Digestive Organ Size and Behavior of Red Knots (Calidris Canutus) Indicate the Quality of Their Benthic Food Stocks

Abstract

Digestive organ size anD behavior of reD knots (Calidris Canutus) inDicate the quality of their benthic fooD stocksAbSTRACT Assuming that animals respond optimally to environmental changes, both behavior and physiology should be useful indicators of the way that animals perceive the quality of their environment. For verification, we examined foraging time and gizzard size of the red knot (Calidris canutus), a long-distance migrant shorebird that ingests hard-shelled mollusk prey whole and therefore readily faces digestive constraints. Nevertheless, even when digestively bottlenecked, knots can still enhance their daily energy intake by flexibly increasing the size of their digestive system, notably the gizzard, and/or by feeding longer per day. Whether such adjustments are actually necessary depends on the quality (condition) of their food, i.e., the ratio of flesh to shell mass. Hence, gizzard mass and daily foraging time in knots may be reliable indicators of the quality of their invertebrate prey. This idea is explored by using field data on radio-marked knots during late summer in the Dutch Wadden Sea. Their gizzards were measured using ultrasonography and the knots' movements and working hours were monitored using handheld receivers and automated radio-tracking systems. Indeed, gizzard mass alone predicted annual variation in food quality satisfactorily; however, adding estimates for daily foraging time significantly improved the predictions. We discuss and conceptualize how knots may trade off digestive organ size against foraging time upon arrival in the Wadden Sea, given the quality and quantity of the food on offer, and why these findings are relevant in the context of conservation of migration systems.

Adaptive trade-offs in juvenile salmonid metabolism associated with habitat partitioning between coho salmon and steelhead trout in coastal streams

“…This suggests that adaptive trade‐offs between these species involve different digestion strategies that influence growth efficiency, where steelhead may have a high food consumption strategy that strips out more labile energy with lower efficiency than coho, allowing them to achieve higher growth at high food abundance. Different digestive strategies to maximize energy assimilation are common among vertebrates (Milton 1981; Hume 1989; van Gils et al. 2007), but remain poorly documented among fish (see Nicieza, Reiriz & Brana (1994) and Millidine, Armstrong & Metcalfe (2009) for exceptions), but we suggest that they may underlie many of the adaptive trade‐offs related to growth that allow for differentiation and coexistence of ecologically similar species like salmonids.…”

Small-scale demographic structure suggests preemptive behavior in a flocking shorebird

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Beyond the information centre hypothesis: communal roosting for information on food, predators, travel companions and mates?

“…Arrival timing is a function of gizzard size, and could thus give an estimate of the quality of food that roost mates encountered (Fig. 3B, also between years it has been found that food quality determines daily foraging times, van Gils et al 2007). Given that individuals aim to maximise intake rate, thereby minimizing daily required foraging time, naïve or unsuccessful red knots could find the optimal prey type that matches their digestive capacity by following informed roost mates in similar state (i.e.…”