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Subspecies Calidris canutus islandica of the Red Knot breeds on the arctic tundra of northeastern Canada and northern Greenland and winters along the coasts of northwestern Europe. During northward migration, it stops over in either Iceland or northern Norway. It has been assumed that it does the same during southward migration. Using ratios of stable carbon isotopes ([delta][sup.13]C) in whole blood, blood cells, and plasma, we investigated evidence for a stopover in Iceland en route from the breeding grounds to the Dutch Wadden Sea. With the expected diet (shellfish) and stopover duration at Iceland (12-15 days, maximum 17 days) and the turnover rates of blood cells (15.1 days) and plasma (6.0 days), Red Knots that stopped in Iceland should arrive with a blood (cell) [delta][sup.13]C midway between a tundra (-24.7[per thousand]) and a marine value (-14.0[per thousand]) and a plasma [delta][sup.13]C approaching the marine value (-15.3[per thousand]). However, many adults arriving at the Wadden Sea had [delta][sup.13]C ratios in blood (cells) and plasma below these levels, and some arrived with clear tundra signals in blood cells, suggesting that they skipped Iceland during southward migration. Surprisingly, available data suggest this also to be true for juveniles during their first southward migration. The [delta][sup.13]C signature of second-year birds confirmed that they oversummered in the Wadden Sea. Our findings contradict the largely untested idea that juvenile shorebirds make more stopovers than adults as well as the idea that the migration between the Nearctic and Europe is necessarily a two-leg process. Key words: age effect, stable carbon isotope, migration, sex, shorebird, stopover ecology, Red Knot, Calidris canutus. Calidris canutus islandica se reproduce en la tundra artica en el noreste de Canada y norte de Groenlandia y pasa el invierno a lo largo de la costa del noroeste de Europa. Durante la migracion hacia el norte, los individuos de esta subespecie hacen paradas migratorias en Islandia o en el norte de Noruega. Para la migracion hacia el sur, se ha asumido el mismo patron de paradas migratorias. Utilizando cocientes de isotopos estables de carbono ([delta][sup.13]C) en la sangre, celulas sanguineas y plasma sanguineo investigamos si existe evidencia a favor de paradas en Islandia durante la ruta desde las areas de cria hacia el Mar de Wadden en Holanda. Con base en la dieta esperada (mariscos) y en la duracion esperada de la parada en Islandia (12-15 dias, maximo 17 dias) y en las tasas de recambio de las celulas sanguineas (15.1 dias) y plasmaticas (6.0 dias), los individuos de C. c. islandica que habrian parado en Islandia deberian llegar con un [delta][sup.13]C en la sangre (celulas) con un valor medio entre tundra (-24.7[por millar]) y marino (-14.0[por millar]) y con [delta][sup.13]C plasmatico cercano al valor marino (-15.3[por millar]). Sin embargo, muchos adultos que llegaron al Mar de Wadden tuvieron un cociente de [delta][sup.13]C en la sangre (celulas) y en el plasma menor al de los niveles esperados, y algunos llegaron con senales claras de tundra en las celulas sanguineas, lo que sugiere que estos individuos se saltaron la parada en Islandia durante la ruta migratoria hacia el sur. Sorpresivamente, los datos disponibles tambien surgieren este mismo patron para los juveniles durante su primera migracion hacia el sur. La senal del [delta][sup.13]C de los individuos en su segundo ano confirmo que estos prolongaron su estadia durante el verano en el Mar de Wadden. Nuestros resultados contradicen la idea no corroborada de que los juveniles de aves costeras hacen mas paradas que los adultos, como tambien la idea de que la migracion entre el Neartico y Europa es un proceso que necesita dos etapas. DOI: 10.1525/cond.2010.090139

In the calidrine sandpiper red knot (Calidris canutus), the weeks preceding takeoff for long-distance migration are characterized by a rapid increase in body mass, largely made up of fat but also including a significant proportion of lean tissue. Before takeoff, the pectoral muscles are known to hypertrophy in preparation for endurance flight without any specific training. Because birds facing cold environments counterbalance heat loss through shivering thermogenesis, and since pectoral muscles represent a large proportion of avian body mass, we asked the question whether muscle hypertrophy in preparation for long-distance endurance flight would induce improvements in thermogenic capacity. We acclimated red knots to different controlled thermal environments: 26[degrees]C, 5[degrees]C, and variable conditions tracking outdoor temperatures. We then studied within-individual variations in body mass, pectoral muscle size (measured by ultrasound), and metabolic parameters [basal metabolic rate (BMR) and summit metabolic rate ([M.sub.sum])] throughout a 3-mo period enclosing the migratory gain and loss of mass. The gain in body mass during the fattening period was associated with increases in pectoral muscle thickness and thermogenic capacity independent of thermal acclimation. Regardless of their thermal treatment, birds showing the largest increases in body mass also exhibited the largest increases in [M.sub.sum.]. We conclude that migratory fattening is accompanied by thermoregulatory side effects. The gain of body mass and muscle hypertrophy improve thermogenic capacity independent of thermal acclimation in this species. Whether this represents an ecological advantage depends on the ambient temperature at the time of fattening. muscle mass; summit metabolic rate; basal metabolic rate; thermogenic capacity; phenotypic flexibility

