Prediction of bird‐day carrying capacity on a staging site: a test of depletion models

Abstract

Summary 1.The carrying capacity of a site for migratory water birds, expressed in bird-days, can be of particular conservation value. Several attempts have been made to model this carrying capacity using ideal free distribution models such as, for instance, depletion models, in which the distribution is fully determined by exploitative competition. 2. In the tests of depletion models carried out so far, no alternative models were compared; rather, one specific model was tested. We tested whether bird-days were more in accordance with birds depleting the food resource (a1) until a critical food density which just enabled survival or (a2) until a threshold food density which renders the site as profitable as an alternative site; and birds (b1) satisfying their daily requirements or (b2) maximizing daily intake. 3. We studied Bewick's swans feeding on below-ground tubers of fennel pondweed in one part of an autumn staging site. In most years between 1995 and 2005, we measured tuber biomass densities around September, November and March, and counted swans daily during their stopover in October. 4. The best fit between observed and predicted bird-days was obtained by assuming that the swans were maximizing their daily intake and depleting the tubers until a threshold biomass density (which yielded the same energetic return as the alternative food source after accounting for a small part of the initial tuber biomass being out of reach of the swans). Also in line with daily intake maximization, the daily feeding time did not differ from 10 h day −1 , the value predicted for Bewick's swans based on their feeding costs. 5. Our results suggests that the applicable model to calculate carrying capacity may depend strongly on whether birds use a site to stopover or to winter, because it determines whether the birds are more likely to use a threshold or critical food density, and to behave as energy maximizers or satisficers.

Herbivory on freshwater and marine macrophytes: A review and perspective

“…At the end of the summer, below-ground tubers are being formed depending on the amount of above-ground plant material (Van Wijk, 1988). Thus, more and, notably, earlier waterfowl grazing in summer results in less tuber biomass in autumn (Klaassen et al, 2006;Gyimesi et al, 2011). In autumn, tubers are being depleted by migratory swans (Bewick's swans Cygnus columbianus) and diving ducks (mainly tufted ducks Aythya fuligula and pochards Aythya ferina) until a grazing threshold (Nolet et al, 2001).…”

Underuse of stopover site by migratory swans

“…In a previous analysis, the positive relationship between initial (D i ) and final (D f ) tuber densities was ascribed to part of the initial tuber biomass being buried deep, inaccessible to the swans, and the slope of the regression of D f on D i was interpreted as the fraction out of reach of the swans (Nolet et al 2006b). The current analysis suggests that this positive relationship results from periods with high water, which implies that the final tuber density in Lauwersmeer would be similar in all years, independent of initial tuber density, if water levels would remain low throughout the stopover period.…”

“…Hence, the difference between the initial and final tuber density should roughly equal the consumption C by the swans, and the observed patterns of depletion and stopover duration should be reproducible from the functional response of swans feeding on tubers (Nolet et al 2006b). This functional response model indeed yields very similar patterns as the statistical models, both for D f (cf.…”

“…where A is the area of pondweed in the inlet (17.7 9 10 4 m 2 ) (Nolet et al 2006b). Realized final tuber density D f (h) is obtained after re-arranging:…”

“…Because accessibility is directly influenced by water levels, we hypothesize that inter-annual (i.e., temporal) variation in GUD at the stopover site is related to yearly differences in initial food density and water level. As the difference between initial and final biomass represents the biomass consumed by swans (Nolet et al 2006b), both the initial food density and water levels should also influence stopover duration (at the individual or population level). Hence, we predict that lower food accessibility leads to shorter stopover duration and higher GUDs (Fig.…”