Adaptive timing of digestion and digestion-related thermogenesis in the pigeon

Pigeons were allowed to feed for 1 h either 2 h after lights on (morning pulse) or 3 h before lights off (evening pulse). Body temperature was measured radiotelemetrically. Faecal excretion, as an index of rate of digestion, was measured using load cells. At 22 degrees C, faecal excretion peaked just after lights-on in morning-pulse condition, but not in evening-pulse condition. In the cold (+5 degrees C), the peak was absent. We conclude that at thermoneutrality, pigeons are able to postpone a major part of digestion until late in the dark phase when their body temperature is increasing to the diurnal level. Thus, the extra heat from digestion-related thermogenesis that otherwise would be dissipated into the environment can be used for rewarming. In the cold, such a delay is not necessarily advantageous as the extra heat can always be used to substitute for thermoregulatory thermogenesis. The occurrence of concentrated period of digestion only in the morning-pulse condition was correlated to a lower food intake and lower body temperature as compared with the evening-pulse condition. Restricted feeding may thus be needed to induce adaptive timing of digestion as a mechanism of energy sparing. In addition to storage of food, timing of digestion may be a significant factor in the evolution of the crop in birds.