FOOD AS A CIRCADIAN ZEITGEBER FOR HOUSE SPARROWS - THE EFFECT OF DIFFERENT FOOD ACCESS DURATIONS

House sparrows (Passer domesticus) can synchronize their circadian rhy thms of locomotion and feeding with times of periodic food availabilit y. In contrast to most mammals, which synchronize only a specific part of their circadian system with feeding cycles and thus express two di stinct activity components, house sparrows synchronize their circadian activity rhythms as a whole with the food zeitgeber. Previous results had indicated that feeding cycles act as comparatively weak zeitgeber s for house sparrows. In the present study, therefore, we investigate whether feeding schedules are weak zeitgebers in general or whether th eir impact on the circadian system of the birds depends on the degree of food restriction. A detailed analysis of the synchronization patter n under the different experimental conditions should help to clarify w hether house sparrows use a different mechanism for food-synchronizati on than mammals. House sparrows were kept in continuous dim light and exposed to different feeding schedules with daily food access duration s ranging from 8 to 20 h. Many birds lost synchronization and exhibite d free-running rhythms in locomotor and feeding activity when the dail y food access duration was lengthened but became synchronized when the feeding duration was shortened. The interpretation that short food ac cess durations represent stronger zeitgebers than long food access dur ations was supported by the occurrence of large negative phase-angle d ifferences during long daily feeding schedules, contrasting with small and sometimes positive phase-angle differences under short food acces s durations. There were no indications that house sparrows possess a s pecific food-entrainable circadian oscillator as mammals do. Rather, p eriodic food availability seems to be a zeitgeber for the whole circad ian system, which, hence, can be synchronized both by light and food. An explanation for such different mechanisms of food-synchronization i s offered in the feeding ecology of these animals. Birds may evaluate the importance of a specific feeding schedule as a zeitgeber either fr om temporal information on the duration of the daily food access time or from energetic considerations. The phase-angle differences associat ed with the different feeding schedules and the maintenance of daily a ctivity times may ensure an appropriate temporal integration of behavi or with specific conditions. Nonsynchronized birds exhibited masking-i nduced feeding activity, which might represent an alternative means of adjusting to feeding cycles when synchronization cannot occur.