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First published online October 21, 2005
Journal of Experimental Biology 208, 4099-4108 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01855

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Metabolic adjustments to increasing foraging costs of starlings in a closed economy

Popko Wiersma^*, H. Martijn Salomons and Simon Verhulst

Zoological Laboratory, University of Groningen, PO Box 14, 9750 AA Haren, The Netherlands

View larger version (13K): [in a new window]   Fig. 1. Daily energy expenditure in relation to food availability manipulations. (A) Caloric restriction results in a decrease in energy expenditure. (B) Increasing foraging costs per reward: when metabolic rate during foraging is higher than when not foraging, and everything else remains constant (mass, nocturnal energy expenditure, etc.), daily energy expenditure is an accelerating function of foraging costs per reward.

View larger version (28K): [in a new window]   Fig. 2. The hourly averaged metabolic rates (A) and mass-specific metabolic rates (B) for different food availabilities (night from 0 to 10 h, i.e. 14:00 h to 00:00 h local time). Values are means ± S.E.M., N=7. The lines are fitted using 3rd-order polynomials.

View larger version (24K): [in a new window]   Fig. 3. Body mass in different foraging environments, shown separately for birds that started in the rich environment (closed symbols, broken lines) and poor environment (open symbols, solid lines). Arrows indicate treatment order. Treatment and time had an effect on mass, while order of treatment had not (Table 1).

View larger version (13K): [in a new window]   Fig. 4. Association between metabolic rates (measured at ±20.8°C) of starlings measured during the day (MRday) and the night (MRnight) during a 24 h respirometer measurement. MRday and MRnight were strongly correlated (r=0.75, N=24, P<0.001, controlling for mass); the regression line is shown. The grey lines show the average values of MRnight for our birds and the estimated values of MRday in the rich and poor environment.

View larger version (27K): [in a new window]   Fig. 5. Daily energy expenditure (DEE) in different foraging environments. The shaded area indicates the range of DEE values measured in brood provisioning starlings in the field (Westerterp et al., 1982; Ricklefs and Williams, 1984). Symbols refer to the same individuals as in Fig. 3, and the arrowheads indicate treatment order.

View larger version (26K): [in a new window]   Fig. 6. Energy budgets and flight times in different environments. Also shown (far right) is the hypothetical budget and flight time in the poor environment of starlings that maintain high body mass the same as in the rich environment. Eflight is the energy spent on flying, BMRnight is the energy spent during the night on BMR only, Enight is the total energy spent during the night, and Enonfly day is what is spent during day-time when not flying. The broken lines in the hypothetical energy budget indicate the surplus energy that would have been spent due to the extra time spent flying (and less time spent not flying), and the extra energy that is spent on flying due to the body mass increase. The total daily energy expenditure (DEE) budget is shown on top of each bar.