Besides the 'normal' challenge of obtaining adequate intake rates in a patchy and dangerous world, shorebirds foraging in intertidal habitats face additional environmental hurdles. The tide forces them to commute between a roosting site and feeding grounds, twice a day. Moreover, because intertidal food patches are not all available at the same time, shorebirds should follow itineraries along the best patches available at a given time. Finally, shorebirds need additional energy stores in order to survive unpredictable periods of bad weather, during which food patches are covered by extreme tides. In order to model such tide-specific decisions, we applied stochastic dynamic programming in a spatially explicit context. Two assumptions were varied, leading to four models. First, birds had either perfect (ideal) or no (non-ideal) information about the intake rate at each site. Second, traveling between sites was either for free or incurred time and energy costs (non-free). Predictions were generated for three aspects of foraging: area use, foraging routines, and energy stores. In general, non-ideal foragers should feed most intensely and should maintain low energy stores. If traveling for such birds is free, they should feed at a random site; otherwise, they should feed close to their roost. Ideal foragers should concentrate their feeding around low tide (especially when free) and should maintain larger energy stores (especially when non-free). If traveling for such birds is free, they should feed at the site offering the highest intake rate; otherwise, they should trade off travel costs and intake rate. Models were parameterized for Red Knots (Calidris canutus) living in the Dutch Wadden Sea in late summer, an area for which detailed, spatially explicit data on prey densities and tidal heights are available. Observations of radio-marked knots (area use) and unmarked knots (foraging routines, energy stores) showed the closest match with the ideal/non-free model. We conclude that knots make state-dependent decisions by trading off starvation against foraging-associated risks, including predation. Presumably, knots share public information about resource quality that enables them to behave in a more or less ideal manner. We suggest that our modeling approach may be applicable in other systems where resources fluctuate in space and time. Key words: Calidris canutus; daily routine; foraging; patch use; Red Knot; state-dependent decisions; stochastic dynamic programming; tidal regime; Wadden Sea.

Great Knots (Calidris tenuirostris) make one of the longest migratory flights in the avian world, flying almost 5500 km from Australia to China during northward migration. We measured basal metabolic rate (BMR) and body composition in birds before and after this flight and found that BMR decreased 42%. The mass-specific BMR based on lean mass decreased 33%. We also starved a group of pre-migratory Great Knots in captivity to determine whether they showed the same reduction in BMR without having undergone the hard work of flight. The captive birds showed a similar range and reduction of BMR values as the wild birds. Exponents of relationships between BMR and body mass in different comparisons were high, indicating large changes in BMR as a function of body mass. Analysis of the body composition of ten wild and three captive birds found that the flight muscle mass and intestine mass positively correlated with BMR. Key words: BMR, Calidris tenuirostris, fasting, metabolism, migration, 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.

Visser, G. Henk, Anne Dekinga, Bart Achterkamp, and Theunis Piersma. Ingested water equilibrates isotopically with the body water pool of a shorebird with unrivaled water fluxes. Am J Physiol Regulatory Integrative Comp Physiol 279: R1795-R1804, 2000.--We investigated the applicability of [sup.2]H to measure the amount of body water (TBW) and water fluxes in relation to diet type and level of food intake in a mollusk-eating shorebird, the Red Knot (Calidris canutus). Six birds were exposed to eight experimental indoor conditions. Average fractional [sup.2]H turnover rates ranged between 0.182 [day.sup.-1] (SD = 0.0219) for fasting birds and 7.759 [day.sup.-1] (SD = 0.4535) for birds feeding on cockles (Cerastoderma edule). Average TBW estimates obtained with the plateau method were within the narrow range of 75.985.4 g (or between 64.6 and 70.1% of the body mass). Those obtained with the extrapolation method showed strong day-to-day variations (range 55.7-83.7 g, or between 49.7 and 65.5%). Average difference between the two calculation methods ranged between 0.6% and 36.3%, and this difference was strongly negatively correlated with water flux rate. Average water influx rates ranged between 15.5 g/day (fasting) and 624.5 g/day (feeding on cockles). The latter value is at 26.6 times the allometrically predicted value and is the highest reported to date. Differences in [sup.2]H concentrations between the blood and feces (i.e., biological fractionation) were small but significant (-3.4% when fed a pellet diet, and - 1.1% for all the other diets), and did not relate to the rate of water flux ([[Chi].sup.2].sub.1] = 0.058, P [is less than] 0.81). We conclude that the ingested water equilibrated rapidly with the body water pool even in an avian species that shows record water flux rates when living on ingested marine bivalves. stable isotopes; water fluxes; evaporative water loss; isotope kinetics