University of Groningen INTERACTIONS BETWEEN STOMACH STRUCTURE AND DIET CHOICE IN SHOREBIRDS

-Captive Red Knots (Calidris canutus) fed soft food pellets developed atrophied stomachs, and were reluctant to eat their usual hard-shelled mollusc prey. An interspecific comparison among shorebirds showed that wild Red Knots and other intact-mollusc-eating species have gizzards with relatively great mass but very small proventriculi. Within six different shorebird species, the heavier individuals usually had the heavier stomachs as well, but in Red Knots and Bar-tailed Godwits (Limosa lapponica) we identified heavy premigrant individuals with reduced stomach masses, suggesting a reallocation of protein reserves before long-distance flights. In both species reduced stomach mass was associated with a relatively soft diet. We were unable to show that during adjustment of stomachs to hard-shelled prey, such prey are broken down to smaller fragments. We attribute this to the counteractive influence of the pylorus during adjustment. We summarize the suggested stomach/diet interactions as a network of causal relationships and feedback loops involving the type of diet and gizzard mass. We identify two basic modifiers of gizzard mass: one working via endurance training and disuse atrophy; and another involving endocrine and/or neural mechanisms. It is likely that, in the course of their annual cycle, shorebirds are prevented from achieving maximal digestive performance owing to seasonal changes in feeding habitats and diet enforced by their long-distance migrations. Received 28 May 1992, accepted 25 November 1992. ANIMAL BODIES are dynamic systems showing large changes in composition in relation to season, nutritional condition and diet (e.g. Allison and Wannemacher 1965, King 1972, Goldberg et al. 1974, 1975, Raveling 1979, Cherel et al. 1988, Blem 1990, Gaunt et al. 1990). Variation in the form and mass of stomachs in relation to diet has long attracted the attention of researchers (e.g. Darwin 1885; summary in Ziswiler and Farner 1972), and a series of detailed studies indicates that associations between stomach mass and type of diet often represent speciesand individual-specific adaptations (Table 1). For example, eating more fibrous food usually leads to increased gizzard mass (i.e. the muscular part of stomach), as has been shown in experimental as well as in observational studies. Having a particular stomach morphology because of exposure to a particular diet might constrain the ingestion rate, or the digestive yield, of other food types (Moss 1983), at least during the period in which the stomach is adjusting. Although fiber-eating wildfowl and galliforms are widely recognized as birds with large and muscular stomachs, species with similar stomach types that feed on hard-shelled animal prey ingested whole have escaped detailed attention; only casual remarks on mollusc-eating sea ducks have been made (Bellrose 1976, Barnes and Thomas 1987, Brown and Frederickson 1987). Some shorebird species, such as Red Knots (Calidris canutus), also feed on whole bivalves and gastropods, which they capture by probing in soft intertidal sediments (Prater 1972, GossCustard et al. 1977, Boere and Smit 1980b, Piersma 1991, Zwarts and Blomert 1992). During experimental studies on the feeding behavior of Common Eiders (Somateria mollissima; P. Duiven and C. Swennen pers. comm.) and Red Knots, we repeatedly have found that individuals conditioned to soft food pellets did not eat their natural hard-shelled prey for a couple of days. Such observations, and those on Wild Turkeys (Meleagris gallopavo) and Surfbirds (Aphriza virgata) listed in Table 1, further suggested that gizzard mass may influence diet rather than vice versa. Because most shorebirds show considerable seasonal variation in the types of habitat frequented (e.g. marine intertidal in winter, tundra in summer), and because these shifts coincide with changes in prey type (e.g. hard-shelled molluscs in winter, soft arthropods in summer), we examined the relationship between diet and gizzard morphology in this group of birds. We first describe some behavioral observations on captive Red Knots that led us to associate diet with stomach mass in shorebirds, both