Shorebirds show large interspecific variation in the relative size of the stomach, and especially of the muscular part, the gizzard. Much of this variation can be explained by their diet. Species feeding mainly on hard-shelled prey such as bivalves and gastropods have large stomachs; those feeding on soft-bodied prey such as worms have small stomachs. Within a species, diet- and migration-induced changes in stomach size can occur. Our studies on this intraspecific variation have focused on two mollusc-specialists, the Red Knot Calidris canutus and the Great Knot C. tenuirostris. Both are renowned for long-range flights between their arctic or sub-arctic alpine breeding grounds and a variety of coastal wetlands. Feeding mainly on shellfish ingested whole, both knot species have large stomachs, but changing diets easily lead to apparently adaptive modifications. In addition, the demands imposed by flights of many thousand kilometres may induce reductions in stomach size. Using ultrasonography we have begun to experimentally disentangle the causal relationships between diet, season and stomach size in Red Knots. A soft diet can induce stomach reductions of 50% within a week, and such changes are reversible. Studies on radiomarked birds in the Wadden Sea emphasize that variations in stomach size are correlated with prey and patch choice in the field.
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