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Metabolic response to wind of downy chicks of Arctic-breeding shorebirds (Scolopacidae)

George S. Bakken^1,*, Joseph B. Williams² and Robert E. Ricklefs³

¹ Department of Life Sciences, Indiana State University, Terre Haute, Indiana 47809, USA
² Department of Evolution, Ecology and Organismal Biology, Ohio State University, 1735 Niel Avenue, Columbus, Ohio 43210-1293, USA
³ Department of Biology, University of Missouri — St Louis, 8001 Natural Bridge Road, St Louis, Missouri 63121-4499, USA

View larger version (26K): [in a new window]   Fig. 1. Logarithmic plots of daily total evaporative water loss (g day-1) as a function of body mass. As evaporative water loss from birds is sensitive to ambient temperature, Equation 2 was used to adjust both plotted data and the regression line to a nearly thermally neutral 25°C for comparison with simple and phylogenetic allometric regression models for birds in thermally neutral conditions (equations 2 and 7 in Williams, 1996).

View larger version (25K): [in a new window]   Fig. 2. Logarithmic plots of thermal conductance versus body mass for downy shorebird chicks showing that conductance increases markedly with wind, especially for smaller chicks. To show trends, LOWESS (locally weighted scatterplot smoother) regressions have been fitted to data taken at wind speeds of 0.1 m s-1 (solid red line), 0.8 m s-1 (broken orange line) 1.8 m s-1 (broken green line) and 3.0 m s-1 (dotted blue line). (A) Dry conductance Ko=(M-E)/(Tb-Ta). The dashed black line is a regression to data on adult nonpasserines (Calder and King, 1974). For comparison, the conductance of 1-day-old mallard ducklings exposed to 0.1 m s-1 wind is indicated with a black asterisk (Bakken et al., 1999). (B) Wet conductance Kow=M/(Tb-Ta). The results for our 7- to 10-day-old chicks at 0.1 m s-1 are consistent with data on newly hatched (<24 h-old) shorebirds in still air (dotted red line; Kow=0.0125m0.371) (Visser and Ricklefs, 1993). The conductances of notably cold-resistant downy chicks are shown for comparison, including 1-day-old mallards (asterisk; Bakken et al., 1999), 2-day and 6-day-old Xantus' murrelets (pentagon; Eppley, 1984), and 3-day and 6-day-old capercaillie (diamonds; Pis, 2002). Also shown are some regression models for adult conductance: broken red lines, our regression to data from adult shorebirds (36-540 g; Kersten and Piersma, 1987; Kendeigh et al., 1977); solid black lines, active phase adult nonpasserines (Aschoff, 1981); broken black lines, winter adult nonpasserines (Kendeigh et al., 1977). Model parameters are given in Table 1.

View larger version (18K): [in a new window]   Fig. 3. Comparison of the effect of wind speed on operative temperature Tes given by different Tes scales. Solid lines, 25 g shorebirds (upper line, Equation 3b; lower line, Equation 3c). Long broken lines, dark-eyed junco modeled using the best-fit wind speed dependence of u0.7 (junco data taken from Bakken et al., 1999). Short broken lines, mallard ducklings using the best fit wind-speed dependence of u1.0 (Bakken et al., 1999). Dotted line, Equation 4, based on published data using u0.5 (Bakken, 1990). See text for discussion.

View larger version (24K): [in a new window]   Fig. 4. The ratio of evaporative cooling E to metabolic heat production M for shorebird chicks varies with standard operative temperature Tes, and is independent of body mass. The solid curve is an exponential function fitted to these data (see text), and the broken line is the correlation found by Calder and King (1974).

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