Effects of intake rate on energy expenditure, somatic repair and reproduction of zebra finches

Understanding the effect of food availability on food requirements is critical when linking food availability e.g. to reproduction or habitat selection. Decreasing intake rate (intake per unit foraging effort) can be expected to increase daily energy expenditure (DEE), due to increased foraging costs. However, all the studies we could find that have tested this hypothesis (with one exception) found DEE to be constant or decreasing when intake rate was experimentally decreased. This may be due to the design of the reward schedule, which can be fixed (e.g. 20 units effort required for each reward) or variable (e.g. each unit effort rewarded with probability 1/20). Most studies used fixed reward rates, but foraging motivation is generally higher for variable reward rates, and the only study in which animals increased DEE when intake rate decreased used variable reward rates. To assess the generality of this result, we exposed zebra finches Taeniopygia guttata to different intake rates using variable reward rates. We decreased intake rate by mixing 25 g of seeds with 0, 25 or 75 g of chaff. With increasing chafftseed ratio the time spent foraging increased from 6% to 27%, but this was insufficient to compensate for the lower intake rate, because DEE decreased by 6.6%. Body mass was independent of chafftseed ratio. Effects of intake rate on foraging time and DEE were stronger at lower temperatures, when DEE was higher. The decrease in DEE in adverse conditions raises the question of what prevents such behaviour in benign circumstances. We hypothesize that energy is saved at the expense of 'condition', and we tested this hypothesis in two ways. Firstly, we tested the effect of intake rate on the replacement of two plucked tail feathers (a form of somatic repair). Replacement feathers were shorter when intake rate was low, indicating an effect of intake rate on somatic repair ability. Secondly, we tested for carry-over effects of intake rate on reproduction, by giving pairs the opportunity to reproduce with access ad libitum to food after feeding on one of the three chaff/seed ratios for 6 weeks. The interval until laying the first egg increased with decreasing intake rate in the preceding 6 weeks. The effects of intake rate on somatic maintenance and reproduction may explain why birds sustained higher metabolic rates than apparently necessary, but the physiological mechanisms determining the optimal metabolic rate remain to be discovered.