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Fig. 1. The effect of sugar concentration on sugar intake. Tristram's grackles (A)
and Yellow-vented bulbuls (B) exhibit a typical intake response: their sugar
intake (top) was independent of sugar concentration. They decrease volume of
intake (I) (bottom) as the sugar concentration in food (C)
increases (Martínez del Rio et al.,
2001). The intake response of both species is well described by
power functions (I=543C-0.85,
r2=0.94 and I=285C-0.85,
r2=0.79 for Tristram's grackles and yellow-vented bulbuls,
respectively). Closed circles, low protein concentration in diet; open
circles, high protein concentration in diet (see text for the concentration
values). Because there were no significant effects of protein content on
volumetric intake the power functions were fitted to pooled data.