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Behavioural variation within a species is usually explained as the consequence of individual variation in physiology. However, new evidence suggests that the arrow of causality may well be in the reverse direction: behaviours such as diet preferences cause the differences in physiological and morphological traits. Recently, diet preferences were proposed to underlie consistent differences in digestive organ mass and movement patterns (patch residence times) in red knots (Calidris canutus islandica). Red knots are molluscivorous and migrant shorebirds for which the size of the muscular stomach (gizzard) is critical for the food processing rate. In this study, red knots (C. c. canutus, n = 46) were caught at Banc d'Arguin, an intertidal flat ecosystem in Mauritania, and released with radio-tags after the measurement of gizzard mass. Using a novel tracking system (time-of-arrival), patch residence times were measured over a period of three weeks. Whether or not gizzard mass determined patch residence times was tested experimentally by offering 12 of the 46 tagged red knots soft diets prior to release; this reduced an individual's gizzard mass by 20-60%. To validate whether the observed range of patch residence times would be expected from individual diet preferences, we simulated patch residence times as a function of diet preferences via a simple departure rule. Consistent with previous empirical studies, patch residence times in the field were positively correlated with gizzard mass. The slope of this correlation, as well as the observed range of patch residence times, was in accordance with the simulated values. The 12 birds with reduced gizzard masses did not decrease patch residence times in response to the reduction in gizzard mass. These findings suggest that diet preferences can indeed cause the observed among-individual variation in gizzard mass and patch residence times. We discuss how early diet experiences can have cascading effects on the individual expression of both behavioural and physiomorphic traits. This emphasizes that to understand the ecological consequences of individual differences, knowledge of the environment during development is required., (© 2016 The Authors. Journal of Animal Ecology © 2016 British Ecological Society.)

When a diet switch results in a change in dietary isotopic values, isotope ratios of the consumer's tissues will change until a new equilibrium is reached. This change is generally best described by an exponential decay curve. Indeed, after a diet switch in captive red knot shorebirds (Calidris canutus islandica), the depletion of (13)C in both blood cells and plasma followed an exponential decay curve. Surprisingly, the diet switch with a dietary (15)N/(14)N ratio (δ(15)N) change from 11.4 to 8.8 ‰ had little effect on δ(15)N in the same tissues. The diet-plasma and diet-cellular discrimination factors of (15)N with the initial diet were very low (0.5 and 0.2 ‰, respectively). δ(15)N in blood cells and plasma decreased linearly with increasing body mass, explaining about 40 % of the variation in δ(15)N. δ(15)N in plasma also decreased with increasing body-mass change (r (2)=.07). This suggests that the unusual variation in δ(15)N with time after the diet switch was due to interferences with simultaneous changes in body-protein turnover.

1. Studies of diet choice usually assume maximization of energy intake. The well-known 'contingency model' (CM) additionally assumes that foraging animals only spend time searching or handling prey. Despite considerable empirical support, there are many foraging contexts in which the CM fails, but such cases were considered exceptions rather than the rule. 2. For animals constrained by the rate at which food is digested, CM does not necessarily lead to maximal energy intake rates because the time for digestion is not part of the selection criteria. In the main model developed to explain diet choice under a digestive constraint, the 'digestive rate model' (DRM), time lost to digestive breaks is minimized so that energy intake over total time (searching, handling, digestive breaks) is maximized. 3. It is increasingly acknowledged that most animals may face digestive constraints as prey capture rates vary over time and as it would be a waste to carry around heavy digestive machinery that can rapidly process food under all circumstances: this is only needed in times of high demand, provided that enough food can be found. 4. In molluscivore shorebirds ingesting hard-shelled prey such as red knots (Calidris canutus), the predictions of DRM were held up so far, whereas those of CM were rejected. However, most tests were carried out under controlled experimental conditions. Red knots overwinter in coastal areas over much of Western Europe and we capitalized on this variation by comparing, during a single winter, observed diet composition with predictions of DRM, CM and a null model assuming no prey selection ('no-selection model', NSM). 5. The observed diets were best predicted by DRM followed by CM. NSM poorly predicted observed diet choice. Under the present conditions, diet choice based on DRM would on average have yielded an energy intake rate twice as large as one based on CM. By adjusting the size of their gizzard (held constant in the present simulations), red knots could have lifted their energy intake rate further. We suggest that application of the DRM can help many diet studies forward, especially those previously seen as exceptions to the classical CM-based rule.

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