Circannual rhythms, such as the cycling between breeding and nonbreeding seasons, provide critical timing information for organisms’ major life history events including reproduction cycles, courtship, and migratory restlessness, among other physiological and behavioral changes. Photoperiods are particularly useful indicators of seasonal change because of their consistent nature, which allows organisms to predict and prepare for the onset of the short breeding season. The growth and regression of the neural circuits that control the seasonal production of song in Gamble’s white-crowned sparrow (Zonotrichia leucophrys gambelii) are one such example of the ability of photoperiods to time changes in brain morphology, organismal physiology, and behavior. As sparrows transition into breeding conditions, testosterone levels increase, driving an increase in the volume and neuron number of one song nucleus HVC (proper name) and an increase in song production rate and stereotypy. These seasonal changes can be replicated in a laboratory environment through strict light, temperature, and hormone regimes. However, variability in HVC volume and neuron number, testosterone levels, and song production of sparrows during the replicated breeding and nonbreeding seasons has been observed. The endogenous seasonal rhythm has been observed to persist in constant laboratory conditions across taxa, and thus may persist in white-crowned sparrows as well. We tested whether endogenous seasonal rhythms may contribute to the observed variability between individuals. By examining the persistence of an endogenous seasonal rhythm the measurement of several morphological and physiological traits associated with breeding seasons in sparrows maintained throughout the year in controlled laboratory nonbreeding conditions was collected. Finding seasonal-like differences in morphology and physiology of birds housed in constant nonbreeding conditions would suggest the need for additional experimental controls in future studies of songbirds in order to prevent variability due to endogenous seasonal rhythms.