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First published online August 25, 2003
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A PRICE WORTH PAYING
kathryn{at}biologists.com
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There's no doubt that hen's teeth are rare, but most birds don't need teeth to grind their food; they solve the mashing problem with a powerful gizzard, crushing food with a few contractions. But not all gizzards are equal. In fact, red knots' gizzards grow larger when the birds put on weight ready to migrate. But they also change size throughout the year, and `there was no pattern to this variation' says Jan van Gils. Theunis Piersma and van Gils were puzzled why some birds had large gizzards and others small ones, so they decided to measure how much energy it took a bird to get the most from it's crunchy diet to see if big gizzards gave birds the feeding edge.
The team needed to subtly manipulate the birds diet as they foraged, to find out how hard the gizzards worked cracking shells. Enter the `tidodrome'; a large floodable aviary that van Gils and his colleagues supplied daily with freshly dredged molluscs, to simulate the mudflats of the bird's Wadden Sea feeding grounds. First the team tested how hard the searching birds worked as they probed for cockles, cracking them open in their gizzards. Then they varied the birds' diets to see how much effort it took to get their fill from meals where they had to find each morsel, through to a minimum effort meal of unshelled cockles presented to them in the roost. When van Gils balanced the budget, he was amazed; the bird's probing search seemed to take an enormous amount of effort, while the birds' gizzards barely used any energy at all as they crunched (p. 3361). So larger gizzards wouldn't give the birds a metabolic advantage; in fact, they incur a significant metabolic cost, so why bother upsizing in the first place? Something else must be driving the gizzard's growth. Could it have something to do with the enormous amounts of shell the birds have to process to get a well-balanced meal?
This time the team knew that the birds would naturally adjust the size of their gizzards, depending on the hardness of their diet. van Gils took advantage of this to manipulate the gizzard's sizes, then he could see whether having a large gizzard allowed the bird to get through the large amounts of waste shell they had to shift before they got their fill.
Serving knots with large and small gizzards a selection of hard intact molluscs and soft mollusc meat, van Gils filmed the birds as they ate. The knots with large gizzards consumed far more hard molluscs in their shells than the birds with smaller gizzards (p. 3369). van Gils also offered the birds a shell-heavy diet, but even the birds with the largest gizzards needed to feed for 16 hours a day to sustain their weight! Birds with smaller gizzards simply couldn't feed fast enough. By allowing them to crush more shell per gizzardfull, larger gizzards gave birds the edge.
van Gils suspects that even though it is costly for the knots to maintain a larger gizzard, when the bird needs to get the most out of its crunchy diet, it's a price worth paying. So, the birds' gizzards enlarge as they fatten for migration, and also when they need stay warm as the temperature falls. But van Gils was most surprised that the knot's gizzards also enlarged when the molluscs begin shrivelling, as their winter food supply dwindles. He explains that molluscs' shells stay the same size as the molluscs shrink, increasing the amount of shell a bird must process to eat its fill. But with their larger gizzards, the birds can still make the most of even the crunchiest winter diet! Which is `this is thrilling for ecologists, since here's a great link between the ecophysiology of the prey and its predator' says van Gils.
References
Piersma, T., Dekinga, A., van Gils, J. A., Achterkamp, B. and
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. J. Exp. Biol.
206,3361
-3368.
van Gils, J. A., Piersma, T., Dekinga, A. and Dietz, M. W.
(2003). Cost-benefit analysis of mollusc-eating in a shorebird.
II. Optimizing gizzard size in the face of seasonal demands. J.
Exp. Biol. 206,3369
-3380.
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