BRANTA — Jan A. van Gils
Foraging decisions in a digestively constrained long-distance migrant, the Red Knot (Calidris canutus)
Institution: University of Groningen, the Netherlands
Supervisors: T Piersma (also Netherlands Institute for Sea Research (NIOZ))
Details: PhD 2004 (Completed)
Address: Netherlands Institute of Ecology (NIOO-KNAW), Centre for Limnology, Rijksstraatweg 6, 3631 AC Nieuwersluis, The Netherlands (Oct 2005) Email
Subject Keywords: Digestive ecology, foraging, intertidal mudflats, migration, patch and prey choice, shellfish, shorebirds
Species Keywords: Red Knot Calidris canutus
This thesis is about foraging decisions in red knots (Calidris canutus). During their non-breeding season, knots are found at intertidal mudflats, where they mainly feed on armoured mollusc prey, which they typically swallow whole! Among waders, knots possess a relatively large muscular gizzard, which allows them to crush their hard-shelled prey ‘internally’. This way of feeding comes at a price as large gizzards require large transport and maintenance costs. However, knots seem to have found a way out by flexibly reducing the size of their gizzard (and other nutritional organs) at times when feeding is impossible, such as during their long-distance flight (5,000-16,000 km) between their high-arctic breeding grounds and their coastal wintering grounds. However, such reductions (before departure) and enlargements (upon arrival) of the digestive system take time and therefore knots in their migratory period often have too small gizzards to (re)fuel at full speed. How red knots cope with this problem is the main theme of this work.
The use of ultrasonography enabled us to estimate gizzard mass in live knots. Apart from major ethical advantages, this allowed us to study experimentally and observationally how foraging behaviour is affected by the size of the gizzard.
By applying a diet model that takes a (flexible) digestive constraint into account, we predicted knots with relatively small gizzards to feed on easy-to-digest, soft-bodied prey (crustaceans) and knots with larger gizzards to feed on more abundant but harder-to-digest, hard-shelled prey (bivalves). As the model predicted birds with the smallest gizzards to obtain the lowest intake rates, we expected those birds to make the longest working days.
Indeed, knots lived up to our expectations. Radio-marked, free-living knots that had relatively small gizzards (i.e. those that were about to depart or had just arrived) were found in soft-bodied food patches, while birds with the largest gizzards were found in hard-shelled food patches. Diet composition varied accordingly (measured by detailed analysis of the faeces), with the softest diets found in knots with the tiniest gizzards. Finally, length of the foraging day declined with gizzard mass, with birds possessing the smallest gizzards stretching their working day to almost 17-h by moving along with the tide in an eastward direction.
In addition, we found that, within and between years, knots fine-tuned the size of their gizzard to the local quality of the prey. Knots possessed large gizzards when their bivalve prey contained little amounts of flesh relative to the amount of shell material, while they possessed smaller gizzards when prey contained relatively good amounts of flesh. However, we observed limits to these flexible adjustments of gizzard size. We calculated that in years with low prey quality the required size adjustments would be too large for some birds to be able to cope. In accordance with this prediction, our colour-marking programme revealed that birds with too small gizzards disappeared and likely died after such years. We were able to show that mechanical cockle-fisheries were the direct cause of the observed decline in prey quality.
Van Gils, J.A., Battley, P.F., Piersma, T. & Drent, R. 2005. Reinterpretation of gizzard sizes of red knots world-wide emphasises overriding importance of prey quality at migratory stopover sites. Proceedings of the Royal Society of London, Series B in press.
Van Gils, J.A., De Rooij, S.R., Van Belle, J., Van der Meer, J., Dekinga, A., Piersma, T. & Drent, R. 2005. Digestive bottleneck affects foraging decisions in red knots (Calidris canutus). I. Prey choice. Journal of Animal Ecology 74:105-119.
Van Gils, J.A., Dekinga, A., Spaans, B., Vahl, W.K. & Piersma, T. 2005. Digestive bottleneck affects foraging decisions in red knots (Calidris canutus). II. Patch choice and length of working day. Journal of Animal Ecology74:120-130.
Van Gils, J.A. & Piersma, T. 2004. Digestively constrained predators evade the cost of interference competition.Journal of Animal Ecology 73:386-398.
Van Gils, J.A., Edelaar, P., Escudero, G. & Piersma, T. 2004. Carrying capacity models should not use fixed prey density thresholds: a plea for using more tools of behavioural ecology. Oikos 104:197-204.
Piersma, T., Dekinga, A., Van Gils, J.A., Achterkamp, B. & Visser, G.H. 2003. Cost-benefit analysis of mollusc-eating in a shorebird. I. Foraging and processing costs estimated by the doubly labelled water method.Journal of Experimental Biology 206: 3361-3368.
Van Gils, J.A., Piersma, T., Dekinga, A. & Dietz, M.W. 2003. Cost-benefit analysis of mollusc-eating in a shorebird. II. Optimizing gizzard size in the face of seasonal demands. Journal of Experimental Biology 206: 3369-3380.
Van Gils, J.A., Schenk, I.W., Bos, O. & Piersma, T. 2003. Incompletely informed shorebirds that face a digestive constraint maximize net energy gain when exploiting patches. American Naturalist 161:777-793.