foraging. However, because the feeding time and trampling time changed concurrently, the rate of energy expenditure was only slightly higher in deep water (1.03–1.06 times). This is a conservative estimate since it does not take into account that the feeding costs of up‐ending are possibly higher than that of head‐dipping. Second, we compared captive swans foraging on sandy and clayey sediments. We found that the harvest rate on clayey sediment was only 0.6 times that on sandy sediment and that the power requirements for foraging were 1.2–1.4 times greater. Our results are in qualitative agreement with the hypothesis that the large spatial variation in giving‐up densities was caused by differences in net rates of energy intake. This potentially has important implications for the prey dynamics, because plant regrowth has been shown to be related to the same habitat factors (water depth and sediment type).

Summary 1.Vertebrates are important seed dispersers for many plants, particularly those inhabiting naturally fragmented habitats such as lakes and wetlands. Such dispersal often takes place through the transport of ingested seeds (endozoochory). 2. Endozoochorous passage of seeds is likely to vary among both disperser and dispersed species. We hypothesized that seed retention time and survival of gut passage varies among disperser species (here Anas ducks) and is influenced by intraspecific differences in seed size. 3. Wigeongrass ( Ruppia maritima ) seeds were ingested by five duck species; Teal, Wigeon, Shoveler, Pintail and Mallard. Defecated seeds were recovered and germinated. 4. Total retrieval and germination of seeds, patterns of retrieval over time and seed weight before and after gut passage did not differ among duck species. Hence interspecific differences among Anas ducks and intraspecific differences in seed weight do not affect seed retention time or the response of seeds to gut passage. 5. Germination of retrieved seeds was influenced by the retention time in the gut, with seeds voided earlier more likely to germinate than those voided later. 6. The probability of dispersal at different retention times by any given duck was low. However, when considering the thousands of ducks moving among wetlands, the dispersal probabilities of seeds become significant. 7. Estimation of seed dispersal distance as a function of retention time suggested higher dispersal probabilities for seeds voided earlier. Based on average flight speeds ranging from 10 to 70 km h − 1 , most probable dispersal distances range from 40 to 280 km. Dispersal over greater distances is possible, but less likely.