House sparrows show a clear circadian rhythm with regard to their activity levels.
They are active diurnally and inactive nocturnally. Investigators were able to quantify activity levels of the sparrows by observing their perch-hopping. Thus, the sparrow was a suitable organism to use for the study of photoreception as it relates to photoperiodism. This experiment created a bit of skepticism, since the existence of photoreceptors outside of the retina was scoffed at by members of the field. If their eyes were the only means of photoreception, one would expect that the sparrows would quickly begin to free run in the absence of light cues to entrain them.