Individual diet differences in a molluscivore shorebird are associated with the size of body instruments for internal processing rather than for feeding

Especially in birds, it is widely found that the size of individual prey items follows the size of the instruments of prey capture, handling and processing, i.e. bill size. In fact, this is the natural history basis of major discoveries on adaptive evolution in the face of changing food resources. In some birds, e.g. the molluscivore shorebirds ingesting hard-shelled prey, most of the prey processing takes place within the digestive tract. This study of a salvaged sample of actively feeding great knots Calidris tenuirostris accidentally drowned in fishing nets in northern China, is the first documentation of diet selection at the level of the individual in previously well-studied molluscivore shorebirds. Diet composition was not associated with the length of the bill, but with the mass of the muscular gizzard. Gizzard mass, which unlike bill length is a phenotypically flexible trait, enables great knots to adjust to changing food resources as an individual, i.e. instantly responding to the food on offer. For migratory species like great knots which rely on seasonal sequences of interdistant feeding areas offering prey with a variety of characteristics, the capacity to individually adjust appears a key adaptation.