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First published online August 30, 2006
Journal of Experimental Biology 209, 3489-3498 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02385

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Energetic influence on gull flight strategy selection

Judy Shamoun-Baranes^* and Emiel van Loon

Computational Bio- and Physical Geography, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands

View larger version (16K): [in a new window]   Fig. 1. Schematic representation of Eqn 1 and 2, showing our conceptualization of a bird's energy balance. Birds gain energy during foraging and lose energy when traveling between food patches. The switch between these modes is instantaneous. (A) The general case where energy gain, En(tp), and the power required for flight, P(tt), are functions of time (Eqn 1). (B) The simplification used in this study where En and P are constant over time (Eqn 2). The total travel time is in both cases calculated by the sum tt=itt(i) and the time spent foraging is calculated by tp=itp(i). Note that P is given in positive values in Eqn 1 and 2, so that -P is used on the negative part of the vertical axis.

View larger version (19K): [in a new window]   Fig. 2. Net rate of energy gain (W) of black-headed gulls (A,C,E) and lesser black-backed gulls (B,D,F) for flapping (broken line) and soaring flight (solid line) solved for equal air speeds and ground speeds (m s-1). (A,B) En=20 W, D=10000 m, (C,D) En=20 W, D=5000 m, (E,F) En=10 W, D=10000 m. tp is kept constant because its effect on R is inverse to that on D.

View larger version (61K): [in a new window]   Fig. 3. The net energy gain (W) for flapping (Rflap; A,B) and soaring (Rsoar; C,D) and the difference between Rsoar and Rflap (E,F) considering different combinations of air speed (Va m s-1) and ground speed (Vg m s-1) for black-headed gulls (A,C,E) and lesser black-backed gulls (B,D,F). Parameter estimates for calculating R are as follows: En=20 W, D=10000 m, tp=1800 s.

View larger version (26K): [in a new window]   Fig. 4. Observed ground speeds (Vg, m s-1) and observed air speeds (Va, m s-1) during flapping (+) and soaring () flight for the black-headed gull (A) and the lesser black-backed gull (B). Regression lines are shown for each species and each flight strategy (solid line for soaring, broken line for flapping; the 1:1 line is included for reference purposes). The frequency distributions of Va and Vg during soaring and flapping flight are presented along the respective axes at the right and top. The lines of the frequency distributions are shifted slightly along the category axis for display purposes.

View larger version (16K): [in a new window]   Fig. 5. The predicted net energy gain (R, in W) calculated with observed ground speed (Vg), air speed (Va) and flight strategy combinations. Different symbols (+ flapping, soaring) represent the predicted R for a measured combination of Vg, Va and flight strategy for black-headed gulls (A) and lesser black-backed gulls (B). Parameter estimates for R calculations are as follows: En=20 W, D=10000 m, tp=1800 s. As observed Va and Vg values are highly confounded (see Fig. 4), R was not plotted against both Va and Vg.

View larger version (17K): [in a new window]   Fig. 6. The predicted differences in net energy gain between soaring () and flapping (+) flight (Rs-Rf) calculated with observed combinations of air speed (Va, m s-1), ground speed (Vg, m s-1) and flight strategy, for black-headed gulls (A) and lesser black-backed gulls (B). Parameter estimates for R are as follows: En=20 W, D=10000 m, tp=1800 s. As observed Va and Vg values are highly confounded (as shown in Fig. 4), Rs-Rf was not plotted against both Va and Vg.

View larger version (22K): [in a new window]   Fig. 7. The estimated normal probability distribution of air speeds (Va, m s-1) during flapping (broken line) and soaring flight (solid line) for the black-headed gulls (A) and lesser black-backed gulls (B). This simplification of the data can be used to predict, for example, the range of Va where the soar/flap ratio is <1 (area shaded in gray). Thick solid lines at the top of each figure represent alternative selection criteria that can be used to determine the soar/flap ratio <1 (area shaded in gray): for example, where Rdiff<1.9 W and Rs>11.6 (black-headed gull).