Melatonin’s darkness-only expression profile makes it an intellectually appealing molecule to serve as a transmitter-of-time to vertebrates on two scales: circadian and seasonal. The degree to which this occurs varies across vertebrate taxa, as an example, pineal melatonin has no effect on the size of the gonads in perching songbirds. Where melatonin does play a seasonal effect is via gating a seasonal expansion in courtship behavior.

Seasonally breeding animals concentrate their courtship behavior to a particular time of year such that their offspring will be reared in a favorable environment. In house sparrows, Passer domesticus, removal of the pineal gland alters the photostimulated increase in vocal rate and complexity. Additionally, administration of long durations of melatonin, indicative of short days of winter, prevents seasonal recrudescence of song control nuclei in photostimulated house sparrows.

In this highlighted study, long durations of melatonin delivered to pinealectomized birds prevented the expansion to a spring-like vocal state observed in the birds receiving a short duration of melatonin, indicative of summer, or a vehicle control. The daily distribution of locomotor activity, beak color (a correlate to circulating testosterone), and testis size were relatively unaffected by treatment. Vocal state change was defined by our measures in two capacities: (i) increased dawn and dusk choruses, and (ii) an increase in calls associated with territory and mate attraction compared to the winter-like “social song,” as demonstrated by the above video.

We conclude that house sparrows use the calendar information provided by melatonin duration to control seasonal vocalization behavior, independent of effects on and of the gonads.