Spatial Variation in Tuber Depletion by Swans Explained by Differences in Net Intake Rates

Abstract

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).

The Sunk-cost Effect as an Optimal Rate-maximizing Behavior

“…However, by giving a foraging theoretic explanation, we show that the fallacy is actually an optimal behavior. This explanation is also consistent with observations of animals that commit longer feeding times after moving into areas where prey requires greater energy to acquire (Nolet et al 2001). Additionally, if a forager's uncertainty in estimating patch quality is modeled as an initial average cost associated with each patch type, the extended foraging durations predicted by our augmented OFT is consistent with the ''wait for good news'' behaviors predicted and observed in supposed Bayesian foragers (Olsson and Holmgren 1998;van Gils et al 2003;Olsson and Brown 2006).…”

Animal–plant–microbe interactions: direct and indirect effects of swan foraging behaviour modulate methane cycling in temperate shallow wetlands

“…time that the swans had their heads under water) over the enclosed area of 12.0 foraging swan hours. The choice of the number of swans used was based on observations on free-living swans in the Lauwersmeer area (Nolet et al 2001). The grazing pressure observed in the cage is in the same range as observed in the field (Nolet 2004;Nolet et al 2001).…”

“…The choice of the number of swans used was based on observations on free-living swans in the Lauwersmeer area (Nolet et al 2001). The grazing pressure observed in the cage is in the same range as observed in the field (Nolet 2004;Nolet et al 2001). Subsequently, swans were removed from the enclosure on 17 November and cores for tuber and soil samples were taken again in the plot (as above).…”

“…Tubers were then separated, counted and weighed individually [fresh weight (f.w. ), accuracy 0.001 g], as described by (Nolet et al 2001). Although both (i.e.…”

“…Sandy and clay-rich sediment were obtained from swan foraging fields at the Lauwersmeer (near Nieuwe Robbengat and Zoutkamp, respectively) after the swans had left the foraging areas. Tubers, stones and other large particles were removed using a 2-mm sieve [for a description of sediment granulometry at those sites, see Nolet et al (2001)]. Sterile sediment was obtained by autoclaving wet sediment samples twice (1 h at 121°C, on 2 subsequent days).…”