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.