The ability to learn and perform complex motor skills varies with the time of day. One complex motor skill that oscillates daily is song produced by songbirds. The study of song is particularly useful for understanding mechanisms driving the learning of vocalizations and the cellular and morphological changes that accompany changes in song production rate and quality. In one such songbird, Gambel’s White-crowned Sparrow (Zonotrichia leucophrys gambelii), the production of song and the underlying neural circuit that controls this song change dramatically between breeding and nonbreeding seasons. We sought to determine whether song also changed on a daily time scale. We analyzed song production rate, spectral features, and stereotypy of more than 1500 songs produced over the course of two consecutive days by sparrows maintained in breeding conditions. We identified daily patterns in song rate, frequency, entropy, power, and energy of breeding song within both individual birds and pooled bird data. The demonstration of daily oscillations in song rate and spectral features song supports both descriptions of ‘the dawn chorus’ and growing evidence on the modulation of the song control circuit by the internal circadian clock. Having shown daily changes in song production in white-crowned sparrows, we can now begin to study the relationship between song production, cellular changes like neuronal birth and death that occur within the song circuit, and the endogenous circadian clock.

The balance between neuronal birth and death is a fundamental process of adult neural plasticity that mediates the maintenance and production of behavior. Aberration in this balance often coincides with neurodegenerative diseases including Parkinson’s, chronic depression, and stroke. Songbirds are an excellent model for exploring the dynamics of neuronal turnover, i.e. the balance between neuronal birth and death, and its effects on behavior, as seasonal production of song is under the control of a well-defined plastic neural circuit. This circuit includes the song control nucleus HVC (proper name) and its target, the robust nucleus of the arcopallium. Seasonal plasticity of HVC in Gambel’s white-crowned sparrows (Zonotrichia leucophrys gambelli) involves pronounced changes in neuron number; HVC neuron number changes 25% (>68,000 neurons) between breeding and nonbreeding seasons. The dramatic differences in neuron number between breeding and nonbreeding conditions suggest that dynamics between neuronal birth and death also differ between seasons. To determine if there are seasonal differences in neuronal turnover, I labeled two cohorts of new HVC neurons separated by one, two, and four months with two thymidine analogs, BrdU and Edu, in both breeding and nonbreeding condition birds. I determined turnover rate by quantifying the number of new neurons from each cohort that were present in HVC at the different intervals. Preliminary data indicates that in breeding birds more new neurons belonged to the first cohort, whereas in nonbreeding birds more new neurons belonged to the second cohort. The replacement of first cohort neurons by the second cohort neurons increases with time in non-breeding birds. This suggests that testosterone in breeding birds promotes survival of the first wave of new neurons entering HVC. This study demonstrates that the seasonal changes in HVC neuron number results from a dynamic shifting balance between neuronal birth and death.

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.