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Figures
- 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.
- Table 1.
Biometrics, activity and energetics in different foraging environments
Foraging conditions Rich Intermediate Poor Treatment Time Order Mass and flight muscle Body mass (g) 79.8±2.4 72.7±2.1 64.2±1.4 F1,12=104, P<0.001 −*** ns Muscle depression (mm) 2.31±0.24 3.21±0.17 3.39±0.25 F1,12=18.0, P<0.005 ns ns Activity Travel distance (km day−1) 7.84±0.38 20.38±2.16 31.84±2.74 F1,12=97.1, P<0.001 ns ns Flight time (min day−1) 32.2±2.1 85.4±9.3 136.1±12.8 F1,12=95.8, P<0.001 ns ns Flight speed (m s−1) 4.08±0.08 3.92±0.09 F1,202=17.6, P<0.001 +* ns Measured energetics Assimilation efficiency 0.826±0.010 0.843±0.014 0.840±0.016 F1,12=2.76, P=0.12 +** ns MEI (kJ day−1) 149.8±5.6 189.2±20.3 215.5±9.5 F1,12=14.4, P<0.005 ns ns DEE (kJ day−1) 153.8±5.1 192.3±16.2 220.2±10.1 F1,12=22.9, P<0.001 ns ns Enight (kJ day−1) 37.79±1.01 31.92±1.22 27.54±1.17 F1,12=65.8, P<0.001 ns ns BMR (W) 0.861±0.019 0.776±0.019 0.692±0.029 F1,12=53.3, P<0.001 ns ns BMRms (mW g−1) 10.85±0.40 10.69±0.27 10.76±0.28 F2112=0.03, P=0.88 ns +* -
Values are means ± s.e.m. (N=7 birds).
MEI, metabolisable energy intake; DEE, daily energy expenditure; E energy expended; BMRms, mass-specific BMR.
Flight speed was only measured in the poor and rich environment.
Test statistics are from GLMMs with bird identity as a random effect (see Materials and methods). `Time' is time elapsed since the start of the experiments and `order' is whether the first measurements were in the poor or rich environment (`+' indicates higher values in rich environment). ns, P>0.05, *P<0.05, **P<0.01, ***P<0.001.
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- 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.
- 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).
- Table 2.
Energy budget calculations based on either (A) fixed flight costs of 20.5 W* and a minimum requirement during the daylight period when not flying (E=RMR), or on (B) estimated energy expenditure when not flying during the daylight period
Foraging conditions Rich Intermediate Poor (A) Flight costs fixed at 20.5 W* Eflight (kJ day−1) 39.7±2.6 105.0±11.5 167.4±15.8 Enonflight day (kJ day−1) 51.7±1.1 43.5±1.0 36.2±1.4 MRnonflight day (×BMR) 1.84±0.10 1.59±0.19 0.87±0.26 Budget fit (kJ day−1) 24.6±4.0 11.8±6.4 −10.9±7.5 (B) Flight costs estimated Enonflight day (kJ day−1) 82.4±2.2 65.0±2.4 52.2±2.1 Eflight (kJ day−1) 33.6±6.0 95.4±16.9 140.4±9.8 MRflight (W) 16.99±2.37 17.90±1.56 17.50±0.78 Budget fit (kJ day−1) 0 0 0 -
RMR, resting metabolic rate (MR); E, energy expenditure.
The calculated energy budget components are within bold borders. Budget fit shows the difference between actual and calculated budgets. Values (mean± s.e.m.) are averages from calculations for each individual for each treatment.
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↵* Fixed flight costs in A are taken from Hambly et al. (2004).
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- 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.
- 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.
- 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.
- Table 3.
Two foraging currencies, net rate and efficiency of energy intake, estimated for different foraging environments and for the hypothetical case of birds with high mass in the poor environment
Net intake (W) Efficiency Environment Actual mass High mass Actual mass High mass Rich 49.8 6.45 Intermediate 21.2 3.17 Poor 13.3 12.4 2.52 2.17 -
Net intake is the difference between the rates of metabolisable energy gain and energy expenditure during a foraging cycle (W). Efficiency is the ratio of metabolisable energy gain over energy expenditure during a foraging cycle (dimensionless). A foraging cycle included the flying time needed to obtain one pellet, the perch time following these flights and the time for handling and eating a pellet (the latter two were estimated at 2 s, but within reasonable limits these estimates had negligible effect). Computations used data from Table 1, and assumed that MRflight=20.5 W in the rich and intermediate environment and MRflight=17.5 W in the poor environment.
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