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First published online August 25, 2003
Cost-benefit analysis of mollusc-eating in a shorebird II. Optimizing gizzard size in the face of seasonal demands
1 Department of Marine Ecology and Evolution, Royal Netherlands Institute
for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, Texel, The
Netherlands
2 Animal Ecology Group, Centre for Ecological and Evolutionary Studies
(CEES), University of Groningen, PO Box 14, 9750 AA Haren, The
Netherlands
* Author for correspondence (e-mail: janvg{at}nioz.nl)
Accepted 18 June 2003
Aiming to interpret functionally the large variation in gizzard masses of red knots Calidris canutus, we experimentally studied how the digestive processing rate is influenced by the size of the gizzard. During their non-breeding season, red knots feed on hard-shelled molluscs, which they ingest whole and crush in their gizzard. In three experiments with captive birds we tested predictions of the hypothesis that gizzard size, via the rate of shell crushing and processing, constrains intake rate in red knots (against the alternative idea that external handling times constrain intake rate). Gizzard size within individual birds was manipulated by varying the hardness of the diet on offer, and was confirmed by ultrasonography. The results upheld the `shell-crushing hypothesis' and rejected the `handling time hypothesis'. Intake rates on with-shell prey increased with gizzard size, and decreased with shell mass per prey. Intake rates on soft (without shell) prey were higher than on with-shell prey and were unaffected by gizzard size. Offering prey that were heavily shelled relative to their flesh mass led to energy intake rates that were marginally sufficient to balance the daily energy budget within the time that is naturally available in a tidal system. We predicted the optimal gizzard sizes that are required to either (1) balance energy income with energy expenditure, or (2) to maximise net daily energy intake. The gizzard mass of free-living red knots in the Wadden Sea is such that it maximises daily net energy intake in spring when fuelling for migration, while it balances energy budget throughout the remainder of the year.
Key words: gizzard, digestive constraint, intake rate, red knot, Calidris canutus, ultrasonography, optimization, phenotypic flexibility
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