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1 Centre for Ecological and Evolutionary Studies, University of Groningen, 9750 AA Haren; 2 Centre for Isotope Research, University of Groningen, 9747 AG Groningen; and 3 Netherlands Institute for Sea Research, 1790 AB Den Burg, Texel, The Netherlands
We investigated the
applicability of 2H 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 2H turnover rates ranged
between 0.182 day
1 (SD = 0.0219) for fasting
birds and 7.759 day
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
75.9-85.4 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 2H
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 (
21 = 0.058, P < 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
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